go/types,cmd/compile/internal/types2: better diagnostic for type shadowing

This change causes the "x is not a type" diagnostic to describe
x's actual kind, helping to reveal when shadowing is at work.

(The kind description could improve other errors too.)

Fixes #76877

Change-Id: Ia3484998bb384ff570c20b6792cf8461c60aa38c
Reviewed-on: https://go-review.googlesource.com/c/go/+/731180
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>

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

@@ -671,3 +671,52 @@ func writeFuncName(buf *bytes.Buffer, f *Func, qf Qualifier) {
 	}
 	buf.WriteString(f.name)
 }
+
+// objectKind returns a description of the object's kind.
+func objectKind(obj Object) string {
+	switch obj := obj.(type) {
+	case *PkgName:
+		return "package name"
+	case *Const:
+		return "constant"
+	case *TypeName:
+		if obj.IsAlias() {
+			return "type alias"
+		} else if _, ok := obj.Type().(*TypeParam); ok {
+			return "type parameter"
+		} else {
+			return "defined type"
+		}
+	case *Var:
+		switch obj.Kind() {
+		case PackageVar:
+			return "package-level variable"
+		case LocalVar:
+			return "local variable"
+		case RecvVar:
+			return "receiver"
+		case ParamVar:
+			return "parameter"
+		case ResultVar:
+			return "result variable"
+		case FieldVar:
+			return "struct field"
+		}
+	case *Func:
+		if obj.Signature().Recv() != nil {
+			return "method"
+		} else {
+			return "function"
+		}
+	case *Label:
+		return "label"
+	case *Builtin:
+		return "built-in function"
+	case *Nil:
+		return "untyped nil"
+	}
+	if debug {
+		panic(fmt.Sprintf("unknown symbol (%T)", obj))
+	}
+	return "unknown symbol"
+}

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

@@ -50,7 +50,8 @@ func (check *Checker) ident(x *operand, e *syntax.Name, wantType bool) {
 	// (see go.dev/issue/65344).
 	_, gotType := obj.(*TypeName)
 	if !gotType && wantType {
-		check.errorf(e, NotAType, "%s is not a type", obj.Name())
+		check.errorf(e, NotAType, "%s (%s) is not a type", obj.Name(), objectKind(obj))
+
 		// avoid "declared but not used" errors
 		// (don't use Checker.use - we don't want to evaluate too much)
 		if v, _ := obj.(*Var); v != nil && v.pkg == check.pkg /* see Checker.use1 */ {

src/go/types/object.go

@@ -674,3 +674,52 @@ func writeFuncName(buf *bytes.Buffer, f *Func, qf Qualifier) {
 	}
 	buf.WriteString(f.name)
 }
+
+// objectKind returns a description of the object's kind.
+func objectKind(obj Object) string {
+	switch obj := obj.(type) {
+	case *PkgName:
+		return "package name"
+	case *Const:
+		return "constant"
+	case *TypeName:
+		if obj.IsAlias() {
+			return "type alias"
+		} else if _, ok := obj.Type().(*TypeParam); ok {
+			return "type parameter"
+		} else {
+			return "defined type"
+		}
+	case *Var:
+		switch obj.Kind() {
+		case PackageVar:
+			return "package-level variable"
+		case LocalVar:
+			return "local variable"
+		case RecvVar:
+			return "receiver"
+		case ParamVar:
+			return "parameter"
+		case ResultVar:
+			return "result variable"
+		case FieldVar:
+			return "struct field"
+		}
+	case *Func:
+		if obj.Signature().Recv() != nil {
+			return "method"
+		} else {
+			return "function"
+		}
+	case *Label:
+		return "label"
+	case *Builtin:
+		return "built-in function"
+	case *Nil:
+		return "untyped nil"
+	}
+	if debug {
+		panic(fmt.Sprintf("unknown symbol (%T)", obj))
+	}
+	return "unknown symbol"
+}

src/go/types/typexpr.go

@@ -49,7 +49,8 @@ func (check *Checker) ident(x *operand, e *ast.Ident, wantType bool) {
 	// (see go.dev/issue/65344).
 	_, gotType := obj.(*TypeName)
 	if !gotType && wantType {
-		check.errorf(e, NotAType, "%s is not a type", obj.Name())
+		check.errorf(e, NotAType, "%s (%s) is not a type", obj.Name(), objectKind(obj))
+
 		// avoid "declared but not used" errors
 		// (don't use Checker.use - we don't want to evaluate too much)
 		if v, _ := obj.(*Var); v != nil && v.pkg == check.pkg /* see Checker.use1 */ {

src/internal/types/testdata/check/cycles5.go

@@ -162,7 +162,7 @@ func makeArray() (res T12) { return }
 
 // issue #20770
 var r = newReader()
-func newReader() r // ERROR "r is not a type"
+func newReader() r // ERROR "r (package-level variable) is not a type"
 
 // variations of the theme of #8699 and #20770
 var arr /* ERROR "cycle" */ = f()

src/internal/types/testdata/check/cycles5a.go

@@ -162,7 +162,7 @@ func makeArray() (res T12) { return }
 
 // issue #20770
 var r = newReader()
-func newReader() r // ERROR "r is not a type"
+func newReader() r // ERROR "r (package-level variable) is not a type"
 
 // variations of the theme of #8699 and #20770
 var arr /* ERROR "cycle" */ = f()

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

@@ -63,7 +63,7 @@ var (
 	t12 complex64 = -(u + *t11) / *&v
 	t13 int = a /* ERROR "shifted operand" */ << d
 	t14 int = i << j
-	t15 math /* ERROR "math is not a type" */
+	t15 math /* ERROR "math (package name) is not a type" */
 	t16 math.xxx /* ERROR "undefined" */
 	t17 math /* ERROR "not a type" */ .Pi
 	t18 float64 = math.Pi * 10.0
@@ -144,3 +144,10 @@ func init /* ERROR "no arguments and no return values" */ (int) {}
 func init /* ERROR "no arguments and no return values" */ () int { return 0 }
 func init /* ERROR "no arguments and no return values" */ (int) int { return 0 }
 func (T) init(int) int { return 0 }
+
+func _() {
+	var error error
+	var _ error /* ERROR "error (local variable) is not a type" */
+	_ = error
+}
+

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

@@ -104,7 +104,7 @@ func issue10979() {
 
 // issue11347
 // These should not crash.
-var a1, b1, c1 /* ERROR "cycle" */ b1 /* ERROR "b1 is not a type" */ = 0 > 0<<""[""[c1]]>c1
+var a1, b1, c1 /* ERROR "cycle" */ b1 /* ERROR "b1 (package-level variable) is not a type" */ = 0 > 0<<""[""[c1]]>c1
 var a2, b2 /* ERROR "cycle" */ = 0 /* ERROR "assignment mismatch" */ /* ERROR "assignment mismatch" */ > 0<<""[b2]
 var a3, b3 /* ERROR "cycle" */ = int /* ERROR "assignment mismatch" */ /* ERROR "assignment mismatch" */ (1<<""[b3])
 

src/internal/types/testdata/check/issues1.go

@@ -246,5 +246,5 @@ var _ = append[context.CancelFunc, []context.CancelFunc, context.CancelFunc](can
 func g[T any](T) T { panic(0) }
 
 var _ = g[int]
-var _ = g[nil /* ERROR "is not a type" */ ]
+var _ = g[nil /* ERROR "nil (untyped nil) is not a type" */ ]
 var _ = g(0)

src/internal/types/testdata/fixedbugs/issue39634.go

@@ -43,7 +43,7 @@ type foo9[A any] interface { foo9 /* ERROR "invalid recursive type" */ [A] }
 func _() { var _ = new(foo9[int]) }
 
 // crash 12
-var u, i [func /* ERROR "used as value" */ /* ERROR "used as value" */ (u /* ERROR "u is not a type" */ /* ERROR "u is not a type" */ , c /* ERROR "undefined" */ /* ERROR "undefined" */ ) {}(0, len /* ERROR "must be called" */ /* ERROR "must be called" */ )]c /* ERROR "undefined" */ /* ERROR "undefined" */
+var u, i [func /* ERROR "used as value" */ /* ERROR "used as value" */ (u /* ERROR "u (package-level variable) is not a type" */ /* ERROR "u (package-level variable) is not a type" */ , c /* ERROR "undefined" */ /* ERROR "undefined" */ ) {}(0, len /* ERROR "must be called" */ /* ERROR "must be called" */ )]c /* ERROR "undefined" */ /* ERROR "undefined" */
 
 // crash 15
 func y15() { var a /* ERROR "declared and not used" */ interface{ p() } = G15[string]{} }

src/internal/types/testdata/fixedbugs/issue65344.go

@@ -4,12 +4,12 @@
 
 package p
 
-type T1 C /* ERROR "C is not a type" */
+type T1 C /* ERROR "C (constant) is not a type" */
 
 // TODO(gri) try to avoid this follow-on error
 const C = T1(0 /* ERROR "cannot convert 0 (untyped int constant) to type T1" */)
 
-type T2 V /* ERROR "V is not a type" */
+type T2 V /* ERROR "V (package-level variable) is not a type" */
 
 var V T2
 

test/fixedbugs/issue4610.go

@@ -14,4 +14,3 @@ func main() {
 	var foo bar
 	_ = &foo{} // ERROR "is not a type|expected .;."
 } // GCCGO_ERROR "expected declaration"
-