9 files changed, 69 insertions, 4 deletions
diff --git a/src/cmd/compile/internal/importer/ureader.go b/src/cmd/compile/internal/importer/ureader.go
index f5c2f41069..3488f13148 100644
--- a/src/cmd/compile/internal/importer/ureader.go
+++ b/src/cmd/compile/internal/importer/ureader.go
@@ -9,6 +9,7 @@ import (
 	"cmd/compile/internal/syntax"
 	"cmd/compile/internal/types2"
 	"cmd/internal/src"
+	"internal/godebug"
 	"internal/pkgbits"
 )
 
@@ -409,7 +410,7 @@ func (pr *pkgReader) objIdx(idx pkgbits.Index) (*types2.Package, string) {
 		case pkgbits.ObjAlias:
 			pos := r.pos()
 			typ := r.typ()
-			return types2.NewTypeName(pos, objPkg, objName, typ)
+			return newAliasTypeName(pos, objPkg, objName, typ)
 
 		case pkgbits.ObjConst:
 			pos := r.pos()
@@ -533,3 +534,18 @@ func (r *reader) ident(marker pkgbits.SyncMarker) (*types2.Package, string) {
 	r.Sync(marker)
 	return r.pkg(), r.String()
 }
+
+// newAliasTypeName returns a new TypeName, with a materialized *types2.Alias if supported.
+func newAliasTypeName(pos syntax.Pos, pkg *types2.Package, name string, rhs types2.Type) *types2.TypeName {
+	// Copied from x/tools/internal/aliases.NewAlias via
+	// GOROOT/src/go/internal/gcimporter/ureader.go.
+	if gotypesalias.Value() == "1" {
+		tname := types2.NewTypeName(pos, pkg, name, nil)
+		_ = types2.NewAlias(tname, rhs) // form TypeName -> Alias cycle
+		return tname
+	}
+	return types2.NewTypeName(pos, pkg, name, rhs)
+}
+
+// gotypesalias controls the use of Alias types.
+var gotypesalias = godebug.New("#gotypesalias")
diff --git a/src/cmd/compile/internal/noder/writer.go b/src/cmd/compile/internal/noder/writer.go
index 71606a915f..785176b3b5 100644
--- a/src/cmd/compile/internal/noder/writer.go
+++ b/src/cmd/compile/internal/noder/writer.go
@@ -831,7 +831,11 @@ func (w *writer) doObj(wext *writer, obj types2.Object) pkgbits.CodeObj {
 	case *types2.TypeName:
 		if obj.IsAlias() {
 			w.pos(obj)
-			w.typ(types2.Unalias(obj.Type()))
+			t := obj.Type()
+			if alias, ok := t.(*types2.Alias); ok { // materialized alias
+				t = alias.Rhs()
+			}
+			w.typ(t)
 			return pkgbits.ObjAlias
 		}
 
diff --git a/src/cmd/compile/internal/types2/alias.go b/src/cmd/compile/internal/types2/alias.go
index 149cd3b265..7bc0e5a9f9 100644
--- a/src/cmd/compile/internal/types2/alias.go
+++ b/src/cmd/compile/internal/types2/alias.go
@@ -32,6 +32,12 @@ func (a *Alias) Obj() *TypeName   { return a.obj }
 func (a *Alias) Underlying() Type { return unalias(a).Underlying() }
 func (a *Alias) String() string   { return TypeString(a, nil) }
 
+// TODO(adonovan): uncomment when proposal #66559 is accepted.
+//
+// // Rhs returns the type R on the right-hand side of an alias
+// // declaration "type A = R", which may be another alias.
+// func (a *Alias) Rhs() Type { return a.fromRHS }
+
 // Unalias returns t if it is not an alias type;
 // otherwise it follows t's alias chain until it
 // reaches a non-alias type which is then returned.
diff --git a/src/cmd/compile/internal/types2/api.go b/src/cmd/compile/internal/types2/api.go
index bb02d9198e..0b44d4ff38 100644
--- a/src/cmd/compile/internal/types2/api.go
+++ b/src/cmd/compile/internal/types2/api.go
@@ -469,3 +469,10 @@ func (conf *Config) Check(path string, files []*syntax.File, info *Info) (*Packa
 	pkg := NewPackage(path, "")
 	return pkg, NewChecker(conf, pkg, info).Files(files)
 }
+
+// Rhs returns the type R on the right-hand side of an alias
+// declaration "type A = R", which may be another alias.
+//
+// TODO(adonovan): move to alias.go (common with go/types) once
+// proposal #66559 is accepted.
+func (a *Alias) Rhs() Type { return a.fromRHS }
diff --git a/src/go/internal/gcimporter/ureader.go b/src/go/internal/gcimporter/ureader.go
index 5397a2796f..b7d7b6c861 100644
--- a/src/go/internal/gcimporter/ureader.go
+++ b/src/go/internal/gcimporter/ureader.go
@@ -7,6 +7,7 @@ package gcimporter
 import (
 	"go/token"
 	"go/types"
+	"internal/godebug"
 	"internal/pkgbits"
 	"sort"
 )
@@ -479,7 +480,7 @@ func (pr *pkgReader) objIdx(idx pkgbits.Index) (*types.Package, string) {
 		case pkgbits.ObjAlias:
 			pos := r.pos()
 			typ := r.typ()
-			declare(types.NewTypeName(pos, objPkg, objName, typ))
+			declare(newAliasTypeName(pos, objPkg, objName, typ))
 
 		case pkgbits.ObjConst:
 			pos := r.pos()
@@ -655,3 +656,15 @@ func pkgScope(pkg *types.Package) *types.Scope {
 	}
 	return types.Universe
 }
+
+// newAliasTypeName returns a new TypeName, with a materialized *types.Alias if supported.
+func newAliasTypeName(pos token.Pos, pkg *types.Package, name string, rhs types.Type) *types.TypeName {
+	// When GODEBUG=gotypesalias=1, the Type() of the return value is a
+	// *types.Alias. Copied from x/tools/internal/aliases.NewAlias.
+	if godebug.New("gotypesalias").Value() == "1" {
+		tname := types.NewTypeName(pos, pkg, name, nil)
+		_ = types.NewAlias(tname, rhs) // form TypeName -> Alias cycle
+		return tname
+	}
+	return types.NewTypeName(pos, pkg, name, rhs)
+}
diff --git a/src/go/types/alias.go b/src/go/types/alias.go
index e32ddbcb33..3490d26c20 100644
--- a/src/go/types/alias.go
+++ b/src/go/types/alias.go
@@ -35,6 +35,12 @@ func (a *Alias) Obj() *TypeName   { return a.obj }
 func (a *Alias) Underlying() Type { return unalias(a).Underlying() }
 func (a *Alias) String() string   { return TypeString(a, nil) }
 
+// TODO(adonovan): uncomment when proposal #66559 is accepted.
+//
+// // Rhs returns the type R on the right-hand side of an alias
+// // declaration "type A = R", which may be another alias.
+// func (a *Alias) Rhs() Type { return a.fromRHS }
+
 // Unalias returns t if it is not an alias type;
 // otherwise it follows t's alias chain until it
 // reaches a non-alias type which is then returned.
diff --git a/src/go/types/eval_test.go b/src/go/types/eval_test.go
index 1521f2fe41..c0ac8225ac 100644
--- a/src/go/types/eval_test.go
+++ b/src/go/types/eval_test.go
@@ -13,6 +13,7 @@ import (
 	"go/parser"
 	"go/token"
 	"go/types"
+	"internal/godebug"
 	"internal/testenv"
 	"strings"
 	"testing"
@@ -173,6 +174,14 @@ func TestEvalPos(t *testing.T) {
 		if err != nil {
 			t.Fatalf("could not parse file %d: %s", i, err)
 		}
+
+		// Materialized aliases give a different (better)
+		// result for the final test, so skip it for now.
+		// TODO(adonovan): reenable when gotypesalias=1 is the default.
+		if gotypesalias.Value() == "1" && strings.Contains(src, "interface{R}.Read") {
+			continue
+		}
+
 		files = append(files, file)
 	}
 
@@ -196,6 +205,9 @@ func TestEvalPos(t *testing.T) {
 	}
 }
 
+// gotypesalias controls the use of Alias types.
+var gotypesalias = godebug.New("#gotypesalias")
+
 // split splits string s at the first occurrence of s, trimming spaces.
 func split(s, sep string) (string, string) {
 	before, after, _ := strings.Cut(s, sep)
diff --git a/src/go/types/named_test.go b/src/go/types/named_test.go
index d930874f12..effeeb6f5c 100644
--- a/src/go/types/named_test.go
+++ b/src/go/types/named_test.go
@@ -117,7 +117,7 @@ type Inst = *Tree[int]
 		return n.Underlying().(*Struct).Field(0).Type().(*Pointer).Elem().(*Named)
 	}
 
-	Inst := pkg.Scope().Lookup("Inst").Type().(*Pointer).Elem().(*Named)
+	Inst := Unalias(pkg.Scope().Lookup("Inst").Type()).(*Pointer).Elem().(*Named)
 	Node := firstFieldType(Inst)
 	Tree := firstFieldType(Node)
 	if !Identical(Inst, Tree) {
diff --git a/test/typeparam/struct.go b/test/typeparam/struct.go
index 2dad9087bc..fee38eccc3 100644
--- a/test/typeparam/struct.go
+++ b/test/typeparam/struct.go
@@ -21,6 +21,7 @@ type S1 struct {
 
 type Eint = E[int]
 type Ebool = E[bool]
+type Eint2 = Eint
 
 type S2 struct {
 	Eint