go/types, types2: better errors for == when type sets are empty

For #51525.

Change-Id: I3762bc4a48a1aaab3b006b1ad1400f866892243c
Reviewed-on: https://go-review.googlesource.com/c/go/+/413934
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -147,7 +147,17 @@ func comparable(T Type, dynamic bool, seen map[Type]bool, reportf func(string, .
 		}
 		return true
 	case *Interface:
-		return dynamic && !isTypeParam(T) || t.typeSet().IsComparable(seen)
+		if dynamic && !isTypeParam(T) || t.typeSet().IsComparable(seen) {
+			return true
+		}
+		if reportf != nil {
+			if t.typeSet().IsEmpty() {
+				reportf("empty type set")
+			} else {
+				reportf("incomparable types in type set")
+			}
+		}
+		// fallthrough
 	}
 	return false
 }

src/cmd/compile/internal/types2/testdata/fixedbugs/issue48712.go

@@ -23,18 +23,18 @@ func _[P comparable](x P, y any) {
 }
 
 func _[P any](x, y P) {
-	_ = x /* ERROR type parameter P is not comparable with == */ == x
+	_ = x /* ERROR incomparable types in type set */ == x
-	_ = x /* ERROR type parameter P is not comparable with == */ == y
+	_ = x /* ERROR incomparable types in type set */ == y
-	_ = y /* ERROR type parameter P is not comparable with == */ == x
+	_ = y /* ERROR incomparable types in type set */ == x
-	_ = y /* ERROR type parameter P is not comparable with == */ == y
+	_ = y /* ERROR incomparable types in type set */ == y
 
 	_ = x /* ERROR type parameter P is not comparable with < */ < y
 }
 
 func _[P any](x P, y any) {
-	_ = x /* ERROR type parameter P is not comparable with == */ == x
+	_ = x /* ERROR incomparable types in type set */ == x
-	_ = x /* ERROR type parameter P is not comparable with == */ == y
+	_ = x /* ERROR incomparable types in type set */ == y
-	_ = y == x // ERROR type parameter P is not comparable with ==
+	_ = y == x // ERROR incomparable types in type set
 	_ = y == y
 
 	_ = x /* ERROR type parameter P is not comparable with < */ < y

src/cmd/compile/internal/types2/testdata/fixedbugs/issue51525.go

@@ -0,0 +1,16 @@
+// Copyright 2022 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 _[T interface {
+	int
+	string
+}](x T) {
+	_ = x /* ERROR empty type set */ == x
+}
+
+func _[T interface{ int | []byte }](x T) {
+	_ = x /* ERROR incomparable types in type set */ == x
+}

src/cmd/compile/internal/types2/testdata/spec/comparisons.go

@@ -86,14 +86,14 @@ func _[
 	c C,
 ) {
 	_ = b == b
-	_ = a /* ERROR type parameter A is not comparable with == */ == a
+	_ = a /* ERROR incomparable types in type set */ == a
-	_ = l /* ERROR type parameter L is not comparable with == */ == l
+	_ = l /* ERROR incomparable types in type set */ == l
-	_ = s /* ERROR type parameter S is not comparable with == */ == s
+	_ = s /* ERROR incomparable types in type set */ == s
 	_ = p == p
-	_ = f /* ERROR type parameter F is not comparable with == */ == f
+	_ = f /* ERROR incomparable types in type set */ == f
-	_ = i /* ERROR type parameter I is not comparable with == */ == i
+	_ = i /* ERROR incomparable types in type set */ == i
 	_ = j == j
-	_ = m /* ERROR type parameter M is not comparable with == */ == m
+	_ = m /* ERROR incomparable types in type set */ == m
 	_ = c == c
 
 	_ = b /* ERROR mismatched types */ == nil

src/go/types/predicates.go

@@ -149,7 +149,17 @@ func comparable(T Type, dynamic bool, seen map[Type]bool, reportf func(string, .
 		}
 		return true
 	case *Interface:
-		return dynamic && !isTypeParam(T) || t.typeSet().IsComparable(seen)
+		if dynamic && !isTypeParam(T) || t.typeSet().IsComparable(seen) {
+			return true
+		}
+		if reportf != nil {
+			if t.typeSet().IsEmpty() {
+				reportf("empty type set")
+			} else {
+				reportf("incomparable types in type set")
+			}
+		}
+		// fallthrough
 	}
 	return false
 }

src/go/types/testdata/fixedbugs/issue48712.go

@@ -23,18 +23,18 @@ func _[P comparable](x P, y any) {
 }
 
 func _[P any](x, y P) {
-	_ = x /* ERROR type parameter P is not comparable with == */ == x
+	_ = x /* ERROR incomparable types in type set */ == x
-	_ = x /* ERROR type parameter P is not comparable with == */ == y
+	_ = x /* ERROR incomparable types in type set */ == y
-	_ = y /* ERROR type parameter P is not comparable with == */ == x
+	_ = y /* ERROR incomparable types in type set */ == x
-	_ = y /* ERROR type parameter P is not comparable with == */ == y
+	_ = y /* ERROR incomparable types in type set */ == y
 
 	_ = x /* ERROR type parameter P is not comparable with < */ < y
 }
 
 func _[P any](x P, y any) {
-	_ = x /* ERROR type parameter P is not comparable with == */ == x
+	_ = x /* ERROR incomparable types in type set */ == x
-	_ = x /* ERROR type parameter P is not comparable with == */ == y
+	_ = x /* ERROR incomparable types in type set */ == y
-	_ = y == x // ERROR type parameter P is not comparable with ==
+	_ = y == x // ERROR incomparable types in type set
 	_ = y == y
 
 	_ = x /* ERROR type parameter P is not comparable with < */ < y

src/go/types/testdata/fixedbugs/issue51525.go

@@ -0,0 +1,16 @@
+// Copyright 2022 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 _[T interface {
+	int
+	string
+}](x T) {
+	_ = x /* ERROR empty type set */ == x
+}
+
+func _[T interface{ int | []byte }](x T) {
+	_ = x /* ERROR incomparable types in type set */ == x
+}

src/go/types/testdata/spec/comparisons.go

@@ -86,14 +86,14 @@ func _[
 	c C,
 ) {
 	_ = b == b
-	_ = a /* ERROR type parameter A is not comparable with == */ == a
+	_ = a /* ERROR incomparable types in type set */ == a
-	_ = l /* ERROR type parameter L is not comparable with == */ == l
+	_ = l /* ERROR incomparable types in type set */ == l
-	_ = s /* ERROR type parameter S is not comparable with == */ == s
+	_ = s /* ERROR incomparable types in type set */ == s
 	_ = p == p
-	_ = f /* ERROR type parameter F is not comparable with == */ == f
+	_ = f /* ERROR incomparable types in type set */ == f
-	_ = i /* ERROR type parameter I is not comparable with == */ == i
+	_ = i /* ERROR incomparable types in type set */ == i
 	_ = j == j
-	_ = m /* ERROR type parameter M is not comparable with == */ == m
+	_ = m /* ERROR incomparable types in type set */ == m
 	_ = c == c
 
 	_ = b /* ERROR mismatched types */ == nil