[dev.typeparams] cmd/compile: export/import of recursive generic types.

Deal with export/import of recursive generic types. This includes
typeparams which have bounds that reference the typeparam.

There are three main changes:

  - Change export/import of typeparams to have an implicit "declaration"
    (doDecl). We need to do a declaration of typeparams (via the
    typeparam's package and unique name), because it may be referenced
    within its bound during its own definition.

  - We delay most of the processing of the Instantiate call until we
    finish the creation of the top-most type (similar to the way we
    delay CheckSize). This is because we can't do the full instantiation
    properly until the base type is fully defined (with methods). The
    functions delayDoInst() and resumeDoInst() delay and resume the
    processing of the instantiations.

  - To do the full needed type substitutions for type instantiations
    during import, I had to separate out the type subster in stencil.go
    and move it to subr.go in the typecheck package. The subster in
    stencil.go now does node substitution and makes use of the type
    subster to do type substitutions.

Notable other changes:
 - In types/builtins.go, put the newly defined typeparam for a union type
   (related to use of real/imag, etc.) in the current package, rather
   than the builtin package, so exports/imports work properly.

 - In types2, allowed NewTypeParam() to be called with a nil bound, and
   allow setting the bound later. (Needed to import a typeparam whose
   bound refers to the typeparam itself.)

 - During import of typeparams in types2 (importer/import.go), we need
   to keep an index of the typeparams by their package and unique name
   (with id). Use a new map typParamIndex[] for that. Again, this is
   needed to deal with typeparams whose bounds refer to the typeparam
   itself.

 - Added several new tests absdiffimp.go and orderedmapsimp.go. Some of
   the orderemapsimp tests are commented out for now, because there are
   some issues with closures inside instantiations (relating to unexported
   names of closure structs).

 - Renamed some typeparams in test value.go to make them all T (to make
   typeparam uniqueness is working fine).

Change-Id: Ib47ed9471c19ee8e9fbb34e8506907dad3021e5a
Reviewed-on: https://go-review.googlesource.com/c/go/+/323029
Trust: Dan Scales <danscales@google.com>
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

8 files changed, 620 insertions, 430 deletions

diff --git a/src/cmd/compile/internal/importer/iimport.go b/src/cmd/compile/internal/importer/iimport.go
index fd48bfc179..fb39e93073 100644
--- a/src/cmd/compile/internal/importer/iimport.go
+++ b/src/cmd/compile/internal/importer/iimport.go
@@ -51,6 +51,11 @@ const (
 	iexportVersionCurrent = iexportVersionGenerics + 1
 )
 
+type ident struct {
+	pkg  string
+	name string
+}
+
 const predeclReserved = 32
 
 type itag uint64
@@ -124,6 +129,9 @@ func iImportData(imports map[string]*types2.Package, data []byte, path string) (
 		declData: declData,
 		pkgIndex: make(map[*types2.Package]map[string]uint64),
 		typCache: make(map[uint64]types2.Type),
+		// Separate map for typeparams, keyed by their package and unique
+		// name (name with subscript).
+		tparamIndex: make(map[ident]types2.Type),
 	}
 
 	for i, pt := range predeclared {
@@ -202,9 +210,10 @@ type iimporter struct {
 	pkgCache     map[uint64]*types2.Package
 	posBaseCache map[uint64]*syntax.PosBase
 
-	declData []byte
-	pkgIndex map[*types2.Package]map[string]uint64
-	typCache map[uint64]types2.Type
+	declData    []byte
+	pkgIndex    map[*types2.Package]map[string]uint64
+	typCache    map[uint64]types2.Type
+	tparamIndex map[ident]types2.Type
 
 	interfaceList []*types2.Interface
 }
@@ -358,6 +367,28 @@ func (r *importReader) obj(name string) {
 			}
 		}
 
+	case 'P':
+		// We need to "declare" a typeparam in order to have a name that
+		// can be referenced recursively (if needed) in the type param's
+		// bound.
+		if r.p.exportVersion < iexportVersionGenerics {
+			errorf("unexpected type param type")
+		}
+		index := int(r.int64())
+		name0, sub := parseSubscript(name)
+		tn := types2.NewTypeName(pos, r.currPkg, name0, nil)
+		t := (*types2.Checker)(nil).NewTypeParam(tn, index, nil)
+		if sub == 0 {
+			errorf("missing subscript")
+		}
+		t.SetId(sub)
+		// To handle recursive references to the typeparam within its
+		// bound, save the partial type in tparamIndex before reading the bounds.
+		id := ident{r.currPkg.Name(), name}
+		r.p.tparamIndex[id] = t
+
+		t.SetBound(r.typ())
+
 	case 'V':
 		typ := r.typ()
 
@@ -617,34 +648,15 @@ func (r *importReader) doType(base *types2.Named) types2.Type {
 		if r.p.exportVersion < iexportVersionGenerics {
 			errorf("unexpected type param type")
 		}
-		r.currPkg = r.pkg()
-		pos := r.pos()
-		name := r.string()
-
-		// Extract the subscript value from the type param name. We export
-		// and import the subscript value, so that all type params have
-		// unique names.
-		sub := uint64(0)
-		startsub := -1
-		for i, r := range name {
-			if '₀' <= r && r < '₀'+10 {
-				if startsub == -1 {
-					startsub = i
-				}
-				sub = sub*10 + uint64(r-'₀')
-			}
-		}
-		if startsub >= 0 {
-			name = name[:startsub]
-		}
-		index := int(r.int64())
-		bound := r.typ()
-		tn := types2.NewTypeName(pos, r.currPkg, name, nil)
-		t := (*types2.Checker)(nil).NewTypeParam(tn, index, bound)
-		if sub >= 0 {
-			t.SetId(sub)
+		pkg, name := r.qualifiedIdent()
+		id := ident{pkg.Name(), name}
+		if t, ok := r.p.tparamIndex[id]; ok {
+			// We're already in the process of importing this typeparam.
+			return t
 		}
-		return t
+		// Otherwise, import the definition of the typeparam now.
+		r.p.doDecl(pkg, name)
+		return r.p.tparamIndex[id]
 
 	case instType:
 		if r.p.exportVersion < iexportVersionGenerics {
@@ -753,3 +765,23 @@ func baseType(typ types2.Type) *types2.Named {
 	n, _ := typ.(*types2.Named)
 	return n
 }
+
+func parseSubscript(name string) (string, uint64) {
+	// Extract the subscript value from the type param name. We export
+	// and import the subscript value, so that all type params have
+	// unique names.
+	sub := uint64(0)
+	startsub := -1
+	for i, r := range name {
+		if '₀' <= r && r < '₀'+10 {
+			if startsub == -1 {
+				startsub = i
+			}
+			sub = sub*10 + uint64(r-'₀')
+		}
+	}
+	if startsub >= 0 {
+		name = name[:startsub]
+	}
+	return name, sub
+}
diff --git a/src/cmd/compile/internal/noder/stencil.go b/src/cmd/compile/internal/noder/stencil.go
index 08c09c6fb1..3ba364f67c 100644
--- a/src/cmd/compile/internal/noder/stencil.go
+++ b/src/cmd/compile/internal/noder/stencil.go
@@ -15,7 +15,6 @@ import (
 	"cmd/compile/internal/types"
 	"fmt"
 	"go/constant"
-	"strings"
 )
 
 // For catching problems as we add more features
@@ -425,13 +424,6 @@ func (g *irgen) instantiateMethods() {
 
 }
 
-// genericSym returns the name of the base generic type for the type named by
-// sym. It simply returns the name obtained by removing everything after the
-// first bracket ("[").
-func genericTypeName(sym *types.Sym) string {
-	return sym.Name[0:strings.Index(sym.Name, "[")]
-}
-
 // getInstantiationForNode returns the function/method instantiation for a
 // InstExpr node inst.
 func (g *irgen) getInstantiationForNode(inst *ir.InstExpr) *ir.Func {
@@ -479,11 +471,7 @@ type subster struct {
 	g        *irgen
 	isMethod bool     // If a method is being instantiated
 	newf     *ir.Func // Func node for the new stenciled function
-	tparams  []*types.Type
-	targs    []*types.Type
-	// The substitution map from name nodes in the generic function to the
-	// name nodes in the new stenciled function.
-	vars map[*ir.Name]*ir.Name
+	ts       typecheck.Tsubster
 }
 
 // genericSubst returns a new function with name newsym. The function is an
@@ -526,9 +514,11 @@ func (g *irgen) genericSubst(newsym *types.Sym, nameNode *ir.Name, targs []*type
 		g:        g,
 		isMethod: isMethod,
 		newf:     newf,
-		tparams:  tparams,
-		targs:    targs,
-		vars:     make(map[*ir.Name]*ir.Name),
+		ts: typecheck.Tsubster{
+			Tparams: tparams,
+			Targs:   targs,
+			Vars:    make(map[*ir.Name]*ir.Name),
+		},
 	}
 
 	newf.Dcl = make([]*ir.Name, 0, len(gf.Dcl)+1)
@@ -574,6 +564,9 @@ func (g *irgen) genericSubst(newsym *types.Sym, nameNode *ir.Name, targs []*type
 	newf.Body.Prepend(g.checkDictionary(dictionaryName, targs)...)
 
 	ir.CurFunc = savef
+	// Add any new, fully instantiated types seen during the substitution to
+	// g.instTypeList.
+	g.instTypeList = append(g.instTypeList, subst.ts.InstTypeList...)
 
 	return newf
 }
@@ -586,13 +579,13 @@ func (subst *subster) localvar(name *ir.Name) *ir.Name {
 	if name.IsClosureVar() {
 		m.SetIsClosureVar(true)
 	}
-	m.SetType(subst.typ(name.Type()))
+	m.SetType(subst.ts.Typ(name.Type()))
 	m.BuiltinOp = name.BuiltinOp
 	m.Curfn = subst.newf
 	m.Class = name.Class
 	assert(name.Class != ir.PEXTERN && name.Class != ir.PFUNC)
 	m.Func = name.Func
-	subst.vars[name] = m
+	subst.ts.Vars[name] = m
 	m.SetTypecheck(1)
 	return m
 }
@@ -635,7 +628,7 @@ func (g *irgen) checkDictionary(name *ir.Name, targs []*types.Type) (code []ir.N
 	return
 }
 
-// node is like DeepCopy(), but substitutes ONAME nodes based on subst.vars, and
+// node is like DeepCopy(), but substitutes ONAME nodes based on subst.ts.vars, and
 // also descends into closures. It substitutes type arguments for type parameters
 // in all the new nodes.
 func (subst *subster) node(n ir.Node) ir.Node {
@@ -644,10 +637,10 @@ func (subst *subster) node(n ir.Node) ir.Node {
 	edit = func(x ir.Node) ir.Node {
 		switch x.Op() {
 		case ir.OTYPE:
-			return ir.TypeNode(subst.typ(x.Type()))
+			return ir.TypeNode(subst.ts.Typ(x.Type()))
 
 		case ir.ONAME:
-			if v := subst.vars[x.(*ir.Name)]; v != nil {
+			if v := subst.ts.Vars[x.(*ir.Name)]; v != nil {
 				return v
 			}
 			return x
@@ -673,7 +666,7 @@ func (subst *subster) node(n ir.Node) ir.Node {
 					base.Fatalf(fmt.Sprintf("Nil type for %v", x))
 				}
 			} else if x.Op() != ir.OCLOSURE {
-				m.SetType(subst.typ(x.Type()))
+				m.SetType(subst.ts.Typ(x.Type()))
 			}
 		}
 		ir.EditChildren(m, edit)
@@ -815,7 +808,7 @@ func (subst *subster) node(n ir.Node) ir.Node {
 			m.(*ir.ClosureExpr).Func = newfn
 			// Closure name can already have brackets, if it derives
 			// from a generic method
-			newsym := typecheck.MakeInstName(oldfn.Nname.Sym(), subst.targs, subst.isMethod)
+			newsym := typecheck.MakeInstName(oldfn.Nname.Sym(), subst.ts.Targs, subst.isMethod)
 			newfn.Nname = ir.NewNameAt(oldfn.Nname.Pos(), newsym)
 			newfn.Nname.Func = newfn
 			newfn.Nname.Defn = newfn
@@ -828,7 +821,7 @@ func (subst *subster) node(n ir.Node) ir.Node {
 			newfn.Dcl = subst.namelist(oldfn.Dcl)
 			newfn.ClosureVars = subst.namelist(oldfn.ClosureVars)
 
-			typed(subst.typ(oldfn.Nname.Type()), newfn.Nname)
+			typed(subst.ts.Typ(oldfn.Nname.Type()), newfn.Nname)
 			typed(newfn.Nname.Type(), m)
 			newfn.SetTypecheck(1)
 
@@ -867,298 +860,6 @@ func (subst *subster) list(l []ir.Node) []ir.Node {
 	return s
 }
 
-// tstruct substitutes type params in types of the fields of a structure type. For
-// each field, tstruct copies the Nname, and translates it if Nname is in
-// subst.vars. To always force the creation of a new (top-level) struct,
-// regardless of whether anything changed with the types or names of the struct's
-// fields, set force to true.
-func (subst *subster) tstruct(t *types.Type, force bool) *types.Type {
-	if t.NumFields() == 0 {
-		if t.HasTParam() {
-			// For an empty struct, we need to return a new type,
-			// since it may now be fully instantiated (HasTParam
-			// becomes false).
-			return types.NewStruct(t.Pkg(), nil)
-		}
-		return t
-	}
-	var newfields []*types.Field
-	if force {
-		newfields = make([]*types.Field, t.NumFields())
-	}
-	for i, f := range t.Fields().Slice() {
-		t2 := subst.typ(f.Type)
-		if (t2 != f.Type || f.Nname != nil) && newfields == nil {
-			newfields = make([]*types.Field, t.NumFields())
-			for j := 0; j < i; j++ {
-				newfields[j] = t.Field(j)
-			}
-		}
-		if newfields != nil {
-			// TODO(danscales): make sure this works for the field
-			// names of embedded types (which should keep the name of
-			// the type param, not the instantiated type).
-			newfields[i] = types.NewField(f.Pos, f.Sym, t2)
-			if f.Nname != nil {
-				v := subst.vars[f.Nname.(*ir.Name)]
-				if v != nil {
-					// This is the case where we are
-					// translating the type of the function we
-					// are substituting, so its dcls are in
-					// the subst.vars table, and we want to
-					// change to reference the new dcl.
-					newfields[i].Nname = v
-				} else {
-					// This is the case where we are
-					// translating the type of a function
-					// reference inside the function we are
-					// substituting, so we leave the Nname
-					// value as is.
-					newfields[i].Nname = f.Nname
-				}
-			}
-		}
-	}
-	if newfields != nil {
-		return types.NewStruct(t.Pkg(), newfields)
-	}
-	return t
-
-}
-
-// tinter substitutes type params in types of the methods of an interface type.
-func (subst *subster) tinter(t *types.Type) *types.Type {
-	if t.Methods().Len() == 0 {
-		return t
-	}
-	var newfields []*types.Field
-	for i, f := range t.Methods().Slice() {
-		t2 := subst.typ(f.Type)
-		if (t2 != f.Type || f.Nname != nil) && newfields == nil {
-			newfields = make([]*types.Field, t.Methods().Len())
-			for j := 0; j < i; j++ {
-				newfields[j] = t.Methods().Index(j)
-			}
-		}
-		if newfields != nil {
-			newfields[i] = types.NewField(f.Pos, f.Sym, t2)
-		}
-	}
-	if newfields != nil {
-		return types.NewInterface(t.Pkg(), newfields)
-	}
-	return t
-}
-
-// typ computes the type obtained by substituting any type parameter in t with the
-// corresponding type argument in subst. If t contains no type parameters, the
-// result is t; otherwise the result is a new type. It deals with recursive types
-// by using TFORW types and finding partially or fully created types via sym.Def.
-func (subst *subster) typ(t *types.Type) *types.Type {
-	if !t.HasTParam() && t.Kind() != types.TFUNC {
-		// Note: function types need to be copied regardless, as the
-		// types of closures may contain declarations that need
-		// to be copied. See #45738.
-		return t
-	}
-
-	if t.Kind() == types.TTYPEPARAM {
-		for i, tp := range subst.tparams {
-			if tp == t {
-				return subst.targs[i]
-			}
-		}
-		// If t is a simple typeparam T, then t has the name/symbol 'T'
-		// and t.Underlying() == t.
-		//
-		// However, consider the type definition: 'type P[T any] T'. We
-		// might use this definition so we can have a variant of type T
-		// that we can add new methods to. Suppose t is a reference to
-		// P[T]. t has the name 'P[T]', but its kind is TTYPEPARAM,
-		// because P[T] is defined as T. If we look at t.Underlying(), it
-		// is different, because the name of t.Underlying() is 'T' rather
-		// than 'P[T]'. But the kind of t.Underlying() is also TTYPEPARAM.
-		// In this case, we do the needed recursive substitution in the
-		// case statement below.
-		if t.Underlying() == t {
-			// t is a simple typeparam that didn't match anything in tparam
-			return t
-		}
-		// t is a more complex typeparam (e.g. P[T], as above, whose
-		// definition is just T).
-		assert(t.Sym() != nil)
-	}
-
-	var newsym *types.Sym
-	var neededTargs []*types.Type
-	var forw *types.Type
-
-	if t.Sym() != nil {
-		// Translate the type params for this type according to
-		// the tparam/targs mapping from subst.
-		neededTargs = make([]*types.Type, len(t.RParams()))
-		for i, rparam := range t.RParams() {
-			neededTargs[i] = subst.typ(rparam)
-		}
-		// For a named (defined) type, we have to change the name of the
-		// type as well. We do this first, so we can look up if we've
-		// already seen this type during this substitution or other
-		// definitions/substitutions.
-		genName := genericTypeName(t.Sym())
-		newsym = t.Sym().Pkg.Lookup(typecheck.InstTypeName(genName, neededTargs))
-		if newsym.Def != nil {
-			// We've already created this instantiated defined type.
-			return newsym.Def.Type()
-		}
-
-		// In order to deal with recursive generic types, create a TFORW
-		// type initially and set the Def field of its sym, so it can be
-		// found if this type appears recursively within the type.
-		forw = typecheck.NewIncompleteNamedType(t.Pos(), newsym)
-		//println("Creating new type by sub", newsym.Name, forw.HasTParam())
-		forw.SetRParams(neededTargs)
-		// Copy the OrigSym from the re-instantiated type (which is the sym of
-		// the base generic type).
-		assert(t.OrigSym != nil)
-		forw.OrigSym = t.OrigSym
-	}
-
-	var newt *types.Type
-
-	switch t.Kind() {
-	case types.TTYPEPARAM:
-		if t.Sym() == newsym {
-			// The substitution did not change the type.
-			return t
-		}
-		// Substitute the underlying typeparam (e.g. T in P[T], see
-		// the example describing type P[T] above).
-		newt = subst.typ(t.Underlying())
-		assert(newt != t)
-
-	case types.TARRAY:
-		elem := t.Elem()
-		newelem := subst.typ(elem)
-		if newelem != elem {
-			newt = types.NewArray(newelem, t.NumElem())
-		}
-
-	case types.TPTR:
-		elem := t.Elem()
-		newelem := subst.typ(elem)
-		if newelem != elem {
-			newt = types.NewPtr(newelem)
-		}
-
-	case types.TSLICE:
-		elem := t.Elem()
-		newelem := subst.typ(elem)
-		if newelem != elem {
-			newt = types.NewSlice(newelem)
-		}
-
-	case types.TSTRUCT:
-		newt = subst.tstruct(t, false)
-		if newt == t {
-			newt = nil
-		}
-
-	case types.TFUNC:
-		newrecvs := subst.tstruct(t.Recvs(), false)
-		newparams := subst.tstruct(t.Params(), false)
-		newresults := subst.tstruct(t.Results(), false)
-		if newrecvs != t.Recvs() || newparams != t.Params() || newresults != t.Results() {
-			// If any types have changed, then the all the fields of
-			// of recv, params, and results must be copied, because they have
-			// offset fields that are dependent, and so must have an
-			// independent copy for each new signature.
-			var newrecv *types.Field
-			if newrecvs.NumFields() > 0 {
-				if newrecvs == t.Recvs() {
-					newrecvs = subst.tstruct(t.Recvs(), true)
-				}
-				newrecv = newrecvs.Field(0)
-			}
-			if newparams == t.Params() {
-				newparams = subst.tstruct(t.Params(), true)
-			}
-			if newresults == t.Results() {
-				newresults = subst.tstruct(t.Results(), true)
-			}
-			newt = types.NewSignature(t.Pkg(), newrecv, t.TParams().FieldSlice(), newparams.FieldSlice(), newresults.FieldSlice())
-		}
-
-	case types.TINTER:
-		newt = subst.tinter(t)
-		if newt == t {
-			newt = nil
-		}
-
-	case types.TMAP:
-		newkey := subst.typ(t.Key())
-		newval := subst.typ(t.Elem())
-		if newkey != t.Key() || newval != t.Elem() {
-			newt = types.NewMap(newkey, newval)
-		}
-
-	case types.TCHAN:
-		elem := t.Elem()
-		newelem := subst.typ(elem)
-		if newelem != elem {
-			newt = types.NewChan(newelem, t.ChanDir())
-			if !newt.HasTParam() {
-				// TODO(danscales): not sure why I have to do this
-				// only for channels.....
-				types.CheckSize(newt)
-			}
-		}
-	}
-	if newt == nil {
-		// Even though there were typeparams in the type, there may be no
-		// change if this is a function type for a function call (which will
-		// have its own tparams/targs in the function instantiation).
-		return t
-	}
-
-	if t.Sym() == nil {
-		// Not a named type, so there was no forwarding type and there are
-		// no methods to substitute.
-		assert(t.Methods().Len() == 0)
-		return newt
-	}
-
-	forw.SetUnderlying(newt)
-	newt = forw
-
-	if t.Kind() != types.TINTER && t.Methods().Len() > 0 {
-		// Fill in the method info for the new type.
-		var newfields []*types.Field
-		newfields = make([]*types.Field, t.Methods().Len())
-		for i, f := range t.Methods().Slice() {
-			t2 := subst.typ(f.Type)
-			oldsym := f.Nname.Sym()
-			newsym := typecheck.MakeInstName(oldsym, subst.targs, true)
-			// TODO: use newsym?
-			var nname *ir.Name
-			if newsym.Def != nil {
-				nname = newsym.Def.(*ir.Name)
-			} else {
-				nname = ir.NewNameAt(f.Pos, oldsym)
-				nname.SetType(t2)
-				oldsym.Def = nname
-			}
-			newfields[i] = types.NewField(f.Pos, f.Sym, t2)
-			newfields[i].Nname = nname
-		}
-		newt.Methods().Set(newfields)
-		if !newt.HasTParam() {
-			// Generate all the methods for a new fully-instantiated type.
-			subst.g.instTypeList = append(subst.g.instTypeList, newt)
-		}
-	}
-	return newt
-}
-
 // fields sets the Nname field for the Field nodes inside a type signature, based
 // on the corresponding in/out parameters in dcl. It depends on the in and out
 // parameters being in order in dcl.
@@ -1178,7 +879,7 @@ func (subst *subster) fields(class ir.Class, oldfields []*types.Field, dcl []*ir
 	newfields := make([]*types.Field, len(oldfields))
 	for j := range oldfields {
 		newfields[j] = oldfields[j].Copy()
-		newfields[j].Type = subst.typ(oldfields[j].Type)
+		newfields[j].Type = subst.ts.Typ(oldfields[j].Type)
 		// A PPARAM field will be missing from dcl if its name is
 		// unspecified or specified as "_". So, we compare the dcl sym
 		// with the field sym (or sym of the field's Nname node). (Unnamed
diff --git a/src/cmd/compile/internal/noder/types.go b/src/cmd/compile/internal/noder/types.go
index f0061e79d7..b37793b2d0 100644
--- a/src/cmd/compile/internal/noder/types.go
+++ b/src/cmd/compile/internal/noder/types.go
@@ -316,13 +316,16 @@ func (g *irgen) fillinMethods(typ *types2.Named, ntyp *types.Type) {
 						tparams[i] = g.typ1(rparam.Type())
 					}
 					assert(len(tparams) == len(targs))
-					subst := &subster{
-						g:       g,
-						tparams: tparams,
-						targs:   targs,
+					ts := typecheck.Tsubster{
+						Tparams: tparams,
+						Targs:   targs,
 					}
 					// Do the substitution of the type
-					meth2.SetType(subst.typ(meth.Type()))
+					meth2.SetType(ts.Typ(meth.Type()))
+					// Add any new fully instantiated types
+					// seen during the substitution to
+					// g.instTypeList.
+					g.instTypeList = append(g.instTypeList, ts.InstTypeList...)
 					newsym.Def = meth2
 				}
 				meth = meth2
diff --git a/src/cmd/compile/internal/typecheck/iexport.go b/src/cmd/compile/internal/typecheck/iexport.go
index 66c356ee7c..f635b79ada 100644
--- a/src/cmd/compile/internal/typecheck/iexport.go
+++ b/src/cmd/compile/internal/typecheck/iexport.go
@@ -506,6 +506,20 @@ func (p *iexporter) doDecl(n *ir.Name) {
 		w.constExt(n)
 
 	case ir.OTYPE:
+		if n.Type().Kind() == types.TTYPEPARAM && n.Type().Underlying() == n.Type() {
+			// Even though it has local scope, a typeparam requires a
+			// declaration via its package and unique name, because it
+			// may be referenced within its type bound during its own
+			// definition.
+			w.tag('P')
+			// A typeparam has a name, and has a type bound rather
+			// than an underlying type.
+			w.pos(n.Pos())
+			w.int64(int64(n.Type().Index()))
+			w.typ(n.Type().Bound())
+			break
+		}
+
 		if n.Alias() {
 			// Alias.
 			w.tag('A')
@@ -519,9 +533,10 @@ func (p *iexporter) doDecl(n *ir.Name) {
 		w.pos(n.Pos())
 
 		if base.Flag.G > 0 {
-			// Export any new typeparams needed for this type
+			// Export type parameters, if any, needed for this type
 			w.typeList(n.Type().RParams())
 		}
+
 		underlying := n.Type().Underlying()
 		if underlying == types.ErrorType.Underlying() {
 			// For "type T error", use error as the
@@ -837,26 +852,6 @@ func (w *exportWriter) startType(k itag) {
 }
 
 func (w *exportWriter) doTyp(t *types.Type) {
-	if t.Kind() == types.TTYPEPARAM {
-		assert(base.Flag.G > 0)
-		// A typeparam has a name, but doesn't have an underlying type.
-		// Just write out the details of the type param here. All other
-		// uses of this typeparam type will be written out as its unique
-		// type offset.
-		w.startType(typeParamType)
-		s := t.Sym()
-		w.setPkg(s.Pkg, true)
-		w.pos(t.Pos())
-
-		// We are writing out the name with the subscript, so that the
-		// typeparam name is unique.
-		w.string(s.Name)
-		w.int64(int64(t.Index()))
-
-		w.typ(t.Bound())
-		return
-	}
-
 	s := t.Sym()
 	if s != nil && t.OrigSym != nil {
 		assert(base.Flag.G > 0)
@@ -880,6 +875,21 @@ func (w *exportWriter) doTyp(t *types.Type) {
 		return
 	}
 
+	// The 't.Underlying() == t' check is to confirm this is a base typeparam
+	// type, rather than a defined type with typeparam underlying type, like:
+	// type orderedAbs[T any] T
+	if t.Kind() == types.TTYPEPARAM && t.Underlying() == t {
+		assert(base.Flag.G > 0)
+		if s.Pkg == types.BuiltinPkg || s.Pkg == ir.Pkgs.Unsafe {
+			base.Fatalf("builtin type missing from typIndex: %v", t)
+		}
+		// Write out the first use of a type param as a qualified ident.
+		// This will force a "declaration" of the type param.
+		w.startType(typeParamType)
+		w.qualifiedIdent(t.Obj().(*ir.Name))
+		return
+	}
+
 	if s != nil {
 		if s.Pkg == types.BuiltinPkg || s.Pkg == ir.Pkgs.Unsafe {
 			base.Fatalf("builtin type missing from typIndex: %v", t)
@@ -1325,14 +1335,20 @@ func (w *exportWriter) funcExt(n *ir.Name) {
 	// Inline body.
 	if n.Type().HasTParam() {
 		if n.Func.Inl != nil {
-			base.FatalfAt(n.Pos(), "generic function is marked inlineable")
-		}
-		// Populate n.Func.Inl, so body of exported generic function will
-		// be written out.
-		n.Func.Inl = &ir.Inline{
-			Cost: 1,
-			Dcl:  n.Func.Dcl,
-			Body: n.Func.Body,
+			// n.Func.Inl may already be set on a generic function if
+			// we imported it from another package, but shouldn't be
+			// set for a generic function in the local package.
+			if n.Sym().Pkg == types.LocalPkg {
+				base.FatalfAt(n.Pos(), "generic function is marked inlineable")
+			}
+		} else {
+			// Populate n.Func.Inl, so body of exported generic function will
+			// be written out.
+			n.Func.Inl = &ir.Inline{
+				Cost: 1,
+				Dcl:  n.Func.Dcl,
+				Body: n.Func.Body,
+			}
 		}
 	}
 	if n.Func.Inl != nil {
diff --git a/src/cmd/compile/internal/typecheck/iimport.go b/src/cmd/compile/internal/typecheck/iimport.go
index 96107b657b..9e6115cbf7 100644
--- a/src/cmd/compile/internal/typecheck/iimport.go
+++ b/src/cmd/compile/internal/typecheck/iimport.go
@@ -342,19 +342,22 @@ func (r *importReader) doDecl(sym *types.Sym) *ir.Name {
 		// declaration before recursing.
 		n := importtype(r.p.ipkg, pos, sym)
 		t := n.Type()
+		if rparams != nil {
+			t.SetRParams(rparams)
+		}
 
 		// We also need to defer width calculations until
 		// after the underlying type has been assigned.
 		types.DeferCheckSize()
+		deferDoInst()
 		underlying := r.typ()
 		t.SetUnderlying(underlying)
-		types.ResumeCheckSize()
-
-		if rparams != nil {
-			t.SetRParams(rparams)
-		}
 
 		if underlying.IsInterface() {
+			// Finish up all type instantiations and CheckSize calls
+			// now that a top-level type is fully constructed.
+			resumeDoInst()
+			types.ResumeCheckSize()
 			r.typeExt(t)
 			return n
 		}
@@ -380,12 +383,38 @@ func (r *importReader) doDecl(sym *types.Sym) *ir.Name {
 		}
 		t.Methods().Set(ms)
 
+		// Finish up all instantiations and CheckSize calls now
+		// that a top-level type is fully constructed.
+		resumeDoInst()
+		types.ResumeCheckSize()
+
 		r.typeExt(t)
 		for _, m := range ms {
 			r.methExt(m)
 		}
 		return n
 
+	case 'P':
+		if r.p.exportVersion < iexportVersionGenerics {
+			base.Fatalf("unexpected type param type")
+		}
+		if sym.Def != nil {
+			// Make sure we use the same type param type for the same
+			// name, whether it is created during types1-import or
+			// this types2-to-types1 translation.
+			return sym.Def.(*ir.Name)
+		}
+		index := int(r.int64())
+		t := types.NewTypeParam(sym, index)
+		// Nname needed to save the pos.
+		nname := ir.NewDeclNameAt(pos, ir.OTYPE, sym)
+		sym.Def = nname
+		nname.SetType(t)
+		t.SetNod(nname)
+
+		t.SetBound(r.typ())
+		return nname
+
 	case 'V':
 		typ := r.typ()
 
@@ -545,7 +574,12 @@ func (r *importReader) pos() src.XPos {
 }
 
 func (r *importReader) typ() *types.Type {
-	return r.p.typAt(r.uint64())
+	// If this is a top-level type call, defer type instantiations until the
+	// type is fully constructed.
+	deferDoInst()
+	t := r.p.typAt(r.uint64())
+	resumeDoInst()
+	return t
 }
 
 func (r *importReader) exoticType() *types.Type {
@@ -683,7 +717,13 @@ func (p *iimporter) typAt(off uint64) *types.Type {
 		// are pushed to compile queue, then draining from the queue for compiling.
 		// During this process, the size calculation is disabled, so it is not safe for
 		// calculating size during SSA generation anymore. See issue #44732.
-		types.CheckSize(t)
+		//
+		// No need to calc sizes for re-instantiated generic types, and
+		// they are not necessarily resolved until the top-level type is
+		// defined (because of recursive types).
+		if t.OrigSym == nil || !t.HasTParam() {
+			types.CheckSize(t)
+		}
 		p.typCache[off] = t
 	}
 	return t
@@ -779,27 +819,18 @@ func (r *importReader) typ1() *types.Type {
 		if r.p.exportVersion < iexportVersionGenerics {
 			base.Fatalf("unexpected type param type")
 		}
-		r.setPkg()
-		pos := r.pos()
-		name := r.string()
-		sym := r.currPkg.Lookup(name)
-		index := int(r.int64())
-		bound := r.typ()
-		if sym.Def != nil {
-			// Make sure we use the same type param type for the same
-			// name, whether it is created during types1-import or
-			// this types2-to-types1 translation.
-			return sym.Def.Type()
+		// Similar to code for defined types, since we "declared"
+		// typeparams to deal with recursion (typeparam is used within its
+		// own type bound).
+		ident := r.qualifiedIdent()
+		if ident.Sym().Def != nil {
+			return ident.Sym().Def.(*ir.Name).Type()
 		}
-		t := types.NewTypeParam(sym, index)
-		// Nname needed to save the pos.
-		nname := ir.NewDeclNameAt(pos, ir.OTYPE, sym)
-		sym.Def = nname
-		nname.SetType(t)
-		t.SetNod(nname)
-
-		t.SetBound(bound)
-		return t
+		n := expandDecl(ident)
+		if n.Op() != ir.OTYPE {
+			base.Fatalf("expected OTYPE, got %v: %v, %v", n.Op(), n.Sym(), n)
+		}
+		return n.Type()
 
 	case instType:
 		if r.p.exportVersion < iexportVersionGenerics {
@@ -1758,20 +1789,91 @@ func Instantiate(pos src.XPos, baseType *types.Type, targs []*types.Type) *types
 	name := InstTypeName(baseSym.Name, targs)
 	instSym := baseSym.Pkg.Lookup(name)
 	if instSym.Def != nil {
+		// May match existing type from previous import or
+		// types2-to-types1 conversion, or from in-progress instantiation
+		// in the current type import stack.
 		return instSym.Def.Type()
 	}
 
 	t := NewIncompleteNamedType(baseType.Pos(), instSym)
 	t.SetRParams(targs)
-	// baseType may not yet be complete (since we are in the middle of
-	// importing it), but its underlying type will be updated when baseType's
-	// underlying type is finished.
-	t.SetUnderlying(baseType.Underlying())
-
-	// As with types2, the methods are the generic method signatures (without
-	// substitution).
-	t.Methods().Set(baseType.Methods().Slice())
 	t.OrigSym = baseSym
 
+	// baseType may still be TFORW or its methods may not be fully filled in
+	// (since we are in the middle of importing it). So, delay call to
+	// substInstType until we get back up to the top of the current top-most
+	// type import.
+	deferredInstStack = append(deferredInstStack, t)
+
 	return t
 }
+
+var deferredInstStack []*types.Type
+var deferInst int
+
+// deferDoInst defers substitution on instantiated types until we are at the
+// top-most defined type, so the base types are fully defined.
+func deferDoInst() {
+	deferInst++
+}
+
+func resumeDoInst() {
+	if deferInst == 1 {
+		for len(deferredInstStack) > 0 {
+			t := deferredInstStack[0]
+			deferredInstStack = deferredInstStack[1:]
+			substInstType(t, t.OrigSym.Def.(*ir.Name).Type(), t.RParams())
+		}
+	}
+	deferInst--
+}
+
+// doInst creates a new instantiation type (which will be added to
+// deferredInstStack for completion later) for an incomplete type encountered
+// during a type substitution for an instantiation. This is needed for
+// instantiations of mutually recursive types.
+func doInst(t *types.Type) *types.Type {
+	return Instantiate(t.Pos(), t.OrigSym.Def.(*ir.Name).Type(), t.RParams())
+}
+
+// substInstType completes the instantiation of a generic type by doing a
+// substitution on the underlying type itself and any methods. t is the
+// instantiation being created, baseType is the base generic type, and targs are
+// the type arguments that baseType is being instantiated with.
+func substInstType(t *types.Type, baseType *types.Type, targs []*types.Type) {
+	subst := Tsubster{
+		Tparams:       baseType.RParams(),
+		Targs:         targs,
+		SubstForwFunc: doInst,
+	}
+	t.SetUnderlying(subst.Typ(baseType.Underlying()))
+
+	newfields := make([]*types.Field, baseType.Methods().Len())
+	for i, f := range baseType.Methods().Slice() {
+		recvType := f.Type.Recv().Type
+		if recvType.IsPtr() {
+			recvType = recvType.Elem()
+		}
+		// Substitute in the method using the type params used in the
+		// method (not the type params in the definition of the generic type).
+		subst := Tsubster{
+			Tparams:       recvType.RParams(),
+			Targs:         targs,
+			SubstForwFunc: doInst,
+		}
+		t2 := subst.Typ(f.Type)
+		oldsym := f.Nname.Sym()
+		newsym := MakeInstName(oldsym, targs, true)
+		var nname *ir.Name
+		if newsym.Def != nil {
+			nname = newsym.Def.(*ir.Name)
+		} else {
+			nname = ir.NewNameAt(f.Pos, newsym)
+			nname.SetType(t2)
+			newsym.Def = nname
+		}
+		newfields[i] = types.NewField(f.Pos, f.Sym, t2)
+		newfields[i].Nname = nname
+	}
+	t.Methods().Set(newfields)
+}
diff --git a/src/cmd/compile/internal/typecheck/subr.go b/src/cmd/compile/internal/typecheck/subr.go
index 3e7799b35b..8ef49f91c8 100644
--- a/src/cmd/compile/internal/typecheck/subr.go
+++ b/src/cmd/compile/internal/typecheck/subr.go
@@ -890,7 +890,6 @@ func TypesOf(x []ir.Node) []*types.Type {
 // based on the name of the function fnsym and the targs. It replaces any
 // existing bracket type list in the name. makeInstName asserts that fnsym has
 // brackets in its name if and only if hasBrackets is true.
-// TODO(danscales): remove the assertions and the hasBrackets argument later.
 //
 // Names of declared generic functions have no brackets originally, so hasBrackets
 // should be false. Names of generic methods already have brackets, since the new
@@ -902,12 +901,24 @@ func TypesOf(x []ir.Node) []*types.Type {
 func MakeInstName(fnsym *types.Sym, targs []*types.Type, hasBrackets bool) *types.Sym {
 	b := bytes.NewBufferString("")
 
-	// marker to distinguish generic instantiations from fully stenciled wrapper functions.
+	// Determine if the type args are concrete types or new typeparams.
+	hasTParam := false
+	for _, targ := range targs {
+		if hasTParam {
+			assert(targ.HasTParam())
+		} else if targ.HasTParam() {
+			hasTParam = true
+		}
+	}
+
+	// Marker to distinguish generic instantiations from fully stenciled wrapper functions.
 	// Once we move to GC shape implementations, this prefix will not be necessary as the
 	// GC shape naming will distinguish them.
 	// e.g. f[8bytenonpointer] vs. f[int].
 	// For now, we use .inst.f[int] vs. f[int].
-	b.WriteString(".inst.")
+	if !hasTParam {
+		b.WriteString(".inst.")
+	}
 
 	name := fnsym.Name
 	i := strings.Index(name, "[")
@@ -942,10 +953,325 @@ func MakeInstName(fnsym *types.Sym, targs []*types.Type, hasBrackets bool) *type
 	return fnsym.Pkg.Lookup(b.String())
 }
 
-// For catching problems as we add more features
-// TODO(danscales): remove assertions or replace with base.FatalfAt()
 func assert(p bool) {
 	if !p {
 		panic("assertion failed")
 	}
 }
+
+// General type substituter, for replacing typeparams with type args.
+type Tsubster struct {
+	Tparams []*types.Type
+	Targs   []*types.Type
+	// If non-nil, the substitution map from name nodes in the generic function to the
+	// name nodes in the new stenciled function.
+	Vars map[*ir.Name]*ir.Name
+	// New fully-instantiated generic types whose methods should be instantiated.
+	InstTypeList []*types.Type
+	// If non-nil, function to substitute an incomplete (TFORW) type.
+	SubstForwFunc func(*types.Type) *types.Type
+}
+
+// Typ computes the type obtained by substituting any type parameter in t with the
+// corresponding type argument in subst. If t contains no type parameters, the
+// result is t; otherwise the result is a new type. It deals with recursive types
+// by using TFORW types and finding partially or fully created types via sym.Def.
+func (ts *Tsubster) Typ(t *types.Type) *types.Type {
+	if !t.HasTParam() && t.Kind() != types.TFUNC {
+		// Note: function types need to be copied regardless, as the
+		// types of closures may contain declarations that need
+		// to be copied. See #45738.
+		return t
+	}
+
+	if t.Kind() == types.TTYPEPARAM {
+		for i, tp := range ts.Tparams {
+			if tp == t {
+				return ts.Targs[i]
+			}
+		}
+		// If t is a simple typeparam T, then t has the name/symbol 'T'
+		// and t.Underlying() == t.
+		//
+		// However, consider the type definition: 'type P[T any] T'. We
+		// might use this definition so we can have a variant of type T
+		// that we can add new methods to. Suppose t is a reference to
+		// P[T]. t has the name 'P[T]', but its kind is TTYPEPARAM,
+		// because P[T] is defined as T. If we look at t.Underlying(), it
+		// is different, because the name of t.Underlying() is 'T' rather
+		// than 'P[T]'. But the kind of t.Underlying() is also TTYPEPARAM.
+		// In this case, we do the needed recursive substitution in the
+		// case statement below.
+		if t.Underlying() == t {
+			// t is a simple typeparam that didn't match anything in tparam
+			return t
+		}
+		// t is a more complex typeparam (e.g. P[T], as above, whose
+		// definition is just T).
+		assert(t.Sym() != nil)
+	}
+
+	var newsym *types.Sym
+	var neededTargs []*types.Type
+	var forw *types.Type
+
+	if t.Sym() != nil {
+		// Translate the type params for this type according to
+		// the tparam/targs mapping from subst.
+		neededTargs = make([]*types.Type, len(t.RParams()))
+		for i, rparam := range t.RParams() {
+			neededTargs[i] = ts.Typ(rparam)
+		}
+		// For a named (defined) type, we have to change the name of the
+		// type as well. We do this first, so we can look up if we've
+		// already seen this type during this substitution or other
+		// definitions/substitutions.
+		genName := genericTypeName(t.Sym())
+		newsym = t.Sym().Pkg.Lookup(InstTypeName(genName, neededTargs))
+		if newsym.Def != nil {
+			// We've already created this instantiated defined type.
+			return newsym.Def.Type()
+		}
+
+		// In order to deal with recursive generic types, create a TFORW
+		// type initially and set the Def field of its sym, so it can be
+		// found if this type appears recursively within the type.
+		forw = NewIncompleteNamedType(t.Pos(), newsym)
+		//println("Creating new type by sub", newsym.Name, forw.HasTParam())
+		forw.SetRParams(neededTargs)
+		// Copy the OrigSym from the re-instantiated type (which is the sym of
+		// the base generic type).
+		assert(t.OrigSym != nil)
+		forw.OrigSym = t.OrigSym
+	}
+
+	var newt *types.Type
+
+	switch t.Kind() {
+	case types.TTYPEPARAM:
+		if t.Sym() == newsym {
+			// The substitution did not change the type.
+			return t
+		}
+		// Substitute the underlying typeparam (e.g. T in P[T], see
+		// the example describing type P[T] above).
+		newt = ts.Typ(t.Underlying())
+		assert(newt != t)
+
+	case types.TARRAY:
+		elem := t.Elem()
+		newelem := ts.Typ(elem)
+		if newelem != elem {
+			newt = types.NewArray(newelem, t.NumElem())
+		}
+
+	case types.TPTR:
+		elem := t.Elem()
+		newelem := ts.Typ(elem)
+		if newelem != elem {
+			newt = types.NewPtr(newelem)
+		}
+
+	case types.TSLICE:
+		elem := t.Elem()
+		newelem := ts.Typ(elem)
+		if newelem != elem {
+			newt = types.NewSlice(newelem)
+		}
+
+	case types.TSTRUCT:
+		newt = ts.tstruct(t, false)
+		if newt == t {
+			newt = nil
+		}
+
+	case types.TFUNC:
+		newrecvs := ts.tstruct(t.Recvs(), false)
+		newparams := ts.tstruct(t.Params(), false)
+		newresults := ts.tstruct(t.Results(), false)
+		if newrecvs != t.Recvs() || newparams != t.Params() || newresults != t.Results() {
+			// If any types have changed, then the all the fields of
+			// of recv, params, and results must be copied, because they have
+			// offset fields that are dependent, and so must have an
+			// independent copy for each new signature.
+			var newrecv *types.Field
+			if newrecvs.NumFields() > 0 {
+				if newrecvs == t.Recvs() {
+					newrecvs = ts.tstruct(t.Recvs(), true)
+				}
+				newrecv = newrecvs.Field(0)
+			}
+			if newparams == t.Params() {
+				newparams = ts.tstruct(t.Params(), true)
+			}
+			if newresults == t.Results() {
+				newresults = ts.tstruct(t.Results(), true)
+			}
+			newt = types.NewSignature(t.Pkg(), newrecv, t.TParams().FieldSlice(), newparams.FieldSlice(), newresults.FieldSlice())
+		}
+
+	case types.TINTER:
+		newt = ts.tinter(t)
+		if newt == t {
+			newt = nil
+		}
+
+	case types.TMAP:
+		newkey := ts.Typ(t.Key())
+		newval := ts.Typ(t.Elem())
+		if newkey != t.Key() || newval != t.Elem() {
+			newt = types.NewMap(newkey, newval)
+		}
+
+	case types.TCHAN:
+		elem := t.Elem()
+		newelem := ts.Typ(elem)
+		if newelem != elem {
+			newt = types.NewChan(newelem, t.ChanDir())
+			if !newt.HasTParam() {
+				// TODO(danscales): not sure why I have to do this
+				// only for channels.....
+				types.CheckSize(newt)
+			}
+		}
+	case types.TFORW:
+		if ts.SubstForwFunc != nil {
+			newt = ts.SubstForwFunc(t)
+		} else {
+			assert(false)
+		}
+	}
+	if newt == nil {
+		// Even though there were typeparams in the type, there may be no
+		// change if this is a function type for a function call (which will
+		// have its own tparams/targs in the function instantiation).
+		return t
+	}
+
+	if t.Sym() == nil {
+		// Not a named type, so there was no forwarding type and there are
+		// no methods to substitute.
+		assert(t.Methods().Len() == 0)
+		return newt
+	}
+
+	forw.SetUnderlying(newt)
+	newt = forw
+
+	if t.Kind() != types.TINTER && t.Methods().Len() > 0 {
+		// Fill in the method info for the new type.
+		var newfields []*types.Field
+		newfields = make([]*types.Field, t.Methods().Len())
+		for i, f := range t.Methods().Slice() {
+			t2 := ts.Typ(f.Type)
+			oldsym := f.Nname.Sym()
+			newsym := MakeInstName(oldsym, ts.Targs, true)
+			var nname *ir.Name
+			if newsym.Def != nil {
+				nname = newsym.Def.(*ir.Name)
+			} else {
+				nname = ir.NewNameAt(f.Pos, newsym)
+				nname.SetType(t2)
+				newsym.Def = nname
+			}
+			newfields[i] = types.NewField(f.Pos, f.Sym, t2)
+			newfields[i].Nname = nname
+		}
+		newt.Methods().Set(newfields)
+		if !newt.HasTParam() {
+			// Generate all the methods for a new fully-instantiated type.
+			ts.InstTypeList = append(ts.InstTypeList, newt)
+		}
+	}
+	return newt
+}
+
+// tstruct substitutes type params in types of the fields of a structure type. For
+// each field, tstruct copies the Nname, and translates it if Nname is in
+// ts.vars. To always force the creation of a new (top-level) struct,
+// regardless of whether anything changed with the types or names of the struct's
+// fields, set force to true.
+func (ts *Tsubster) tstruct(t *types.Type, force bool) *types.Type {
+	if t.NumFields() == 0 {
+		if t.HasTParam() {
+			// For an empty struct, we need to return a new type,
+			// since it may now be fully instantiated (HasTParam
+			// becomes false).
+			return types.NewStruct(t.Pkg(), nil)
+		}
+		return t
+	}
+	var newfields []*types.Field
+	if force {
+		newfields = make([]*types.Field, t.NumFields())
+	}
+	for i, f := range t.Fields().Slice() {
+		t2 := ts.Typ(f.Type)
+		if (t2 != f.Type || f.Nname != nil) && newfields == nil {
+			newfields = make([]*types.Field, t.NumFields())
+			for j := 0; j < i; j++ {
+				newfields[j] = t.Field(j)
+			}
+		}
+		if newfields != nil {
+			// TODO(danscales): make sure this works for the field
+			// names of embedded types (which should keep the name of
+			// the type param, not the instantiated type).
+			newfields[i] = types.NewField(f.Pos, f.Sym, t2)
+			if f.Nname != nil && ts.Vars != nil {
+				v := ts.Vars[f.Nname.(*ir.Name)]
+				if v != nil {
+					// This is the case where we are
+					// translating the type of the function we
+					// are substituting, so its dcls are in
+					// the subst.ts.vars table, and we want to
+					// change to reference the new dcl.
+					newfields[i].Nname = v
+				} else {
+					// This is the case where we are
+					// translating the type of a function
+					// reference inside the function we are
+					// substituting, so we leave the Nname
+					// value as is.
+					newfields[i].Nname = f.Nname
+				}
+			}
+		}
+	}
+	if newfields != nil {
+		return types.NewStruct(t.Pkg(), newfields)
+	}
+	return t
+
+}
+
+// tinter substitutes type params in types of the methods of an interface type.
+func (ts *Tsubster) tinter(t *types.Type) *types.Type {
+	if t.Methods().Len() == 0 {
+		return t
+	}
+	var newfields []*types.Field
+	for i, f := range t.Methods().Slice() {
+		t2 := ts.Typ(f.Type)
+		if (t2 != f.Type || f.Nname != nil) && newfields == nil {
+			newfields = make([]*types.Field, t.Methods().Len())
+			for j := 0; j < i; j++ {
+				newfields[j] = t.Methods().Index(j)
+			}
+		}
+		if newfields != nil {
+			newfields[i] = types.NewField(f.Pos, f.Sym, t2)
+		}
+	}
+	if newfields != nil {
+		return types.NewInterface(t.Pkg(), newfields)
+	}
+	return t
+}
+
+// genericSym returns the name of the base generic type for the type named by
+// sym. It simply returns the name obtained by removing everything after the
+// first bracket ("[").
+func genericTypeName(sym *types.Sym) string {
+	return sym.Name[0:strings.Index(sym.Name, "[")]
+}
diff --git a/src/cmd/compile/internal/types2/builtins.go b/src/cmd/compile/internal/types2/builtins.go
index 20c4ff62a1..8e13b0ff18 100644
--- a/src/cmd/compile/internal/types2/builtins.go
+++ b/src/cmd/compile/internal/types2/builtins.go
@@ -767,8 +767,10 @@ func (check *Checker) applyTypeFunc(f func(Type) Type, x Type) Type {
 		//           uses of real() where the result is used to
 		//           define type and initialize a variable?
 
-		// construct a suitable new type parameter
-		tpar := NewTypeName(nopos, nil /* = Universe pkg */, "<type parameter>", nil)
+		// Construct a suitable new type parameter for the sum type. The
+		// type param is placed in the current package so export/import
+		// works as expected.
+		tpar := NewTypeName(nopos, check.pkg, "<type parameter>", nil)
 		ptyp := check.NewTypeParam(tpar, 0, &emptyInterface) // assigns type to tpar as a side-effect
 		tsum := newUnion(rtypes, tildes)
 		ptyp.bound = &Interface{allMethods: markComplete, allTypes: tsum}
diff --git a/src/cmd/compile/internal/types2/type.go b/src/cmd/compile/internal/types2/type.go
index 604520d27f..10cb651d0c 100644
--- a/src/cmd/compile/internal/types2/type.go
+++ b/src/cmd/compile/internal/types2/type.go
@@ -640,9 +640,8 @@ type TypeParam struct {
 // Obj returns the type name for the type parameter t.
 func (t *TypeParam) Obj() *TypeName { return t.obj }
 
-// NewTypeParam returns a new TypeParam.
+// NewTypeParam returns a new TypeParam.  bound can be nil (and set later).
 func (check *Checker) NewTypeParam(obj *TypeName, index int, bound Type) *TypeParam {
-	assert(bound != nil)
 	// Always increment lastID, even if it is not used.
 	id := nextID()
 	if check != nil {
@@ -679,11 +678,20 @@ func (t *TypeParam) Bound() *Interface {
 	return iface
 }
 
-// optype returns a type's operational type. Except for type parameters,
-// the operational type is the same as the underlying type (as returned
-// by under). For Type parameters, the operational type is determined
-// by the corresponding type constraint. The result may be the top type,
-// but it is never the incoming type parameter.
+func (t *TypeParam) SetBound(bound Type) {
+	if bound == nil {
+		panic("types2.TypeParam.SetBound: bound must not be nil")
+	}
+	t.bound = bound
+}
+
+// optype returns a type's operational type. Except for
+// type parameters, the operational type is the same
+// as the underlying type (as returned by under). For
+// Type parameters, the operational type is determined
+// by the corresponding type bound's type list. The
+// result may be the bottom or top type, but it is never
+// the incoming type parameter.
 func optype(typ Type) Type {
 	if t := asTypeParam(typ); t != nil {
 		// If the optype is typ, return the top type as we have