go.tools/go/types: map keys must be comparable

R=adonovan
CC=golang-dev
https://golang.org/cl/12102044

go/types/stdlib_test.go

@@ -60,9 +60,6 @@ func TestStdtest(t *testing.T) {
 		case "goto.go", "label1.go":
 			// TODO(gri) implement missing label checks
 			continue
-		case "map1.go":
-			// TODO(gri) fix map key checking
-			continue
 		case "sizeof.go", "switch.go":
 			// TODO(gri) tone down duplicate checking in expression switches
 			continue

go/types/testdata/cycles.src

@@ -63,7 +63,7 @@ type (
 	I6 interface{ I5 }
 
 	// maps
-	M0 map[M0]M0
+	M0 map[M0 /* ERROR "invalid map key" */ ]M0
 
 	// channels
 	C0 chan C0
@@ -103,7 +103,7 @@ func _() {
 		i0 /* ERROR "cycle" */ interface{ i0 }
 
 		// maps
-		m0 map[m0]m0
+		m0 map[m0 /* ERROR "invalid map key" */ ]m0
 
 		// channels
 		c0 chan c0

go/types/typexpr.go

@@ -293,6 +293,15 @@ func (check *checker) typ0(e ast.Expr, def *Named, cycleOk bool) Type {
 
 		typ.key = check.typ(e.Key, nil, true)
 		typ.elt = check.typ(e.Value, nil, true)
+
+		// spec: "The comparison operators == and != must be fully defined
+		// for operands of the key type; thus the key type must not be a
+		// function, map, or slice."
+		if !isComparable(typ.key) {
+			check.errorf(e.Key.Pos(), "invalid map key type %s", typ.key)
+			// ok to continue
+		}
+
 		return typ
 
 	case *ast.ChanType: