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>

2 files changed, 51 insertions, 1 deletions

diff --git a/src/cmd/compile/internal/types2/object.go b/src/cmd/compile/internal/types2/object.go
index dd2d415790..5d284ee61b 100644
--- a/src/cmd/compile/internal/types2/object.go
+++ b/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"
+}
diff --git a/src/cmd/compile/internal/types2/typexpr.go b/src/cmd/compile/internal/types2/typexpr.go
index a79b54eacc..7d5932eec1 100644
--- a/src/cmd/compile/internal/types2/typexpr.go
+++ b/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 */ {