[dev.typeparams] cmd/compile: lazy import resolution for types2

This CL adds three new functions to the types2 API to support lazy
import resolution:

1. A new Scope.InsertLazy method to allow recording that Objects exist
in a particular Scope (in particular, package scopes) without having
to yet fully construct those objects. Instead, types2 will call the
provided `resolve` function if/when the object is actually needed.

2. Similarly, a new NewTypeNameLazy function to create TypeName
objects without yet instantiating their underlying Named
instance.

3. Finally, an InstantiateLazy method, that allows creating type
instances without requiring any of the types to be expanded right
away. Importantly, this requires providing a types2.Checker argument
to handle recursive types correctly.

The APIs as-is are a bit clumsy (esp. NewTypeNameLazy), but seem to
work well for cmd/compile's needs. In particular, they simplify some
of the complexities of handling recursive type definitions within the
importer.

Also, the current prototype is a bit fragile. It uses sync.Once to
manage concurrent lazy resolution, which is frustrating to debug in
the presence of reentrancy issues. It also means the importer needs to
deal with concurrency as well. These aren't issues for types2 though
as cmd/compile only walks the type-checked AST sequentially.

Finally, it looks like some of the details of lazy type names are
similar to the lazy "instance" stuff used for generics, so maybe
there's opportunity for unifying them under a more general (but still
internal) lazy type mechanism.

I had originally intended for this CL to also update the types2
importer, but (1) it doesn't have access to the types2.Checker
instance needed to call InstantiateLazy, and (2) it creates a new
TypeName/TypeParam at each use rather than reusing them, which
evidently works with types2.Instantiate but not
types2.(*Checker).instantiate (i.e., InstantiateLazy). I spent a while
trying to fix these issues, but kept running into more subtle
issues. Instead, I've included my WIP "unified IR" CL as a followup CL
that demonstrates these Lazy methods (see noder/reader2.go).

Updates #46449.

Change-Id: I4d1e8e649f6325a11790d25fd90c39fa07c8d41d
Reviewed-on: https://go-review.googlesource.com/c/go/+/323569
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

17 files changed, 186 insertions, 43 deletions

diff --git a/src/cmd/compile/internal/types2/check.go b/src/cmd/compile/internal/types2/check.go
index f80a918467..5d3c2c8ad2 100644
--- a/src/cmd/compile/internal/types2/check.go
+++ b/src/cmd/compile/internal/types2/check.go
@@ -71,7 +71,7 @@ type importKey struct {
 // A dotImportKey describes a dot-imported object in the given scope.
 type dotImportKey struct {
 	scope *Scope
-	obj   Object
+	name  string
 }
 
 // A Checker maintains the state of the type checker.
diff --git a/src/cmd/compile/internal/types2/decl.go b/src/cmd/compile/internal/types2/decl.go
index aa70f3880b..00b4ef7010 100644
--- a/src/cmd/compile/internal/types2/decl.go
+++ b/src/cmd/compile/internal/types2/decl.go
@@ -522,7 +522,7 @@ func (check *Checker) varDecl(obj *Var, lhs []*Var, typ, init syntax.Expr) {
 // is detected, the result is Typ[Invalid]. If a cycle is detected and
 // n0.check != nil, the cycle is reported.
 func (n0 *Named) under() Type {
-	u := n0.underlying
+	u := n0.Underlying()
 
 	if u == Typ[Invalid] {
 		return u
@@ -560,7 +560,7 @@ func (n0 *Named) under() Type {
 	seen := map[*Named]int{n0: 0}
 	path := []Object{n0.obj}
 	for {
-		u = n.underlying
+		u = n.Underlying()
 		if u == nil {
 			u = Typ[Invalid]
 			break
@@ -764,7 +764,7 @@ func (check *Checker) collectMethods(obj *TypeName) {
 	// and field names must be distinct."
 	base := asNamed(obj.typ) // shouldn't fail but be conservative
 	if base != nil {
-		if t, _ := base.underlying.(*Struct); t != nil {
+		if t, _ := base.Underlying().(*Struct); t != nil {
 			for _, fld := range t.fields {
 				if fld.name != "_" {
 					assert(mset.insert(fld) == nil)
@@ -806,6 +806,7 @@ func (check *Checker) collectMethods(obj *TypeName) {
 		}
 
 		if base != nil {
+			base.expand() // TODO(mdempsky): Probably unnecessary.
 			base.methods = append(base.methods, m)
 		}
 	}
diff --git a/src/cmd/compile/internal/types2/instantiate.go b/src/cmd/compile/internal/types2/instantiate.go
index 0df52e851c..85c897a909 100644
--- a/src/cmd/compile/internal/types2/instantiate.go
+++ b/src/cmd/compile/internal/types2/instantiate.go
@@ -23,7 +23,7 @@ func Instantiate(pos syntax.Pos, typ Type, targs []Type) (res Type) {
 	var tparams []*TypeName
 	switch t := typ.(type) {
 	case *Named:
-		tparams = t.tparams
+		tparams = t.TParams()
 	case *Signature:
 		tparams = t.tparams
 		defer func() {
@@ -61,3 +61,19 @@ func Instantiate(pos syntax.Pos, typ Type, targs []Type) (res Type) {
 	smap := makeSubstMap(tparams, targs)
 	return (*Checker)(nil).subst(pos, typ, smap)
 }
+
+// InstantiateLazy is like Instantiate, but avoids actually
+// instantiating the type until needed.
+func (check *Checker) InstantiateLazy(pos syntax.Pos, typ Type, targs []Type) (res Type) {
+	base := asNamed(typ)
+	if base == nil {
+		panic(fmt.Sprintf("%v: cannot instantiate %v", pos, typ))
+	}
+
+	return &instance{
+		check: check,
+		pos:   pos,
+		base:  base,
+		targs: targs,
+	}
+}
diff --git a/src/cmd/compile/internal/types2/labels.go b/src/cmd/compile/internal/types2/labels.go
index d3206988b5..6f02e2fc96 100644
--- a/src/cmd/compile/internal/types2/labels.go
+++ b/src/cmd/compile/internal/types2/labels.go
@@ -32,7 +32,8 @@ func (check *Checker) labels(body *syntax.BlockStmt) {
 	}
 
 	// spec: "It is illegal to define a label that is never used."
-	for _, obj := range all.elems {
+	for name, obj := range all.elems {
+		obj = resolve(name, obj)
 		if lbl := obj.(*Label); !lbl.used {
 			check.softErrorf(lbl.pos, "label %s declared but not used", lbl.name)
 		}
diff --git a/src/cmd/compile/internal/types2/lookup.go b/src/cmd/compile/internal/types2/lookup.go
index eb2b17dd4d..93ed620449 100644
--- a/src/cmd/compile/internal/types2/lookup.go
+++ b/src/cmd/compile/internal/types2/lookup.go
@@ -54,7 +54,7 @@ func (check *Checker) lookupFieldOrMethod(T Type, addressable bool, pkg *Package
 	// pointer type but discard the result if it is a method since we would
 	// not have found it for T (see also issue 8590).
 	if t := asNamed(T); t != nil {
-		if p, _ := t.underlying.(*Pointer); p != nil {
+		if p, _ := t.Underlying().(*Pointer); p != nil {
 			obj, index, indirect = check.rawLookupFieldOrMethod(p, false, pkg, name)
 			if _, ok := obj.(*Func); ok {
 				return nil, nil, false
@@ -126,6 +126,7 @@ func (check *Checker) rawLookupFieldOrMethod(T Type, addressable bool, pkg *Pack
 				seen[named] = true
 
 				// look for a matching attached method
+				named.expand()
 				if i, m := lookupMethod(named.methods, pkg, name); m != nil {
 					// potential match
 					// caution: method may not have a proper signature yet
@@ -400,7 +401,7 @@ func (check *Checker) missingMethod(V Type, T *Interface, static bool) (method,
 		// In order to compare the signatures, substitute the receiver
 		// type parameters of ftyp with V's instantiation type arguments.
 		// This lazily instantiates the signature of method f.
-		if Vn != nil && len(Vn.tparams) > 0 {
+		if Vn != nil && len(Vn.TParams()) > 0 {
 			// Be careful: The number of type arguments may not match
 			// the number of receiver parameters. If so, an error was
 			// reported earlier but the length discrepancy is still
diff --git a/src/cmd/compile/internal/types2/object.go b/src/cmd/compile/internal/types2/object.go
index 8ed55f1dbf..82297ff17f 100644
--- a/src/cmd/compile/internal/types2/object.go
+++ b/src/cmd/compile/internal/types2/object.go
@@ -276,6 +276,14 @@ func NewTypeName(pos syntax.Pos, pkg *Package, name string, typ Type) *TypeName
 	return &TypeName{object{nil, pos, pkg, name, typ, 0, colorFor(typ), nopos}}
 }
 
+// NewTypeNameLazy returns a new defined type like NewTypeName, but it
+// lazily calls resolve to finish constructing the Named object.
+func NewTypeNameLazy(pos syntax.Pos, pkg *Package, name string, resolve func(named *Named) (tparams []*TypeName, underlying Type, methods []*Func)) *TypeName {
+	obj := NewTypeName(pos, pkg, name, nil)
+	NewNamed(obj, nil, nil).resolve = resolve
+	return obj
+}
+
 // IsAlias reports whether obj is an alias name for a type.
 func (obj *TypeName) IsAlias() bool {
 	switch t := obj.typ.(type) {
diff --git a/src/cmd/compile/internal/types2/predicates.go b/src/cmd/compile/internal/types2/predicates.go
index 74436836cd..66de249044 100644
--- a/src/cmd/compile/internal/types2/predicates.go
+++ b/src/cmd/compile/internal/types2/predicates.go
@@ -21,7 +21,7 @@ func isNamed(typ Type) bool {
 func isGeneric(typ Type) bool {
 	// A parameterized type is only instantiated if it doesn't have an instantiation already.
 	named, _ := typ.(*Named)
-	return named != nil && named.obj != nil && named.tparams != nil && named.targs == nil
+	return named != nil && named.obj != nil && named.TParams() != nil && named.targs == nil
 }
 
 func is(typ Type, what BasicInfo) bool {
diff --git a/src/cmd/compile/internal/types2/resolver.go b/src/cmd/compile/internal/types2/resolver.go
index 9b1482b14e..018a20cfb2 100644
--- a/src/cmd/compile/internal/types2/resolver.go
+++ b/src/cmd/compile/internal/types2/resolver.go
@@ -308,22 +308,26 @@ func (check *Checker) collectObjects() {
 						check.dotImportMap = make(map[dotImportKey]*PkgName)
 					}
 					// merge imported scope with file scope
-					for _, obj := range imp.scope.elems {
+					for name, obj := range imp.scope.elems {
+						// Note: Avoid eager resolve(name, obj) here, so we only
+						// resolve dot-imported objects as needed.
+
 						// A package scope may contain non-exported objects,
 						// do not import them!
-						if obj.Exported() {
+						if isExported(name) {
 							// declare dot-imported object
 							// (Do not use check.declare because it modifies the object
 							// via Object.setScopePos, which leads to a race condition;
 							// the object may be imported into more than one file scope
 							// concurrently. See issue #32154.)
-							if alt := fileScope.Insert(obj); alt != nil {
+							if alt := fileScope.Lookup(name); alt != nil {
 								var err error_
-								err.errorf(s.LocalPkgName, "%s redeclared in this block", obj.Name())
+								err.errorf(s.LocalPkgName, "%s redeclared in this block", alt.Name())
 								err.recordAltDecl(alt)
 								check.report(&err)
 							} else {
-								check.dotImportMap[dotImportKey{fileScope, obj}] = pkgName
+								fileScope.insert(name, obj)
+								check.dotImportMap[dotImportKey{fileScope, name}] = pkgName
 							}
 						}
 					}
@@ -469,8 +473,9 @@ func (check *Checker) collectObjects() {
 
 	// verify that objects in package and file scopes have different names
 	for _, scope := range fileScopes {
-		for _, obj := range scope.elems {
-			if alt := pkg.scope.Lookup(obj.Name()); alt != nil {
+		for name, obj := range scope.elems {
+			if alt := pkg.scope.Lookup(name); alt != nil {
+				obj = resolve(name, obj)
 				var err error_
 				if pkg, ok := obj.(*PkgName); ok {
 					err.errorf(alt, "%s already declared through import of %s", alt.Name(), pkg.Imported())
diff --git a/src/cmd/compile/internal/types2/sanitize.go b/src/cmd/compile/internal/types2/sanitize.go
index 03aef90fe1..4e654e074f 100644
--- a/src/cmd/compile/internal/types2/sanitize.go
+++ b/src/cmd/compile/internal/types2/sanitize.go
@@ -134,6 +134,7 @@ func (s sanitizer) typ(typ Type) Type {
 		if debug && t.check != nil {
 			panic("internal error: Named.check != nil")
 		}
+		t.expand()
 		if orig := s.typ(t.fromRHS); orig != t.fromRHS {
 			t.fromRHS = orig
 		}
diff --git a/src/cmd/compile/internal/types2/scope.go b/src/cmd/compile/internal/types2/scope.go
index ade0a79b31..2f1814a631 100644
--- a/src/cmd/compile/internal/types2/scope.go
+++ b/src/cmd/compile/internal/types2/scope.go
@@ -13,6 +13,7 @@ import (
 	"io"
 	"sort"
 	"strings"
+	"sync"
 )
 
 // A Scope maintains a set of objects and links to its containing
@@ -66,7 +67,7 @@ func (s *Scope) Child(i int) *Scope { return s.children[i] }
 // Lookup returns the object in scope s with the given name if such an
 // object exists; otherwise the result is nil.
 func (s *Scope) Lookup(name string) Object {
-	return s.elems[name]
+	return resolve(name, s.elems[name])
 }
 
 // LookupParent follows the parent chain of scopes starting with s until
@@ -81,7 +82,7 @@ func (s *Scope) Lookup(name string) Object {
 // whose scope is the scope of the package that exported them.
 func (s *Scope) LookupParent(name string, pos syntax.Pos) (*Scope, Object) {
 	for ; s != nil; s = s.parent {
-		if obj := s.elems[name]; obj != nil && (!pos.IsKnown() || obj.scopePos().Cmp(pos) <= 0) {
+		if obj := s.Lookup(name); obj != nil && (!pos.IsKnown() || obj.scopePos().Cmp(pos) <= 0) {
 			return s, obj
 		}
 	}
@@ -95,19 +96,38 @@ func (s *Scope) LookupParent(name string, pos syntax.Pos) (*Scope, Object) {
 // if not already set, and returns nil.
 func (s *Scope) Insert(obj Object) Object {
 	name := obj.Name()
-	if alt := s.elems[name]; alt != nil {
+	if alt := s.Lookup(name); alt != nil {
 		return alt
 	}
-	if s.elems == nil {
-		s.elems = make(map[string]Object)
-	}
-	s.elems[name] = obj
+	s.insert(name, obj)
 	if obj.Parent() == nil {
 		obj.setParent(s)
 	}
 	return nil
 }
 
+// InsertLazy is like Insert, but allows deferring construction of the
+// inserted object until it's accessed with Lookup. The Object
+// returned by resolve must have the same name as given to InsertLazy.
+// If s already contains an alternative object with the same name,
+// InsertLazy leaves s unchanged and returns false. Otherwise it
+// records the binding and returns true. The object's parent scope
+// will be set to s after resolve is called.
+func (s *Scope) InsertLazy(name string, resolve func() Object) bool {
+	if s.elems[name] != nil {
+		return false
+	}
+	s.insert(name, &lazyObject{parent: s, resolve: resolve})
+	return true
+}
+
+func (s *Scope) insert(name string, obj Object) {
+	if s.elems == nil {
+		s.elems = make(map[string]Object)
+	}
+	s.elems[name] = obj
+}
+
 // Squash merges s with its parent scope p by adding all
 // objects of s to p, adding all children of s to the
 // children of p, and removing s from p's children.
@@ -117,7 +137,8 @@ func (s *Scope) Insert(obj Object) Object {
 func (s *Scope) Squash(err func(obj, alt Object)) {
 	p := s.parent
 	assert(p != nil)
-	for _, obj := range s.elems {
+	for name, obj := range s.elems {
+		obj = resolve(name, obj)
 		obj.setParent(nil)
 		if alt := p.Insert(obj); alt != nil {
 			err(obj, alt)
@@ -196,7 +217,7 @@ func (s *Scope) WriteTo(w io.Writer, n int, recurse bool) {
 
 	indn1 := indn + ind
 	for _, name := range s.Names() {
-		fmt.Fprintf(w, "%s%s\n", indn1, s.elems[name])
+		fmt.Fprintf(w, "%s%s\n", indn1, s.Lookup(name))
 	}
 
 	if recurse {
@@ -214,3 +235,57 @@ func (s *Scope) String() string {
 	s.WriteTo(&buf, 0, false)
 	return buf.String()
 }
+
+// A lazyObject represents an imported Object that has not been fully
+// resolved yet by its importer.
+type lazyObject struct {
+	parent  *Scope
+	resolve func() Object
+	obj     Object
+	once    sync.Once
+}
+
+// resolve returns the Object represented by obj, resolving lazy
+// objects as appropriate.
+func resolve(name string, obj Object) Object {
+	if lazy, ok := obj.(*lazyObject); ok {
+		lazy.once.Do(func() {
+			obj := lazy.resolve()
+
+			if _, ok := obj.(*lazyObject); ok {
+				panic("recursive lazy object")
+			}
+			if obj.Name() != name {
+				panic("lazy object has unexpected name")
+			}
+
+			if obj.Parent() == nil {
+				obj.setParent(lazy.parent)
+			}
+			lazy.obj = obj
+		})
+
+		obj = lazy.obj
+	}
+	return obj
+}
+
+// stub implementations so *lazyObject implements Object and we can
+// store them directly into Scope.elems.
+func (*lazyObject) Parent() *Scope                        { panic("unreachable") }
+func (*lazyObject) Pos() syntax.Pos                       { panic("unreachable") }
+func (*lazyObject) Pkg() *Package                         { panic("unreachable") }
+func (*lazyObject) Name() string                          { panic("unreachable") }
+func (*lazyObject) Type() Type                            { panic("unreachable") }
+func (*lazyObject) Exported() bool                        { panic("unreachable") }
+func (*lazyObject) Id() string                            { panic("unreachable") }
+func (*lazyObject) String() string                        { panic("unreachable") }
+func (*lazyObject) order() uint32                         { panic("unreachable") }
+func (*lazyObject) color() color                          { panic("unreachable") }
+func (*lazyObject) setType(Type)                          { panic("unreachable") }
+func (*lazyObject) setOrder(uint32)                       { panic("unreachable") }
+func (*lazyObject) setColor(color color)                  { panic("unreachable") }
+func (*lazyObject) setParent(*Scope)                      { panic("unreachable") }
+func (*lazyObject) sameId(pkg *Package, name string) bool { panic("unreachable") }
+func (*lazyObject) scopePos() syntax.Pos                  { panic("unreachable") }
+func (*lazyObject) setScopePos(pos syntax.Pos)            { panic("unreachable") }
diff --git a/src/cmd/compile/internal/types2/signature.go b/src/cmd/compile/internal/types2/signature.go
index c8c4cca0a7..a7edc5ac03 100644
--- a/src/cmd/compile/internal/types2/signature.go
+++ b/src/cmd/compile/internal/types2/signature.go
@@ -62,7 +62,7 @@ func (check *Checker) funcType(sig *Signature, recvPar *syntax.Field, tparams []
 				//       again when we type-check the signature.
 				// TODO(gri) maybe the receiver should be marked as invalid instead?
 				if recv := asNamed(check.genericType(rname, false)); recv != nil {
-					recvTParams = recv.tparams
+					recvTParams = recv.TParams()
 				}
 			}
 			// provide type parameter bounds
diff --git a/src/cmd/compile/internal/types2/sizeof_test.go b/src/cmd/compile/internal/types2/sizeof_test.go
index daa039bf92..3cb162764c 100644
--- a/src/cmd/compile/internal/types2/sizeof_test.go
+++ b/src/cmd/compile/internal/types2/sizeof_test.go
@@ -31,7 +31,7 @@ func TestSizeof(t *testing.T) {
 		{Interface{}, 52, 104},
 		{Map{}, 16, 32},
 		{Chan{}, 12, 24},
-		{Named{}, 68, 136},
+		{Named{}, 84, 160},
 		{TypeParam{}, 28, 48},
 		{instance{}, 52, 96},
 		{top{}, 0, 0},
diff --git a/src/cmd/compile/internal/types2/stmt.go b/src/cmd/compile/internal/types2/stmt.go
index e9ffd4f5ca..ab66432126 100644
--- a/src/cmd/compile/internal/types2/stmt.go
+++ b/src/cmd/compile/internal/types2/stmt.go
@@ -64,7 +64,8 @@ func (check *Checker) funcBody(decl *declInfo, name string, sig *Signature, body
 
 func (check *Checker) usage(scope *Scope) {
 	var unused []*Var
-	for _, elem := range scope.elems {
+	for name, elem := range scope.elems {
+		elem = resolve(name, elem)
 		if v, _ := elem.(*Var); v != nil && !v.used {
 			unused = append(unused, v)
 		}
diff --git a/src/cmd/compile/internal/types2/subst.go b/src/cmd/compile/internal/types2/subst.go
index 35ca197d64..dd8dd74161 100644
--- a/src/cmd/compile/internal/types2/subst.go
+++ b/src/cmd/compile/internal/types2/subst.go
@@ -76,7 +76,7 @@ func (check *Checker) instantiate(pos syntax.Pos, typ Type, targs []Type, poslis
 	var tparams []*TypeName
 	switch t := typ.(type) {
 	case *Named:
-		tparams = t.tparams
+		tparams = t.TParams()
 	case *Signature:
 		tparams = t.tparams
 		defer func() {
@@ -347,7 +347,7 @@ func (subst *subster) typ(typ Type) Type {
 			}
 		}
 
-		if t.tparams == nil {
+		if t.TParams() == nil {
 			dump(">>> %s is not parameterized", t)
 			return t // type is not parameterized
 		}
@@ -357,7 +357,7 @@ func (subst *subster) typ(typ Type) Type {
 		if len(t.targs) > 0 {
 			// already instantiated
 			dump(">>> %s already instantiated", t)
-			assert(len(t.targs) == len(t.tparams))
+			assert(len(t.targs) == len(t.TParams()))
 			// For each (existing) type argument targ, determine if it needs
 			// to be substituted; i.e., if it is or contains a type parameter
 			// that has a type argument for it.
@@ -367,7 +367,7 @@ func (subst *subster) typ(typ Type) Type {
 				if new_targ != targ {
 					dump(">>> substituted %d targ %s => %s", i, targ, new_targ)
 					if new_targs == nil {
-						new_targs = make([]Type, len(t.tparams))
+						new_targs = make([]Type, len(t.TParams()))
 						copy(new_targs, t.targs)
 					}
 					new_targs[i] = new_targ
@@ -397,7 +397,7 @@ func (subst *subster) typ(typ Type) Type {
 
 		// create a new named type and populate caches to avoid endless recursion
 		tname := NewTypeName(subst.pos, t.obj.pkg, t.obj.name, nil)
-		named := subst.check.newNamed(tname, t, t.underlying, t.tparams, t.methods) // method signatures are updated lazily
+		named := subst.check.newNamed(tname, t, t.Underlying(), t.TParams(), t.methods) // method signatures are updated lazily
 		named.targs = new_targs
 		if subst.check != nil {
 			subst.check.typMap[h] = named
@@ -406,7 +406,7 @@ func (subst *subster) typ(typ Type) Type {
 
 		// do the substitution
 		dump(">>> subst %s with %s (new: %s)", t.underlying, subst.smap, new_targs)
-		named.underlying = subst.typOrNil(t.underlying)
+		named.underlying = subst.typOrNil(t.Underlying())
 		named.fromRHS = named.underlying // for cycle detection (Checker.validType)
 
 		return named
diff --git a/src/cmd/compile/internal/types2/type.go b/src/cmd/compile/internal/types2/type.go
index 92f35f1279..604520d27f 100644
--- a/src/cmd/compile/internal/types2/type.go
+++ b/src/cmd/compile/internal/types2/type.go
@@ -6,6 +6,7 @@ package types2
 
 import (
 	"cmd/compile/internal/syntax"
+	"sync"
 	"sync/atomic"
 )
 
@@ -497,6 +498,9 @@ type Named struct {
 	tparams    []*TypeName // type parameters, or nil
 	targs      []Type      // type arguments (after instantiation), or nil
 	methods    []*Func     // methods declared for this type (not the method set of this type); signatures are type-checked lazily
+
+	resolve func(*Named) ([]*TypeName, Type, []*Func)
+	once    sync.Once
 }
 
 // NewNamed returns a new named type for the given type name, underlying type, and associated methods.
@@ -509,6 +513,35 @@ func NewNamed(obj *TypeName, underlying Type, methods []*Func) *Named {
 	return (*Checker)(nil).newNamed(obj, nil, underlying, nil, methods)
 }
 
+func (t *Named) expand() *Named {
+	if t.resolve == nil {
+		return t
+	}
+
+	t.once.Do(func() {
+		// TODO(mdempsky): Since we're passing t to resolve anyway
+		// (necessary because types2 expects the receiver type for methods
+		// on defined interface types to be the Named rather than the
+		// underlying Interface), maybe it should just handle calling
+		// SetTParams, SetUnderlying, and AddMethod instead?  Those
+		// methods would need to support reentrant calls though.  It would
+		// also make the API more future-proof towards further extensions
+		// (like SetTParams).
+
+		tparams, underlying, methods := t.resolve(t)
+
+		switch underlying.(type) {
+		case nil, *Named:
+			panic("invalid underlying type")
+		}
+
+		t.tparams = tparams
+		t.underlying = underlying
+		t.methods = methods
+	})
+	return t
+}
+
 // newNamed is like NewNamed but with a *Checker receiver and additional orig argument.
 func (check *Checker) newNamed(obj *TypeName, orig *Named, underlying Type, tparams []*TypeName, methods []*Func) *Named {
 	typ := &Named{check: check, obj: obj, orig: orig, fromRHS: underlying, underlying: underlying, tparams: tparams, methods: methods}
@@ -550,10 +583,10 @@ func (t *Named) Orig() *Named { return t.orig }
 
 // TParams returns the type parameters of the named type t, or nil.
 // The result is non-nil for an (originally) parameterized type even if it is instantiated.
-func (t *Named) TParams() []*TypeName { return t.tparams }
+func (t *Named) TParams() []*TypeName { return t.expand().tparams }
 
 // SetTParams sets the type parameters of the named type t.
-func (t *Named) SetTParams(tparams []*TypeName) { t.tparams = tparams }
+func (t *Named) SetTParams(tparams []*TypeName) { t.expand().tparams = tparams }
 
 // TArgs returns the type arguments after instantiation of the named type t, or nil if not instantiated.
 func (t *Named) TArgs() []Type { return t.targs }
@@ -562,10 +595,10 @@ func (t *Named) TArgs() []Type { return t.targs }
 func (t *Named) SetTArgs(args []Type) { t.targs = args }
 
 // NumMethods returns the number of explicit methods whose receiver is named type t.
-func (t *Named) NumMethods() int { return len(t.methods) }
+func (t *Named) NumMethods() int { return len(t.expand().methods) }
 
 // Method returns the i'th method of named type t for 0 <= i < t.NumMethods().
-func (t *Named) Method(i int) *Func { return t.methods[i] }
+func (t *Named) Method(i int) *Func { return t.expand().methods[i] }
 
 // SetUnderlying sets the underlying type and marks t as complete.
 func (t *Named) SetUnderlying(underlying Type) {
@@ -575,11 +608,12 @@ func (t *Named) SetUnderlying(underlying Type) {
 	if _, ok := underlying.(*Named); ok {
 		panic("types2.Named.SetUnderlying: underlying type must not be *Named")
 	}
-	t.underlying = underlying
+	t.expand().underlying = underlying
 }
 
 // AddMethod adds method m unless it is already in the method list.
 func (t *Named) AddMethod(m *Func) {
+	t.expand()
 	if i, _ := lookupMethod(t.methods, m.pkg, m.name); i < 0 {
 		t.methods = append(t.methods, m)
 	}
@@ -743,7 +777,7 @@ func (t *Signature) Underlying() Type { return t }
 func (t *Interface) Underlying() Type { return t }
 func (t *Map) Underlying() Type       { return t }
 func (t *Chan) Underlying() Type      { return t }
-func (t *Named) Underlying() Type     { return t.underlying }
+func (t *Named) Underlying() Type     { return t.expand().underlying }
 func (t *TypeParam) Underlying() Type { return t }
 func (t *instance) Underlying() Type  { return t }
 func (t *top) Underlying() Type       { return t }
diff --git a/src/cmd/compile/internal/types2/typestring.go b/src/cmd/compile/internal/types2/typestring.go
index 28583b62d9..07ed510d11 100644
--- a/src/cmd/compile/internal/types2/typestring.go
+++ b/src/cmd/compile/internal/types2/typestring.go
@@ -272,9 +272,9 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 			buf.WriteByte('[')
 			writeTypeList(buf, t.targs, qf, visited)
 			buf.WriteByte(']')
-		} else if t.tparams != nil {
+		} else if t.TParams() != nil {
 			// parameterized type
-			writeTParamList(buf, t.tparams, qf, visited)
+			writeTParamList(buf, t.TParams(), qf, visited)
 		}
 
 	case *TypeParam:
diff --git a/src/cmd/compile/internal/types2/typexpr.go b/src/cmd/compile/internal/types2/typexpr.go
index b27b2a00df..583bb464b2 100644
--- a/src/cmd/compile/internal/types2/typexpr.go
+++ b/src/cmd/compile/internal/types2/typexpr.go
@@ -58,7 +58,7 @@ func (check *Checker) ident(x *operand, e *syntax.Name, def *Named, wantType boo
 	// If so, mark the respective package as used.
 	// (This code is only needed for dot-imports. Without them,
 	// we only have to mark variables, see *Var case below).
-	if pkgName := check.dotImportMap[dotImportKey{scope, obj}]; pkgName != nil {
+	if pkgName := check.dotImportMap[dotImportKey{scope, obj.Name()}]; pkgName != nil {
 		pkgName.used = true
 	}