cmd/compile: accept constraint literals with elided interfaces

This change enables the relaxed syntax for constraint literals
as proposed in issue #48424 and adds a simple smoke test for
the compiler. (Most of the relevant changes are in the syntax
and types2 package which have more extensive tests for this.)

This makes it possible to experiment with the new syntax while
we contemplate the fate of #48424.

If #48424 is accepted, this change can remain. If #48424 is
not accepted, reverting this CL will remove this feature in
the compiler.

For #48424.

Change-Id: I624fbb37c2f616ee9ad692e17e4fc75c9d5b06e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/353389
Trust: Robert Griesemer <gri@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Reviewed-by: Dan Scales <danscales@google.com>

src/cmd/compile/internal/noder/noder.go

@@ -36,7 +36,7 @@ func LoadPackage(filenames []string) {
 
 	mode := syntax.CheckBranches
 	if supportsGenerics {
-		mode |= syntax.AllowGenerics
+		mode |= syntax.AllowGenerics | syntax.AllowTypeSets
 	}
 
 	// Limit the number of simultaneously open files.

src/go/internal/gcimporter/gcimporter_test.go

@@ -165,8 +165,9 @@ func TestImportTypeparamTests(t *testing.T) {
 	}
 
 	skip := map[string]string{
-		"equal.go":  "inconsistent embedded sorting", // TODO(rfindley): investigate this.
-		"nested.go": "fails to compile",              // TODO(rfindley): investigate this.
+		"equal.go":      "inconsistent embedded sorting", // TODO(rfindley): investigate this.
+		"nested.go":     "fails to compile",              // TODO(rfindley): investigate this.
+		"issue48424.go": "go/types support missing",      // TODO: need to implement this if #48424 is accepted
 	}
 
 	for _, entry := range list {

test/typeparam/issue48424.go

@@ -0,0 +1,54 @@
+// run
+
+// 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.
+
+// Smoke test for constraint literals with elided interface
+// per issue #48424.
+
+package main
+
+func identity[T int](x T) T {
+	return x
+}
+
+func min[T int|string](x, y T) T {
+	if x < y {
+		return x
+	}
+	return y
+}
+
+func max[T ~float64](x, y T) T {
+	if x > y {
+		return x
+	}
+	return y
+}
+
+func main() {
+	if identity(1) != 1 {
+		panic("identity(1) failed")
+	}
+
+	if min(2, 3) != 2 {
+		panic("min(2, 3) failed")
+	}
+
+	if min("foo", "bar") != "bar" {
+		panic(`min("foo", "bar") failed`)
+	}
+
+	if max(2, 3) != 3 {
+		panic("max(2, 3) failed")
+	}
+}
+
+// Some random type parameter lists with elided interfaces.
+
+type (
+	_ [T struct{}] struct{}
+	_ [M map[K]V, K comparable, V any] struct{}
+	_ [_ interface{}|int] struct{}
+)