go/types, types2: avoid spurious "undefined" errors" for invalid identifiers

The syntax parser complains about invalid identifiers.
Don't report a typechecker error when such an identifier
cannot be found in the current scope.

For now add a local test for types2 only because the
go/parser behaves differently than the syntax parser
which leads to slightly different error positions.

Fixes #68183.

Change-Id: Idbfe62fafcd704886069182744ec5e6b37ffc4e1
Reviewed-on: https://go-review.googlesource.com/c/go/+/602476
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Tim King <taking@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

Author: griesemer

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

@@ -706,14 +706,16 @@ func (check *Checker) selector(x *operand, e *syntax.SelectorExpr, def *TypeName
 					}
 				}
 				if exp == nil {
-					check.errorf(e.Sel, UndeclaredImportedName, "undefined: %s", syntax.Expr(e)) // cast to syntax.Expr to silence vet
+					if isValidName(sel) {
+						check.errorf(e.Sel, UndeclaredImportedName, "undefined: %s", syntax.Expr(e)) // cast to syntax.Expr to silence vet
+					}
 					goto Error
 				}
 				check.objDecl(exp, nil)
 			} else {
 				exp = pkg.scope.Lookup(sel)
 				if exp == nil {
-					if !pkg.fake {
+					if !pkg.fake && isValidName(sel) {
 						check.errorf(e.Sel, UndeclaredImportedName, "undefined: %s", syntax.Expr(e))
 					}
 					goto Error

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

@@ -6,6 +6,8 @@
 
 package types2
 
+import "unicode"
+
 // isValid reports whether t is a valid type.
 func isValid(t Type) bool { return Unalias(t) != Typ[Invalid] }
 
@@ -567,3 +569,13 @@ func clone[P *T, T any](p P) P {
 	c := *p
 	return &c
 }
+
+// isValidName reports whether s is a valid Go identifier.
+func isValidName(s string) bool {
+	for i, ch := range s {
+		if !(unicode.IsLetter(ch) || ch == '_' || i > 0 && unicode.IsDigit(ch)) {
+			return false
+		}
+	}
+	return true
+}

src/cmd/compile/internal/types2/testdata/local/issue68183.go

@@ -0,0 +1,29 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test that invalid identifiers reported by the parser
+// don't lead to additional errors during typechecking.
+
+package p
+
+import "fmt"
+
+var (
+	☹x /* ERROR "invalid character" */ int
+	_ = ☹x // ERROR "invalid character"
+	_ = fmt.☹x // ERROR "invalid character"
+	_ = ☹fmt /* ERROR "invalid character" */ .Println
+	_ = _世界 // ERROR "undefined: _世界"
+	_ = ☹_世界 // ERROR "invalid character"
+)
+
+func ☹m /* ERROR "invalid character" */ () {}
+
+type T struct{}
+func (T) ☹m /* ERROR "invalid character" */ () {}
+
+func _() {
+	var x T
+	x.☹m /* ERROR "invalid character" */ ()
+}

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

@@ -29,7 +29,7 @@ func (check *Checker) ident(x *operand, e *syntax.Name, def *TypeName, wantType
 	case nil:
 		if e.Value == "_" {
 			check.error(e, InvalidBlank, "cannot use _ as value or type")
-		} else {
+		} else if isValidName(e.Value) {
 			check.errorf(e, UndeclaredName, "undefined: %s", e.Value)
 		}
 		return

src/go/types/call.go

@@ -709,14 +709,16 @@ func (check *Checker) selector(x *operand, e *ast.SelectorExpr, def *TypeName, w
 					}
 				}
 				if exp == nil {
-					check.errorf(e.Sel, UndeclaredImportedName, "undefined: %s", ast.Expr(e)) // cast to ast.Expr to silence vet
+					if isValidName(sel) {
+						check.errorf(e.Sel, UndeclaredImportedName, "undefined: %s", ast.Expr(e)) // cast to ast.Expr to silence vet
+					}
 					goto Error
 				}
 				check.objDecl(exp, nil)
 			} else {
 				exp = pkg.scope.Lookup(sel)
 				if exp == nil {
-					if !pkg.fake {
+					if !pkg.fake && isValidName(sel) {
 						check.errorf(e.Sel, UndeclaredImportedName, "undefined: %s", ast.Expr(e))
 					}
 					goto Error

src/go/types/predicates.go

@@ -30,7 +30,7 @@ func (check *Checker) ident(x *operand, e *ast.Ident, def *TypeName, wantType bo
 	case nil:
 		if e.Name == "_" {
 			check.error(e, InvalidBlank, "cannot use _ as value or type")
-		} else {
+		} else if isValidName(e.Name) {
 			check.errorf(e, UndeclaredName, "undefined: %s", e.Name)
 		}
 		return