cmd/compile: remove unneeded "==" method in pre-defined "comparable" interface

Fixes #49421

Change-Id: Iecf3952346ecd278198c1000014a321e230f7fa7
Reviewed-on: https://go-review.googlesource.com/c/go/+/361962
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

src/cmd/compile/internal/reflectdata/reflect.go

@@ -1304,12 +1304,6 @@ func writeITab(lsym *obj.LSym, typ, iface *types.Type) {
 				break
 			}
 		}
-		if sigs[0].Sym.Name == "==" {
-			sigs = sigs[1:]
-			if len(sigs) == 0 {
-				break
-			}
-		}
 	}
 	if len(sigs) != 0 {
 		base.Fatalf("incomplete itab")

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

@@ -148,8 +148,8 @@ func makeErrorInterface() *Type {
 	return NewInterface(NoPkg, []*Field{method}, false)
 }
 
+// makeComparableInterface makes the the predefined "comparable" interface in the
+// built-in package. It has a unique name, but no methods.
 func makeComparableInterface() *Type {
-	sig := NewSignature(NoPkg, FakeRecv(), nil, nil, nil)
-	method := NewField(src.NoXPos, LocalPkg.Lookup("=="), sig)
-	return NewInterface(NoPkg, []*Field{method}, false)
+	return NewInterface(NoPkg, nil, false)
 }

test/typeparam/issue49421.go

@@ -0,0 +1,27 @@
+// run -gcflags=-G=3
+
+// Copyright 2021 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 main
+
+func main() {
+	var a, b foo
+	bar(a, b)
+}
+
+type foo int
+
+func (a foo) less(b foo) bool {
+	return a < b
+}
+
+type lesser[T any] interface {
+	less(T) bool
+	comparable
+}
+
+func bar[T lesser[T]](a, b T) {
+	a.less(b)
+}