[dev.typeparams] cmd/compile/internal/types2: bring over subst.go changes from go/types

This CL make this code match closely with the go/types version
(see https://golang.org/cl/276253).

Change-Id: I2b7841309fdbda7e2c9533768bd98317729b13b4
Reviewed-on: https://go-review.googlesource.com/c/go/+/276814
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/cmd/compile/internal/types2/subst.go

@@ -1,4 +1,3 @@
-// UNREVIEWED
 // Copyright 2018 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.
@@ -29,7 +28,7 @@ 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
+		// 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
@@ -71,7 +70,7 @@ func (check *Checker) instantiate(pos syntax.Pos, typ Type, targs []Type, poslis
 		}()
 	}
 
-	assert(poslist == nil || len(poslist) <= len(targs))
+	assert(len(poslist) <= len(targs))
 
 	// TODO(gri) What is better here: work with TypeParams, or work with TypeNames?
 	var tparams []*TypeName
@@ -81,9 +80,10 @@ func (check *Checker) instantiate(pos syntax.Pos, typ Type, targs []Type, poslis
 	case *Signature:
 		tparams = t.tparams
 		defer func() {
-			// If we had an unexpected failure somewhere don't
+			// If we had an unexpected failure somewhere don't panic below when
-			// panic below when asserting res.(*Signature).
+			// asserting res.(*Signature). Check for *Signature in case Typ[Invalid]
-			if res == nil {
+			// is returned.
+			if _, ok := res.(*Signature); !ok {
 				return
 			}
 			// If the signature doesn't use its type parameters, subst
@@ -153,6 +153,7 @@ func (check *Checker) instantiate(pos syntax.Pos, typ Type, targs []Type, poslis
 			if m, wrong := check.missingMethod(targ, iface, true); m != nil {
 				// TODO(gri) needs to print updated name to avoid major confusion in error message!
 				//           (print warning for now)
+				// Old warning:
 				// check.softErrorf(pos, "%s does not satisfy %s (warning: name not updated) = %s (missing method %s)", targ, tpar.bound, iface, m)
 				if m.name == "==" {
 					// We don't want to report "missing method ==".
@@ -175,7 +176,6 @@ func (check *Checker) instantiate(pos syntax.Pos, typ Type, targs []Type, poslis
 		if iface.allTypes == nil {
 			continue // nothing to do
 		}
-		// iface.allTypes != nil
 
 		// If targ is itself a type parameter, each of its possible types, but at least one, must be in the
 		// list of iface types (i.e., the targ type list must be a non-empty subset of the iface types).
@@ -278,7 +278,9 @@ func (subst *subster) typ(typ Type) Type {
 		results := subst.tuple(t.results)
 		if recv != t.recv || params != t.params || results != t.results {
 			return &Signature{
-				rparams:  t.rparams,
+				rparams: t.rparams,
+				// TODO(gri) Why can't we nil out tparams here, rather than in
+				//           instantiate above?
 				tparams:  t.tparams,
 				scope:    t.scope,
 				recv:     recv,
@@ -343,7 +345,6 @@ func (subst *subster) typ(typ Type) Type {
 		var new_targs []Type
 
 		if len(t.targs) > 0 {
-
 			// already instantiated
 			dump(">>> %s already instantiated", t)
 			assert(len(t.targs) == len(t.tparams))
@@ -367,13 +368,10 @@ func (subst *subster) typ(typ Type) Type {
 				dump(">>> nothing to substitute in %s", t)
 				return t // nothing to substitute
 			}
-
 		} else {
-
 			// not yet instantiated
 			dump(">>> first instantiation of %s", t)
 			new_targs = subst.smap.targs
-
 		}
 
 		// before creating a new named type, check if we have this one already
@@ -419,9 +417,9 @@ func (subst *subster) typ(typ Type) Type {
 func instantiatedHash(typ *Named, targs []Type) string {
 	var buf bytes.Buffer
 	writeTypeName(&buf, typ.obj, nil)
-	buf.WriteByte('(')
+	buf.WriteByte('[')
 	writeTypeList(&buf, targs, nil, nil)
-	buf.WriteByte(')')
+	buf.WriteByte(']')
 
 	// With respect to the represented type, whether a
 	// type is fully expanded or stored as instance