2 files changed, 29 insertions, 3 deletions
diff --git a/src/cmd/compile/internal/typecheck/iimport.go b/src/cmd/compile/internal/typecheck/iimport.go
index 2e8b18c0b7..d5f4bba98b 100644
--- a/src/cmd/compile/internal/typecheck/iimport.go
+++ b/src/cmd/compile/internal/typecheck/iimport.go
@@ -1858,18 +1858,26 @@ func substInstType(t *types.Type, baseType *types.Type, targs []*types.Type) {
 
 	newfields := make([]*types.Field, baseType.Methods().Len())
 	for i, f := range baseType.Methods().Slice() {
+		if !f.IsMethod() || types.IsInterfaceMethod(f.Type) {
+			// Do a normal substitution if this is a non-method (which
+			// means this must be an interface used as a constraint) or
+			// an interface method.
+			t2 := subst.Typ(f.Type)
+			newfields[i] = types.NewField(f.Pos, f.Sym, t2)
+			continue
+		}
 		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{
+		msubst := Tsubster{
 			Tparams:       recvType.RParams(),
 			Targs:         targs,
 			SubstForwFunc: doInst,
 		}
-		t2 := subst.Typ(f.Type)
+		t2 := msubst.Typ(f.Type)
 		oldsym := f.Nname.Sym()
 		newsym := MakeInstName(oldsym, targs, true)
 		var nname *ir.Name
diff --git a/src/cmd/compile/internal/typecheck/subr.go b/src/cmd/compile/internal/typecheck/subr.go
index e840df56dc..e86c4c6bca 100644
--- a/src/cmd/compile/internal/typecheck/subr.go
+++ b/src/cmd/compile/internal/typecheck/subr.go
@@ -1165,7 +1165,7 @@ func (ts *Tsubster) Typ(t *types.Type) *types.Type {
 
 	case types.TINTER:
 		newt = ts.tinter(t)
-		if newt == t {
+		if newt == t && !targsChanged {
 			newt = nil
 		}
 
@@ -1197,6 +1197,24 @@ func (ts *Tsubster) Typ(t *types.Type) *types.Type {
 		types.TUINT, types.TUINT8, types.TUINT16, types.TUINT32, types.TUINT64,
 		types.TUINTPTR, types.TBOOL, types.TSTRING, types.TFLOAT32, types.TFLOAT64, types.TCOMPLEX64, types.TCOMPLEX128:
 		newt = t.Underlying()
+	case types.TUNION:
+		nt := t.NumTerms()
+		newterms := make([]*types.Type, nt)
+		tildes := make([]bool, nt)
+		changed := false
+		for i := 0; i < nt; i++ {
+			term, tilde := t.Term(i)
+			tildes[i] = tilde
+			newterms[i] = ts.Typ(term)
+			if newterms[i] != term {
+				changed = true
+			}
+		}
+		if changed {
+			newt = types.NewUnion(newterms, tildes)
+		}
+	default:
+		panic(fmt.Sprintf("Bad type in (*TSubster).Typ: %v", t.Kind()))
 	}
 	if newt == nil {
 		// Even though there were typeparams in the type, there may be no