[dev.typeparams] cmd/gofmt: add the -G flag to allow generic code

Add support for type parameters to cmd/gofmt, gated behind the -G flag.

The test was based on a test from go/printer, slightly modified to
exercise more formatting.

Change-Id: I489bcb3ad06e1ed4e6d9f5bc79825e60dcfe9953
Reviewed-on: https://go-review.googlesource.com/c/go/+/291011
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/cmd/gofmt/doc.go

@@ -26,6 +26,9 @@ The flags are:
 		Do not print reformatted sources to standard output.
 		If a file's formatting is different from gofmt's, print its name
 		to standard output.
+	-G
+		Allow generic code, using type parameters.
+		See golang.org/issues/43651 for more information.
 	-r rule
 		Apply the rewrite rule to the source before reformatting.
 	-s

src/cmd/gofmt/gofmt.go

@@ -32,6 +32,7 @@ var (
 	simplifyAST     = flag.Bool("s", false, "simplify code")
 	doDiff          = flag.Bool("d", false, "display diffs instead of rewriting files")
 	allErrors       = flag.Bool("e", false, "report all errors (not just the first 10 on different lines)")
+	allowTypeParams = flag.Bool("G", false, "allow generic code")
 
 	// debugging
 	cpuprofile = flag.String("cpuprofile", "", "write cpu profile to this file")
@@ -71,6 +72,9 @@ func initParserMode() {
 	if *allErrors {
 		parserMode |= parser.AllErrors
 	}
+	if *allowTypeParams {
+		parserMode |= parser.ParseTypeParams
+	}
 }
 
 func isGoFile(f fs.DirEntry) bool {

src/cmd/gofmt/gofmt_test.go

@@ -77,6 +77,8 @@ func runTest(t *testing.T, in, out string) {
 		case "-stdin":
 			// fake flag - pretend input is from stdin
 			stdin = true
+		case "-G":
+			*allowTypeParams = true
 		default:
 			t.Errorf("unrecognized flag name: %s", name)
 		}

src/cmd/gofmt/testdata/typeparams.golden

@@ -0,0 +1,35 @@
+// Copyright 2020 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.
+
+//gofmt -G
+
+package typeparams
+
+type T[P any] struct{}
+type T[P1, P2, P3 any] struct{}
+
+type T[P C] struct{}
+type T[P1, P2, P3 C] struct{}
+
+type T[P C[P]] struct{}
+type T[P1, P2, P3 C[P1, P2, P3]] struct{}
+
+func f[P any](x P)
+func f[P1, P2, P3 any](x1 P1, x2 P2, x3 P3) struct{}
+
+func f[P interface{}](x P)
+func f[P1, P2, P3 interface {
+	m1(P1)
+	type P2, P3
+}](x1 P1, x2 P2, x3 P3) struct{}
+func f[P any](T1[P], T2[P]) T3[P]
+
+func (x T[P]) m()
+func (T[P]) m(x T[P]) P
+
+func _() {
+	type _ []T[P]
+	var _ []T[P]
+	_ = []T[P]{}
+}

src/cmd/gofmt/testdata/typeparams.input

@@ -0,0 +1,32 @@
+// Copyright 2020 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.
+
+//gofmt -G
+
+package typeparams
+
+type T[  P any] struct{}
+type T[P1, P2, P3 any] struct{}
+
+type T[P C] struct{}
+type T[P1,P2, P3 C] struct{}
+
+type T[P C[P]] struct{}
+type T[P1, P2, P3 C[P1,P2,P3]] struct{}
+
+func f[P any](x P)
+func f[P1, P2, P3 any](x1 P1, x2 P2, x3 P3) struct{}
+
+func f[P interface{}](x P)
+func f[P1, P2, P3 interface{ m1(P1); type P2, P3 }](x1 P1, x2 P2, x3 P3) struct{}
+func f[P any](T1[P], T2[P]) T3[P]
+
+func (x T[P]) m()
+func ((T[P])) m(x T[P]) P
+
+func _() {
+	type _ []T[P]
+	var _ []T[P]
+	_ = []T[P]{}
+}