go/types, types2: report "undefined: p.x" instead of "x not declared by package p"

This matches the compiler's long-standing behavior.

For #55326.

Change-Id: Icd946b031b1b6e65498fb52bceb4a53807732463
Reviewed-on: https://go-review.googlesource.com/c/go/+/432556
Reviewed-by: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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

@@ -465,7 +465,7 @@ func (check *Checker) selector(x *operand, e *syntax.SelectorExpr, def *Named) {
 					}
 				}
 				if exp == nil {
-					check.errorf(e.Sel, _UndeclaredImportedName, "%s not declared by package C", sel)
+					check.errorf(e.Sel, _UndeclaredImportedName, "undefined: %s", syntax.Expr(e)) // cast to syntax.Expr to silence vet
 					goto Error
 				}
 				check.objDecl(exp, nil)
@@ -473,11 +473,7 @@ func (check *Checker) selector(x *operand, e *syntax.SelectorExpr, def *Named) {
 				exp = pkg.scope.Lookup(sel)
 				if exp == nil {
 					if !pkg.fake {
-						if check.conf.CompilerErrorMessages {
-							check.errorf(e.Sel, _UndeclaredImportedName, "undefined: %s.%s", pkg.name, sel)
-						} else {
-							check.errorf(e.Sel, _UndeclaredImportedName, "%s not declared by package %s", sel, pkg.name)
-						}
+						check.errorf(e.Sel, _UndeclaredImportedName, "undefined: %s", syntax.Expr(e))
 					}
 					goto Error
 				}

src/go/types/call.go

@@ -469,7 +469,7 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr, def *Named) {
 					}
 				}
 				if exp == nil {
-					check.errorf(e.Sel, _UndeclaredImportedName, "%s not declared by package C", sel)
+					check.errorf(e.Sel, _UndeclaredImportedName, "undefined: %s", ast.Expr(e)) // cast to ast.Expr to silence vet
 					goto Error
 				}
 				check.objDecl(exp, nil)
@@ -477,7 +477,7 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr, def *Named) {
 				exp = pkg.scope.Lookup(sel)
 				if exp == nil {
 					if !pkg.fake {
-						check.errorf(e.Sel, _UndeclaredImportedName, "%s not declared by package %s", sel, pkg.name)
+						check.errorf(e.Sel, _UndeclaredImportedName, "undefined: %s", ast.Expr(e))
 					}
 					goto Error
 				}

src/internal/types/testdata/check/decls1.go

@@ -64,7 +64,7 @@ var (
 	t13 int = a /* ERROR "shifted operand" */ << d
 	t14 int = i << j
 	t15 math /* ERROR "not in selector" */
-	t16 math.xxx /* ERROR "not declared" */
+	t16 math.xxx /* ERROR "undefined" */
 	t17 math /* ERROR "not a type" */ .Pi
 	t18 float64 = math.Pi * 10.0
 	t19 int = t1 /* ERROR "cannot call" */ ()

src/internal/types/testdata/check/issues0.go

@@ -91,7 +91,7 @@ func issue10979() {
 		nosuchtype /* ERROR undefined: nosuchtype */
 	}
 	type _ interface {
-		fmt.Nosuchtype /* ERROR Nosuchtype not declared by package fmt */
+		fmt.Nosuchtype /* ERROR undefined: fmt\.Nosuchtype */
 	}
 	type _ interface {
 		nosuchpkg /* ERROR undefined: nosuchpkg */ .Nosuchtype