go/types: a type arg type list must be a subset of the type param type list

The original implementation (commit 65377ae6e0) checked the wrong way around.
Added more tests.

Change-Id: I59e1fd6d7e646f2e8e0ff68a0bbcf9ff9def0eff

src/go/types/examples/contracts.go2

@@ -53,6 +53,8 @@ type AltGraph (type Node NodeFace(Edge), Edge EdgeFace(Node)) struct { }
 
 func AltNew (type Node NodeFace(Edge), Edge EdgeFace(Node)) (nodes []Node) *AltGraph(Node, Edge) { panic("unimplemented") }
 
+// func (g *AltGraph(N, E)) ShortestPath(from, to N) []E { panic("unimplemented") }
+
 type NodeFace(type Edge) interface {
 	Edges() []Edge
 }

src/go/types/subst.go

@@ -97,12 +97,11 @@ func (check *Checker) instantiate(pos token.Pos, typ Type, targs []Type, poslist
 			break // nothing to do
 		}
 
-		// if targ is a type parameter, the list of iface types must be a subset of the
-		// list of targ types
-		if t, _ := targ.Underlying().(*TypeParam); t != nil {
-			targFace := t.Interface()
-			for _, t := range iface.types {
-				if !includesType(t, targFace) {
+		// if targ is itself a type parameter, each of its possible types must be in the
+		// list of iface types (i.e., the targ type list must be a subset of the iface types)
+		if targ, _ := targ.Underlying().(*TypeParam); targ != nil {
+			for _, t := range targ.Interface().types {
+				if !includesType(t, iface) {
 					check.softErrorf(pos, "%s does not satisfy %s (missing type %s)", targ, tpar.bound, t)
 					break
 				}

src/go/types/testdata/contracts.go2

@@ -202,13 +202,19 @@ type T3(type P B1) P
 
 func _(type P B1)(x T3(P))
 
-contract B2(T) {
-	T int
+
+type B2 interface{type int, string}
+
+func _(type P B2)(x T3(P /* ERROR missing type string */ ))
+
+
+contract B3(T) {
+	T int, string
 }
 
-type T4(type P B2) P
+type T4(type P B1) P
 
-func _(type P B2)(x T4(P))
+func _(type P B3)(x T4(P /* ERROR missing type string */ ))
 
 // --------------------------------------------------------------------------------------
 // Type parameters may be from different parameterized objects

src/go/types/testdata/tmp.go2

@@ -3,22 +3,3 @@
 // license that can be found in the LICENSE file.
 
 package p
-
-contract C(T) {
-	T int
-}
-
-type Cm(type T C) T
-
-func (a Cm(T /* ERROR does not satisfy */ )) m() T
-
-// TODO(gri) make this work
-/*
-type C interface {
-	type int
-}
-
-type Cm(type T C) T
-
-func (a Cm(T)) m() T
-*/