go.tools/go/types: use package-local type names in errors referring to operands

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://golang.org/cl/77670043

go/types/errors.go

@@ -30,6 +30,8 @@ func (check *checker) sprintf(format string, args ...interface{}) string {
 			arg = "<nil>"
 		case operand:
 			panic("internal error: should always pass *operand")
+		case *operand:
+			arg = operandString(check.pkg, a)
 		case token.Pos:
 			arg = check.fset.Position(a).String()
 		case ast.Expr:

go/types/operand.go

@@ -94,7 +94,7 @@ func (x *operand) pos() token.Pos {
 // commaok    <expr> (<untyped kind> <mode>                    )
 // commaok    <expr> (               <mode>       of type <typ>)
 //
-func (x *operand) String() string {
+func operandString(this *Package, x *operand) string {
 	var buf bytes.Buffer
 
 	var expr string
@@ -105,7 +105,7 @@ func (x *operand) String() string {
 		case builtin:
 			expr = predeclaredFuncs[x.id].name
 		case typexpr:
-			expr = TypeString(nil, x.typ)
+			expr = TypeString(this, x.typ)
 		case constant:
 			expr = x.val.String()
 		}
@@ -147,7 +147,7 @@ func (x *operand) String() string {
 	if hasType {
 		if x.typ != Typ[Invalid] {
 			buf.WriteString(" of type ")
-			WriteType(&buf, nil, x.typ)
+			WriteType(&buf, this, x.typ)
 		} else {
 			buf.WriteString(" with invalid type")
 		}
@@ -161,6 +161,10 @@ func (x *operand) String() string {
 	return buf.String()
 }
 
+func (x *operand) String() string {
+	return operandString(nil, x)
+}
+
 // setConst sets x to the untyped constant for literal lit.
 func (x *operand) setConst(tok token.Token, lit string) {
 	val := exact.MakeFromLiteral(lit, tok)

go/types/testdata/errors.src

@@ -48,4 +48,8 @@ func _() {
 // Add more tests as needed.
 func _() {
 	if err := foo /* ERROR undeclared */ (); err != nil /* no error here */ {}
-}
+}
+
+// Use unqualified names for package-local objects.
+type T struct{}
+var _ int = T /* ERROR value of type T */ {} // use T in error message rather then errors.T