[dev.typeparams] cmd/compile/internal/types2: merge instance and Named to eliminate sanitization

This is a port of CL 335929 to types2. It differs significantly from
that CL to handle lazy loading, which wasn't tested in go/types.
Additionally, the *Checker field was moved out of instance and back
onto Named. This way we can tell whether a Named type is uninstantiated
simply by checking whether Named.instance is non-nil, which simplified
the code considerably.

Fixes #46151

Change-Id: I617263bcfaa768ac5442213cecad8d567c2749fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/336252
Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

17 files changed, 127 insertions, 329 deletions

diff --git a/src/cmd/compile/internal/types2/builtins.go b/src/cmd/compile/internal/types2/builtins.go
index 2af2679d5e..b9fcf3c898 100644
--- a/src/cmd/compile/internal/types2/builtins.go
+++ b/src/cmd/compile/internal/types2/builtins.go
@@ -798,7 +798,7 @@ func hasVarSize(t Type) bool {
 		}
 	case *TypeParam:
 		return true
-	case *Named, *Union, *instance, *top:
+	case *Named, *Union, *top:
 		unreachable()
 	}
 	return false
diff --git a/src/cmd/compile/internal/types2/check.go b/src/cmd/compile/internal/types2/check.go
index 071afef058..6bc965c497 100644
--- a/src/cmd/compile/internal/types2/check.go
+++ b/src/cmd/compile/internal/types2/check.go
@@ -282,11 +282,6 @@ func (check *Checker) checkFiles(files []*syntax.File) (err error) {
 	print("== recordUntyped ==")
 	check.recordUntyped()
 
-	if check.Info != nil {
-		print("== sanitizeInfo ==")
-		sanitizeInfo(check.Info)
-	}
-
 	check.pkg.complete = true
 
 	// no longer needed - release memory
diff --git a/src/cmd/compile/internal/types2/decl.go b/src/cmd/compile/internal/types2/decl.go
index 4f656e374a..6ca8f75e9a 100644
--- a/src/cmd/compile/internal/types2/decl.go
+++ b/src/cmd/compile/internal/types2/decl.go
@@ -317,6 +317,8 @@ func (check *Checker) validType(typ Type, path []Object) typeInfo {
 		}
 
 	case *Named:
+		t.expand()
+
 		// don't touch the type if it is from a different package or the Universe scope
 		// (doing so would lead to a race condition - was issue #35049)
 		if t.obj.pkg != check.pkg {
@@ -349,9 +351,6 @@ func (check *Checker) validType(typ Type, path []Object) typeInfo {
 			panic("internal error: cycle start not found")
 		}
 		return t.info
-
-	case *instance:
-		return check.validType(t.expand(), path)
 	}
 
 	return valid
@@ -557,7 +556,7 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *Named
 
 	// determine underlying type of named
 	named.fromRHS = check.definedType(tdecl.Type, named)
-
+	assert(named.fromRHS != nil)
 	// The underlying type of named may be itself a named type that is
 	// incomplete:
 	//
@@ -624,7 +623,8 @@ func (check *Checker) boundType(e syntax.Expr) Type {
 
 	bound := check.typ(e)
 	check.later(func() {
-		if _, ok := under(bound).(*Interface); !ok && bound != Typ[Invalid] {
+		u := under(bound)
+		if _, ok := u.(*Interface); !ok && u != Typ[Invalid] {
 			check.errorf(e, "%s is not an interface", bound)
 		}
 	})
@@ -692,7 +692,7 @@ func (check *Checker) collectMethods(obj *TypeName) {
 		}
 
 		if base != nil {
-			base.expand() // TODO(mdempsky): Probably unnecessary.
+			base.load() // TODO(mdempsky): Probably unnecessary.
 			base.methods = append(base.methods, m)
 		}
 	}
diff --git a/src/cmd/compile/internal/types2/infer.go b/src/cmd/compile/internal/types2/infer.go
index b44ff7377a..6e7a217709 100644
--- a/src/cmd/compile/internal/types2/infer.go
+++ b/src/cmd/compile/internal/types2/infer.go
@@ -342,9 +342,6 @@ func (w *tpWalker) isParameterized(typ Type) (res bool) {
 		// t must be one of w.tparams
 		return t.index < len(w.tparams) && w.tparams[t.index].typ == t
 
-	case *instance:
-		return w.isParameterizedList(t.targs)
-
 	default:
 		unreachable()
 	}
diff --git a/src/cmd/compile/internal/types2/instance.go b/src/cmd/compile/internal/types2/instance.go
index 798d58811f..df0fc17ba7 100644
--- a/src/cmd/compile/internal/types2/instance.go
+++ b/src/cmd/compile/internal/types2/instance.go
@@ -4,56 +4,40 @@
 
 package types2
 
+// TODO(rfindley): move this code to named.go.
+
 import "cmd/compile/internal/syntax"
 
-// An instance represents an instantiated generic type syntactically
-// (without expanding the instantiation). Type instances appear only
-// during type-checking and are replaced by their fully instantiated
-// (expanded) types before the end of type-checking.
+// instance holds position information for use in lazy instantiation.
+//
+// TODO(rfindley): come up with a better name for this type, now that its usage
+// has changed.
 type instance struct {
-	check   *Checker     // for lazy instantiation
 	pos     syntax.Pos   // position of type instantiation; for error reporting only
-	base    *Named       // parameterized type to be instantiated
-	targs   []Type       // type arguments
 	posList []syntax.Pos // position of each targ; for error reporting only
-	verify  bool         // if set, constraint satisfaction is verified
-	value   Type         // base[targs...] after instantiation or Typ[Invalid]; nil if not yet set
+	verify  bool         // if set, check constraint satisfaction upon instantiation
 }
 
-// expand returns the instantiated (= expanded) type of t.
-// The result is either an instantiated *Named type, or
-// Typ[Invalid] if there was an error.
-func (t *instance) expand() Type {
-	v := t.value
-	if v == nil {
-		v = t.check.Instantiate(t.pos, t.base, t.targs, t.posList, t.verify)
-		if v == nil {
-			v = Typ[Invalid]
-		}
-		t.value = v
-	}
-	// After instantiation we must have an invalid or a *Named type.
-	if debug && v != Typ[Invalid] {
-		_ = v.(*Named)
+// expand ensures that the underlying type of n is instantiated.
+// The underlying type will be Typ[Invalid] if there was an error.
+func (n *Named) expand() {
+	if n.instance != nil {
+		// n must be loaded before instantiation, in order to have accurate
+		// tparams. This is done implicitly by the call to n.TParams, but making it
+		// explicit is harmless: load is idempotent.
+		n.load()
+		inst := n.check.instantiate(n.instance.pos, n.orig.underlying, n.TParams(), n.targs, n.instance.posList, n.instance.verify)
+		n.underlying = inst
+		n.fromRHS = inst
+		n.instance = nil
 	}
-	return v
 }
 
-// expand expands a type instance into its instantiated
-// type and leaves all other types alone. expand does
-// not recurse.
+// expand expands uninstantiated named types and leaves all other types alone.
+// expand does not recurse.
 func expand(typ Type) Type {
-	if t, _ := typ.(*instance); t != nil {
-		return t.expand()
+	if t, _ := typ.(*Named); t != nil {
+		t.expand()
 	}
 	return typ
 }
-
-// expandf is set to expand.
-// Call expandf when calling expand causes compile-time cycle error.
-var expandf func(Type) Type
-
-func init() { expandf = expand }
-
-func (t *instance) Underlying() Type { return t }
-func (t *instance) String() string   { return TypeString(t, nil) }
diff --git a/src/cmd/compile/internal/types2/instantiate.go b/src/cmd/compile/internal/types2/instantiate.go
index db398c6563..1294b08490 100644
--- a/src/cmd/compile/internal/types2/instantiate.go
+++ b/src/cmd/compile/internal/types2/instantiate.go
@@ -25,28 +25,6 @@ import (
 // Any methods attached to a *Named are simply copied; they are not
 // instantiated.
 func (check *Checker) Instantiate(pos syntax.Pos, typ Type, targs []Type, posList []syntax.Pos, verify bool) (res Type) {
-	if verify && check == nil {
-		panic("cannot have nil receiver if verify is set")
-	}
-
-	if check != nil && check.conf.Trace {
-		check.trace(pos, "-- instantiating %s with %s", typ, typeListString(targs))
-		check.indent++
-		defer func() {
-			check.indent--
-			var under Type
-			if res != nil {
-				// Calling under() here may lead to endless instantiations.
-				// Test case: type T[P any] T[P]
-				// TODO(gri) investigate if that's a bug or to be expected.
-				under = res.Underlying()
-			}
-			check.trace(pos, "=> %s (under = %s)", res, under)
-		}()
-	}
-
-	assert(len(posList) <= len(targs))
-
 	// TODO(gri) What is better here: work with TypeParams, or work with TypeNames?
 	var tparams []*TypeName
 	switch t := typ.(type) {
@@ -76,7 +54,10 @@ func (check *Checker) Instantiate(pos syntax.Pos, typ Type, targs []Type, posLis
 		// only types and functions can be generic
 		panic(fmt.Sprintf("%v: cannot instantiate %v", pos, typ))
 	}
+	return check.instantiate(pos, typ, tparams, targs, posList, verify)
+}
 
+func (check *Checker) instantiate(pos syntax.Pos, typ Type, tparams []*TypeName, targs []Type, posList []syntax.Pos, verify bool) (res Type) {
 	// the number of supplied types must match the number of type parameters
 	if len(targs) != len(tparams) {
 		// TODO(gri) provide better error message
@@ -86,6 +67,27 @@ func (check *Checker) Instantiate(pos syntax.Pos, typ Type, targs []Type, posLis
 		}
 		panic(fmt.Sprintf("%v: got %d arguments but %d type parameters", pos, len(targs), len(tparams)))
 	}
+	if verify && check == nil {
+		panic("cannot have nil receiver if verify is set")
+	}
+
+	if check != nil && check.conf.Trace {
+		check.trace(pos, "-- instantiating %s with %s", typ, typeListString(targs))
+		check.indent++
+		defer func() {
+			check.indent--
+			var under Type
+			if res != nil {
+				// Calling under() here may lead to endless instantiations.
+				// Test case: type T[P any] T[P]
+				// TODO(gri) investigate if that's a bug or to be expected.
+				under = res.Underlying()
+			}
+			check.trace(pos, "=> %s (under = %s)", res, under)
+		}()
+	}
+
+	assert(len(posList) <= len(targs))
 
 	if len(tparams) == 0 {
 		return typ // nothing to do (minor optimization)
@@ -115,19 +117,35 @@ func (check *Checker) Instantiate(pos syntax.Pos, typ Type, targs []Type, posLis
 // instantiating the type until needed. typ must be a *Named
 // type.
 func (check *Checker) InstantiateLazy(pos syntax.Pos, typ Type, targs []Type, posList []syntax.Pos, verify bool) Type {
-	base := asNamed(typ)
+	// Don't use asNamed here: we don't want to expand the base during lazy
+	// instantiation.
+	base := typ.(*Named)
+
 	if base == nil {
 		panic(fmt.Sprintf("%v: cannot instantiate %v", pos, typ))
 	}
+	h := instantiatedHash(base, targs)
+	if check != nil {
+		// typ may already have been instantiated with identical type arguments. In
+		// that case, re-use the existing instance.
+		if named := check.typMap[h]; named != nil {
+			return named
+		}
+	}
 
-	return &instance{
-		check:   check,
+	tname := NewTypeName(pos, base.obj.pkg, base.obj.name, nil)
+	named := check.newNamed(tname, base, nil, nil, nil) // methods and tparams are set when named is loaded.
+	named.targs = targs
+	named.instance = &instance{
 		pos:     pos,
-		base:    base,
-		targs:   targs,
 		posList: posList,
 		verify:  verify,
 	}
+
+	if check != nil {
+		check.typMap[h] = named
+	}
+	return named
 }
 
 // satisfies reports whether the type argument targ satisfies the constraint of type parameter
diff --git a/src/cmd/compile/internal/types2/lookup.go b/src/cmd/compile/internal/types2/lookup.go
index ecf6926c0a..9e9d6dfb29 100644
--- a/src/cmd/compile/internal/types2/lookup.go
+++ b/src/cmd/compile/internal/types2/lookup.go
@@ -119,7 +119,7 @@ func lookupFieldOrMethod(T Type, addressable bool, pkg *Package, name string) (o
 				seen[named] = true
 
 				// look for a matching attached method
-				named.expand()
+				named.load()
 				if i, m := lookupMethod(named.methods, pkg, name); m != nil {
 					// potential match
 					// caution: method may not have a proper signature yet
diff --git a/src/cmd/compile/internal/types2/named.go b/src/cmd/compile/internal/types2/named.go
index da098b58b7..a88aeb0077 100644
--- a/src/cmd/compile/internal/types2/named.go
+++ b/src/cmd/compile/internal/types2/named.go
@@ -10,12 +10,13 @@ import "sync"
 
 // A Named represents a named (defined) type.
 type Named struct {
-	check      *Checker    // for Named.under implementation; nilled once under has been called
+	check      *Checker
 	info       typeInfo    // for cycle detection
 	obj        *TypeName   // corresponding declared object
 	orig       *Named      // original, uninstantiated type
 	fromRHS    Type        // type (on RHS of declaration) this *Named type is derived from (for cycle reporting)
 	underlying Type        // possibly a *Named during setup; never a *Named once set up completely
+	instance   *instance   // position information for lazy instantiation, or nil
 	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
@@ -34,7 +35,19 @@ func NewNamed(obj *TypeName, underlying Type, methods []*Func) *Named {
 	return (*Checker)(nil).newNamed(obj, nil, underlying, nil, methods)
 }
 
-func (t *Named) expand() *Named {
+func (t *Named) load() *Named {
+	// If t is an instantiated type, it derives its methods and tparams from its
+	// base type. Since we expect type parameters and methods to be set after a
+	// call to load, we must load the base and copy here.
+	//
+	// underlying is set when t is expanded.
+	//
+	// By convention, a type instance is loaded iff its tparams are set.
+	if len(t.targs) > 0 && t.tparams == nil {
+		t.orig.load()
+		t.tparams = t.orig.tparams
+		t.methods = t.orig.methods
+	}
 	if t.resolve == nil {
 		return t
 	}
@@ -83,7 +96,7 @@ func (check *Checker) newNamed(obj *TypeName, orig *Named, underlying Type, tpar
 	if check != nil {
 		check.later(func() {
 			switch typ.under().(type) {
-			case *Named, *instance:
+			case *Named:
 				panic("internal error: unexpanded underlying type")
 			}
 			typ.check = nil
@@ -104,10 +117,12 @@ 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.expand().tparams }
+func (t *Named) TParams() []*TypeName {
+	return t.load().tparams
+}
 
 // SetTParams sets the type parameters of the named type t.
-func (t *Named) SetTParams(tparams []*TypeName) { t.expand().tparams = tparams }
+func (t *Named) SetTParams(tparams []*TypeName) { t.load().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 }
@@ -116,10 +131,10 @@ 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.expand().methods) }
+func (t *Named) NumMethods() int { return len(t.load().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.expand().methods[i] }
+func (t *Named) Method(i int) *Func { return t.load().methods[i] }
 
 // SetUnderlying sets the underlying type and marks t as complete.
 func (t *Named) SetUnderlying(underlying Type) {
@@ -129,18 +144,18 @@ func (t *Named) SetUnderlying(underlying Type) {
 	if _, ok := underlying.(*Named); ok {
 		panic("types2.Named.SetUnderlying: underlying type must not be *Named")
 	}
-	t.expand().underlying = underlying
+	t.load().underlying = underlying
 }
 
 // AddMethod adds method m unless it is already in the method list.
 func (t *Named) AddMethod(m *Func) {
-	t.expand()
+	t.load()
 	if i, _ := lookupMethod(t.methods, m.pkg, m.name); i < 0 {
 		t.methods = append(t.methods, m)
 	}
 }
 
-func (t *Named) Underlying() Type { return t.expand().underlying }
+func (t *Named) Underlying() Type { return t.load().underlying }
 func (t *Named) String() string   { return TypeString(t, nil) }
 
 // ----------------------------------------------------------------------------
@@ -153,6 +168,8 @@ func (t *Named) String() string   { return TypeString(t, nil) }
 // 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 {
+	n0.expand()
+
 	u := n0.Underlying()
 
 	if u == Typ[Invalid] {
@@ -168,7 +185,7 @@ func (n0 *Named) under() Type {
 	default:
 		// common case
 		return u
-	case *Named, *instance:
+	case *Named:
 		// handled below
 	}
 
@@ -199,12 +216,8 @@ func (n0 *Named) under() Type {
 		var n1 *Named
 		switch u1 := u.(type) {
 		case *Named:
+			u1.expand()
 			n1 = u1
-		case *instance:
-			n1, _ = u1.expand().(*Named)
-			if n1 == nil {
-				u = Typ[Invalid]
-			}
 		}
 		if n1 == nil {
 			break // end of chain
diff --git a/src/cmd/compile/internal/types2/object.go b/src/cmd/compile/internal/types2/object.go
index 82297ff17f..48fd1e44de 100644
--- a/src/cmd/compile/internal/types2/object.go
+++ b/src/cmd/compile/internal/types2/object.go
@@ -475,6 +475,9 @@ func writeObject(buf *bytes.Buffer, obj Object, qf Qualifier) {
 		if _, ok := typ.(*Basic); ok {
 			return
 		}
+		if named, _ := typ.(*Named); named != nil && len(named.tparams) > 0 {
+			writeTParamList(buf, named.tparams, qf, nil)
+		}
 		if tname.IsAlias() {
 			buf.WriteString(" =")
 		} else {
diff --git a/src/cmd/compile/internal/types2/predicates.go b/src/cmd/compile/internal/types2/predicates.go
index f2215b36cb..e448ade9e5 100644
--- a/src/cmd/compile/internal/types2/predicates.go
+++ b/src/cmd/compile/internal/types2/predicates.go
@@ -10,7 +10,7 @@ package types2
 // isNamed may be called with types that are not fully set up.
 func isNamed(typ Type) bool {
 	switch typ.(type) {
-	case *Basic, *Named, *TypeParam, *instance:
+	case *Basic, *Named, *TypeParam:
 		return true
 	}
 	return false
@@ -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.targs == nil && named.TParams() != nil
 }
 
 func is(typ Type, what BasicInfo) bool {
@@ -144,8 +144,8 @@ func (p *ifacePair) identical(q *ifacePair) bool {
 // For changes to this code the corresponding changes should be made to unifier.nify.
 func identical(x, y Type, cmpTags bool, p *ifacePair) bool {
 	// types must be expanded for comparison
-	x = expandf(x)
-	y = expandf(y)
+	x = expand(x)
+	y = expand(y)
 
 	if x == y {
 		return true
diff --git a/src/cmd/compile/internal/types2/sanitize.go b/src/cmd/compile/internal/types2/sanitize.go
deleted file mode 100644
index 3d2323a0a2..0000000000
--- a/src/cmd/compile/internal/types2/sanitize.go
+++ /dev/null
@@ -1,205 +0,0 @@
-// Copyright 2020 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package types2
-
-// sanitizeInfo walks the types contained in info to ensure that all instances
-// are expanded.
-//
-// This includes some objects that may be shared across concurrent
-// type-checking passes (such as those in the universe scope), so we are
-// careful here not to write types that are already sanitized. This avoids a
-// data race as any shared types should already be sanitized.
-func sanitizeInfo(info *Info) {
-	var s sanitizer = make(map[Type]Type)
-
-	// Note: Some map entries are not references.
-	// If modified, they must be assigned back.
-
-	for e, tv := range info.Types {
-		if typ := s.typ(tv.Type); typ != tv.Type {
-			tv.Type = typ
-			info.Types[e] = tv
-		}
-	}
-
-	for e, inf := range info.Inferred {
-		changed := false
-		for i, targ := range inf.TArgs {
-			if typ := s.typ(targ); typ != targ {
-				inf.TArgs[i] = typ
-				changed = true
-			}
-		}
-		if typ := s.typ(inf.Sig); typ != inf.Sig {
-			inf.Sig = typ.(*Signature)
-			changed = true
-		}
-		if changed {
-			info.Inferred[e] = inf
-		}
-	}
-
-	for _, obj := range info.Defs {
-		if obj != nil {
-			if typ := s.typ(obj.Type()); typ != obj.Type() {
-				obj.setType(typ)
-			}
-		}
-	}
-
-	for _, obj := range info.Uses {
-		if obj != nil {
-			if typ := s.typ(obj.Type()); typ != obj.Type() {
-				obj.setType(typ)
-			}
-		}
-	}
-
-	// TODO(gri) sanitize as needed
-	// - info.Implicits
-	// - info.Selections
-	// - info.Scopes
-	// - info.InitOrder
-}
-
-type sanitizer map[Type]Type
-
-func (s sanitizer) typ(typ Type) Type {
-	if typ == nil {
-		return nil
-	}
-
-	if t, found := s[typ]; found {
-		return t
-	}
-	s[typ] = typ
-
-	switch t := typ.(type) {
-	case *Basic, *top:
-		// nothing to do
-
-	case *Array:
-		if elem := s.typ(t.elem); elem != t.elem {
-			t.elem = elem
-		}
-
-	case *Slice:
-		if elem := s.typ(t.elem); elem != t.elem {
-			t.elem = elem
-		}
-
-	case *Struct:
-		s.varList(t.fields)
-
-	case *Pointer:
-		if base := s.typ(t.base); base != t.base {
-			t.base = base
-		}
-
-	case *Tuple:
-		s.tuple(t)
-
-	case *Signature:
-		s.var_(t.recv)
-		s.tuple(t.params)
-		s.tuple(t.results)
-
-	case *Union:
-		s.typeList(t.types)
-
-	case *Interface:
-		s.funcList(t.methods)
-		s.typeList(t.embeddeds)
-		// TODO(gri) do we need to sanitize type sets?
-		tset := t.typeSet()
-		s.funcList(tset.methods)
-		if types := s.typ(tset.types); types != tset.types {
-			tset.types = types
-		}
-
-	case *Map:
-		if key := s.typ(t.key); key != t.key {
-			t.key = key
-		}
-		if elem := s.typ(t.elem); elem != t.elem {
-			t.elem = elem
-		}
-
-	case *Chan:
-		if elem := s.typ(t.elem); elem != t.elem {
-			t.elem = elem
-		}
-
-	case *Named:
-		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
-		}
-		if under := s.typ(t.underlying); under != t.underlying {
-			t.underlying = under
-		}
-		s.typeList(t.targs)
-		s.funcList(t.methods)
-
-	case *TypeParam:
-		if bound := s.typ(t.bound); bound != t.bound {
-			t.bound = bound
-		}
-
-	case *instance:
-		typ = t.expand()
-		s[t] = typ
-
-	default:
-		unimplemented()
-	}
-
-	return typ
-}
-
-func (s sanitizer) var_(v *Var) {
-	if v != nil {
-		if typ := s.typ(v.typ); typ != v.typ {
-			v.typ = typ
-		}
-	}
-}
-
-func (s sanitizer) varList(list []*Var) {
-	for _, v := range list {
-		s.var_(v)
-	}
-}
-
-func (s sanitizer) tuple(t *Tuple) {
-	if t != nil {
-		s.varList(t.vars)
-	}
-}
-
-func (s sanitizer) func_(f *Func) {
-	if f != nil {
-		if typ := s.typ(f.typ); typ != f.typ {
-			f.typ = typ
-		}
-	}
-}
-
-func (s sanitizer) funcList(list []*Func) {
-	for _, f := range list {
-		s.func_(f)
-	}
-}
-
-func (s sanitizer) typeList(list []Type) {
-	for i, t := range list {
-		if typ := s.typ(t); typ != t {
-			list[i] = typ
-		}
-	}
-}
diff --git a/src/cmd/compile/internal/types2/sizeof_test.go b/src/cmd/compile/internal/types2/sizeof_test.go
index 22ef369683..a62b7cb3e2 100644
--- a/src/cmd/compile/internal/types2/sizeof_test.go
+++ b/src/cmd/compile/internal/types2/sizeof_test.go
@@ -31,9 +31,8 @@ func TestSizeof(t *testing.T) {
 		{Interface{}, 40, 80},
 		{Map{}, 16, 32},
 		{Chan{}, 12, 24},
-		{Named{}, 84, 160},
+		{Named{}, 88, 168},
 		{TypeParam{}, 28, 48},
-		{instance{}, 56, 104},
 		{top{}, 0, 0},
 
 		// Objects
diff --git a/src/cmd/compile/internal/types2/subst.go b/src/cmd/compile/internal/types2/subst.go
index 63b234a60e..87e3e3018e 100644
--- a/src/cmd/compile/internal/types2/subst.go
+++ b/src/cmd/compile/internal/types2/subst.go
@@ -26,12 +26,7 @@ func makeSubstMap(tpars []*TypeName, targs []Type) *substMap {
 	assert(len(tpars) == len(targs))
 	proj := make(map[*TypeParam]Type, len(tpars))
 	for i, tpar := range tpars {
-		// We must expand type arguments otherwise *instance
-		// types end up as components in composite types.
-		// TODO(gri) explain why this causes problems, if it does
-		targ := expand(targs[i]) // possibly nil
-		targs[i] = targ
-		proj[tpar.typ.(*TypeParam)] = targ
+		proj[tpar.typ.(*TypeParam)] = targs[i]
 	}
 	return &substMap{targs, proj}
 }
@@ -83,6 +78,7 @@ func (check *Checker) subst(pos syntax.Pos, typ Type, smap *substMap) Type {
 		// for recursive types (example: type T[P any] *T[P]).
 		subst.typMap = make(map[string]*Named)
 	}
+
 	return subst.typ(typ)
 }
 
@@ -241,10 +237,13 @@ func (subst *subster) typ(typ Type) Type {
 		named := subst.check.newNamed(tname, t, t.Underlying(), t.TParams(), t.methods) // method signatures are updated lazily
 		named.targs = new_targs
 		subst.typMap[h] = named
+		t.expand() // must happen after typMap update to avoid infinite recursion
 
 		// do the substitution
 		dump(">>> subst %s with %s (new: %s)", t.underlying, subst.smap, new_targs)
 		named.underlying = subst.typOrNil(t.Underlying())
+		dump(">>> underlying: %v", named.underlying)
+		assert(named.underlying != nil)
 		named.fromRHS = named.underlying // for cycle detection (Checker.validType)
 
 		return named
@@ -252,10 +251,6 @@ func (subst *subster) typ(typ Type) Type {
 	case *TypeParam:
 		return subst.smap.lookup(t)
 
-	case *instance:
-		// TODO(gri) can we avoid the expansion here and just substitute the type parameters?
-		return subst.typ(t.expand())
-
 	default:
 		unimplemented()
 	}
diff --git a/src/cmd/compile/internal/types2/testdata/check/issues.go2 b/src/cmd/compile/internal/types2/testdata/check/issues.go2
index 1ede383ebe..e29357de0b 100644
--- a/src/cmd/compile/internal/types2/testdata/check/issues.go2
+++ b/src/cmd/compile/internal/types2/testdata/check/issues.go2
@@ -74,8 +74,10 @@ func (u T2[U]) Add1() U {
     return u.s + 1
 }
 
+// TODO(rfindley): we should probably report an error here as well, not
+//                 just when the type is first instantiated.
 func NewT2[U any]() T2[U /* ERROR U has no constraints */ ] {
-    return T2[U /* ERROR U has no constraints */ ]{}
+    return T2[U]{}
 }
 
 func _() {
diff --git a/src/cmd/compile/internal/types2/typeparam.go b/src/cmd/compile/internal/types2/typeparam.go
index 0aca227c0a..b66256cf00 100644
--- a/src/cmd/compile/internal/types2/typeparam.go
+++ b/src/cmd/compile/internal/types2/typeparam.go
@@ -21,7 +21,8 @@ type TypeParam struct {
 	id    uint64    // unique id, for debugging only
 	obj   *TypeName // corresponding type name
 	index int       // type parameter index in source order, starting at 0
-	bound Type      // *Named or *Interface; underlying type is always *Interface
+	// TODO(rfindley): this could also be Typ[Invalid]. Verify that this is handled correctly.
+	bound Type // *Named or *Interface; underlying type is always *Interface
 }
 
 // Obj returns the type name for the type parameter t.
diff --git a/src/cmd/compile/internal/types2/typestring.go b/src/cmd/compile/internal/types2/typestring.go
index 44099133a0..74d2f1dc51 100644
--- a/src/cmd/compile/internal/types2/typestring.go
+++ b/src/cmd/compile/internal/types2/typestring.go
@@ -269,6 +269,9 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 		}
 
 	case *Named:
+		if t.instance != nil {
+			buf.WriteByte(instanceMarker)
+		}
 		writeTypeName(buf, t.obj, qf)
 		if t.targs != nil {
 			// instantiated type
@@ -294,13 +297,6 @@ func writeType(buf *bytes.Buffer, typ Type, qf Qualifier, visited []Type) {
 		}
 		buf.WriteString(s + subscript(t.id))
 
-	case *instance:
-		buf.WriteByte(instanceMarker) // indicate "non-evaluated" syntactic instance
-		writeTypeName(buf, t.base.obj, qf)
-		buf.WriteByte('[')
-		writeTypeList(buf, t.targs, qf, visited)
-		buf.WriteByte(']')
-
 	case *top:
 		buf.WriteString("⊤")
 
diff --git a/src/cmd/compile/internal/types2/typexpr.go b/src/cmd/compile/internal/types2/typexpr.go
index 83cefa19ba..c55d5c093a 100644
--- a/src/cmd/compile/internal/types2/typexpr.go
+++ b/src/cmd/compile/internal/types2/typexpr.go
@@ -446,7 +446,7 @@ func (check *Checker) instantiatedType(x syntax.Expr, targsx []syntax.Expr, def
 	// make sure we check instantiation works at least once
 	// and that the resulting type is valid
 	check.later(func() {
-		t := typ.(*instance).expand()
+		t := expand(typ)
 		check.validType(t, nil)
 	})