go/types, types2: better error message for invalid == on type parameters

Fixes #48712.

Change-Id: I6f214cdfdd1815493f2a04828e8f0097f1d8c124
Reviewed-on: https://go-review.googlesource.com/c/go/+/372734
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

4 files changed, 110 insertions, 8 deletions

diff --git a/src/cmd/compile/internal/types2/expr.go b/src/cmd/compile/internal/types2/expr.go
index 3e3104abb6..0147e2adfd 100644
--- a/src/cmd/compile/internal/types2/expr.go
+++ b/src/cmd/compile/internal/types2/expr.go
@@ -770,10 +770,12 @@ func (check *Checker) comparison(x, y *operand, op syntax.Operator) {
 	xok, _ := x.assignableTo(check, y.typ, nil)
 	yok, _ := y.assignableTo(check, x.typ, nil)
 	if xok || yok {
+		equality := false
 		defined := false
 		switch op {
 		case syntax.Eql, syntax.Neq:
 			// spec: "The equality operators == and != apply to operands that are comparable."
+			equality = true
 			defined = Comparable(x.typ) && Comparable(y.typ) || x.isNil() && hasNil(y.typ) || y.isNil() && hasNil(x.typ)
 		case syntax.Lss, syntax.Leq, syntax.Gtr, syntax.Geq:
 			// spec: The ordering operators <, <=, >, and >= apply to operands that are ordered."
@@ -782,11 +784,19 @@ func (check *Checker) comparison(x, y *operand, op syntax.Operator) {
 			unreachable()
 		}
 		if !defined {
-			typ := x.typ
-			if x.isNil() {
-				typ = y.typ
+			if equality && (isTypeParam(x.typ) || isTypeParam(y.typ)) {
+				typ := x.typ
+				if isTypeParam(y.typ) {
+					typ = y.typ
+				}
+				err = check.sprintf("%s is not comparable", typ)
+			} else {
+				typ := x.typ
+				if x.isNil() {
+					typ = y.typ
+				}
+				err = check.sprintf("operator %s not defined on %s", op, typ)
 			}
-			err = check.sprintf("operator %s not defined on %s", op, typ)
 		}
 	} else {
 		err = check.sprintf("mismatched types %s and %s", x.typ, y.typ)
diff --git a/src/cmd/compile/internal/types2/testdata/fixedbugs/issue48712.go2 b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue48712.go2
new file mode 100644
index 0000000000..bad8712fda
--- /dev/null
+++ b/src/cmd/compile/internal/types2/testdata/fixedbugs/issue48712.go2
@@ -0,0 +1,41 @@
+// 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 _[P comparable](x, y P) {
+	_ = x == x
+	_ = x == y
+	_ = y == x
+	_ = y == y
+
+	_ = x /* ERROR operator < not defined on P */ < y
+}
+
+func _[P comparable](x P, y any) {
+	_ = x == x
+	_ = x == y
+	_ = y == x
+	_ = y == y
+
+	_ = x /* ERROR operator < not defined on P */ < y
+}
+
+func _[P any](x, y P) {
+	_ = x /* ERROR P is not comparable */ == x
+	_ = x /* ERROR P is not comparable */ == y
+	_ = y /* ERROR P is not comparable */ == x
+	_ = y /* ERROR P is not comparable */ == y
+
+	_ = x /* ERROR operator < not defined on P */ < y
+}
+
+func _[P any](x P, y any) {
+	_ = x /* ERROR P is not comparable */ == x
+	_ = x /* ERROR P is not comparable */ == y
+	_ = y /* ERROR P is not comparable */ == x
+	_ = y == y
+
+	_ = x /* ERROR operator < not defined on P */ < y
+}
diff --git a/src/go/types/expr.go b/src/go/types/expr.go
index 8ddfb8de7e..73b01f4aa4 100644
--- a/src/go/types/expr.go
+++ b/src/go/types/expr.go
@@ -729,10 +729,12 @@ func (check *Checker) comparison(x, y *operand, op token.Token) {
 	xok, _ := x.assignableTo(check, y.typ, nil)
 	yok, _ := y.assignableTo(check, x.typ, nil)
 	if xok || yok {
+		equality := false
 		defined := false
 		switch op {
 		case token.EQL, token.NEQ:
 			// spec: "The equality operators == and != apply to operands that are comparable."
+			equality = true
 			defined = Comparable(x.typ) && Comparable(y.typ) || x.isNil() && hasNil(y.typ) || y.isNil() && hasNil(x.typ)
 		case token.LSS, token.LEQ, token.GTR, token.GEQ:
 			// spec: The ordering operators <, <=, >, and >= apply to operands that are ordered."
@@ -741,11 +743,19 @@ func (check *Checker) comparison(x, y *operand, op token.Token) {
 			unreachable()
 		}
 		if !defined {
-			typ := x.typ
-			if x.isNil() {
-				typ = y.typ
+			if equality && (isTypeParam(x.typ) || isTypeParam(y.typ)) {
+				typ := x.typ
+				if isTypeParam(y.typ) {
+					typ = y.typ
+				}
+				err = check.sprintf("%s is not comparable", typ)
+			} else {
+				typ := x.typ
+				if x.isNil() {
+					typ = y.typ
+				}
+				err = check.sprintf("operator %s not defined on %s", op, typ)
 			}
-			err = check.sprintf("operator %s not defined on %s", op, typ)
 			code = _UndefinedOp
 		}
 	} else {
diff --git a/src/go/types/testdata/fixedbugs/issue48712.go2 b/src/go/types/testdata/fixedbugs/issue48712.go2
new file mode 100644
index 0000000000..bad8712fda
--- /dev/null
+++ b/src/go/types/testdata/fixedbugs/issue48712.go2
@@ -0,0 +1,41 @@
+// 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 _[P comparable](x, y P) {
+	_ = x == x
+	_ = x == y
+	_ = y == x
+	_ = y == y
+
+	_ = x /* ERROR operator < not defined on P */ < y
+}
+
+func _[P comparable](x P, y any) {
+	_ = x == x
+	_ = x == y
+	_ = y == x
+	_ = y == y
+
+	_ = x /* ERROR operator < not defined on P */ < y
+}
+
+func _[P any](x, y P) {
+	_ = x /* ERROR P is not comparable */ == x
+	_ = x /* ERROR P is not comparable */ == y
+	_ = y /* ERROR P is not comparable */ == x
+	_ = y /* ERROR P is not comparable */ == y
+
+	_ = x /* ERROR operator < not defined on P */ < y
+}
+
+func _[P any](x P, y any) {
+	_ = x /* ERROR P is not comparable */ == x
+	_ = x /* ERROR P is not comparable */ == y
+	_ = y /* ERROR P is not comparable */ == x
+	_ = y == y
+
+	_ = x /* ERROR operator < not defined on P */ < y
+}