go/types, types2: require iterator yield to return bool (work-around)

The original implementation of the type checkers accepted any boolean
result type for yield, but the compiler's front-end had a problem with
it (#71131).

As a temporary fix (for 1.24), adjust the type checkers to insist on the
spec's literal wording and avoid the compiler panic.

Fixes #71131.
For #71164.

Change-Id: Ie25f9a892e58b5e489d399b0bce2d0af55dc3c48
Reviewed-on: https://go-review.googlesource.com/c/go/+/640599
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Tim King <taking@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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

@@ -1057,8 +1057,13 @@ func rangeKeyVal(typ Type, allowVersion func(goVersion) bool) (key, val Type, ca
 			return bad("func must be func(yield func(...) bool): argument is not func")
 		case cb.Params().Len() > 2:
 			return bad("func must be func(yield func(...) bool): yield func has too many parameters")
-		case cb.Results().Len() != 1 || !isBoolean(cb.Results().At(0).Type()):
-			return bad("func must be func(yield func(...) bool): yield func does not return bool")
+		case cb.Results().Len() != 1 || !Identical(cb.Results().At(0).Type(), universeBool):
+			// see go.dev/issues/71131, go.dev/issues/71164
+			if cb.Results().Len() == 1 && isBoolean(cb.Results().At(0).Type()) {
+				return bad("func must be func(yield func(...) bool): yield func returns user-defined boolean, not bool")
+			} else {
+				return bad("func must be func(yield func(...) bool): yield func does not return bool")
+			}
 		}
 		assert(cb.Recv() == nil)
 		// determine key and value types, if any

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

@@ -21,6 +21,7 @@ var Unsafe *Package
 
 var (
 	universeIota       Object
+	universeBool       Type
 	universeByte       Type // uint8 alias, but has name "byte"
 	universeRune       Type // int32 alias, but has name "rune"
 	universeAnyNoAlias *TypeName
@@ -275,6 +276,7 @@ func init() {
 	defPredeclaredFuncs()
 
 	universeIota = Universe.Lookup("iota")
+	universeBool = Universe.Lookup("bool").Type()
 	universeByte = Universe.Lookup("byte").Type()
 	universeRune = Universe.Lookup("rune").Type()
 	universeError = Universe.Lookup("error").Type()

src/go/types/stmt.go

@@ -1075,8 +1075,13 @@ func rangeKeyVal(typ Type, allowVersion func(goVersion) bool) (key, val Type, ca
 			return bad("func must be func(yield func(...) bool): argument is not func")
 		case cb.Params().Len() > 2:
 			return bad("func must be func(yield func(...) bool): yield func has too many parameters")
-		case cb.Results().Len() != 1 || !isBoolean(cb.Results().At(0).Type()):
-			return bad("func must be func(yield func(...) bool): yield func does not return bool")
+		case cb.Results().Len() != 1 || !Identical(cb.Results().At(0).Type(), universeBool):
+			// see go.dev/issues/71131, go.dev/issues/71164
+			if cb.Results().Len() == 1 && isBoolean(cb.Results().At(0).Type()) {
+				return bad("func must be func(yield func(...) bool): yield func returns user-defined boolean, not bool")
+			} else {
+				return bad("func must be func(yield func(...) bool): yield func does not return bool")
+			}
 		}
 		assert(cb.Recv() == nil)
 		// determine key and value types, if any

src/go/types/universe.go

@@ -24,6 +24,7 @@ var Unsafe *Package
 
 var (
 	universeIota       Object
+	universeBool       Type
 	universeByte       Type // uint8 alias, but has name "byte"
 	universeRune       Type // int32 alias, but has name "rune"
 	universeAnyNoAlias *TypeName
@@ -278,6 +279,7 @@ func init() {
 	defPredeclaredFuncs()
 
 	universeIota = Universe.Lookup("iota")
+	universeBool = Universe.Lookup("bool").Type()
 	universeByte = Universe.Lookup("byte").Type()
 	universeRune = Universe.Lookup("rune").Type()
 	universeError = Universe.Lookup("error").Type()

src/internal/types/testdata/fixedbugs/issue71131.go

@@ -0,0 +1,15 @@
+// Copyright 2025 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.
+
+package p
+
+func _() {
+	type Bool bool
+	for range func /* ERROR "yield func returns user-defined boolean, not bool" */ (func() Bool) {} {
+	}
+	for range func /* ERROR "yield func returns user-defined boolean, not bool" */ (func(int) Bool) {} {
+	}
+	for range func /* ERROR "yield func returns user-defined boolean, not bool" */ (func(int, string) Bool) {} {
+	}
+}

src/internal/types/testdata/spec/range.go

@@ -5,7 +5,7 @@
 package p
 
 type MyInt int32
-type MyBool bool
+type MyBool = bool // TODO(gri) remove alias declaration - see go.dev/issues/71131, go.dev/issues/71164
 type MyString string
 type MyFunc1 func(func(int) bool)
 type MyFunc2 func(int) bool