go/parser,go/types: hide API changes related to type parameters

While the dev.typeparams branch was merged, the type parameter API is
slated for go1.18. Hide these changes to the go/parser and go/types API.

This was done as follows:
 + For APIs that will probably not be needed for go1.18, simply unexport
   them.
 + For APIs that we expect to export in go1.18, prefix symbols with '_',
   so that the planned (upper-cased) symbol name is apparent.
 + For APIs that must be exported for testing, move both API and tests
   to files guarded by the go1.18 build constraint.
 + parser.ParseTypeParams is unexported and copied wherever it is
   needed.
 + The -G flag is removed from gofmt, replaced by enabling type
   parameters if built with the go1.18 build constraint.

Notably, changes related to type parameters in go/ast are currently left
exported. We're looking at the AST API separately.

The new API diff from 1.16 is:
+pkg go/ast, method (*FuncDecl) IsMethod() bool
+pkg go/ast, method (*ListExpr) End() token.Pos
+pkg go/ast, method (*ListExpr) Pos() token.Pos
+pkg go/ast, type FuncType struct, TParams *FieldList
+pkg go/ast, type ListExpr struct
+pkg go/ast, type ListExpr struct, ElemList []Expr
+pkg go/ast, type TypeSpec struct, TParams *FieldList
+pkg go/types, type Config struct, GoVersion string

Change-Id: I1baf67e26279b49092e774309a836c460979774a
Reviewed-on: https://go-review.googlesource.com/c/go/+/295929
Trust: Robert Findley <rfindley@google.com>
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

3 files changed, 30 insertions, 10 deletions

diff --git a/src/cmd/gofmt/gofmt.go b/src/cmd/gofmt/gofmt.go
index b82aa7e7a9..95f537d91e 100644
--- a/src/cmd/gofmt/gofmt.go
+++ b/src/cmd/gofmt/gofmt.go
@@ -26,13 +26,16 @@ import (
 
 var (
 	// main operation modes
-	list            = flag.Bool("l", false, "list files whose formatting differs from gofmt's")
-	write           = flag.Bool("w", false, "write result to (source) file instead of stdout")
-	rewriteRule     = flag.String("r", "", "rewrite rule (e.g., 'a[b:len(a)] -> a[b:]')")
-	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")
+	list        = flag.Bool("l", false, "list files whose formatting differs from gofmt's")
+	write       = flag.Bool("w", false, "write result to (source) file instead of stdout")
+	rewriteRule = flag.String("r", "", "rewrite rule (e.g., 'a[b:len(a)] -> a[b:]')")
+	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 controls whether type parameters are allowed in the code
+	// being formatted. It is enabled for go1.18 in gofmt_go1.18.go.
+	allowTypeParams = false
 
 	// debugging
 	cpuprofile = flag.String("cpuprofile", "", "write cpu profile to this file")
@@ -48,6 +51,10 @@ const (
 	//
 	// This value is defined in go/printer specifically for go/format and cmd/gofmt.
 	printerNormalizeNumbers = 1 << 30
+
+	// parseTypeParams tells go/parser to parse type parameters. Must be kept in
+	// sync with go/parser/interface.go.
+	parseTypeParams parser.Mode = 1 << 30
 )
 
 var (
@@ -72,8 +79,8 @@ func initParserMode() {
 	if *allErrors {
 		parserMode |= parser.AllErrors
 	}
-	if *allowTypeParams {
-		parserMode |= parser.ParseTypeParams
+	if allowTypeParams {
+		parserMode |= parseTypeParams
 	}
 }
 
diff --git a/src/cmd/gofmt/gofmt_go1.18.go b/src/cmd/gofmt/gofmt_go1.18.go
new file mode 100644
index 0000000000..be7b46b5ed
--- /dev/null
+++ b/src/cmd/gofmt/gofmt_go1.18.go
@@ -0,0 +1,12 @@
+// 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.
+
+//go:build go1.18
+// +build go1.18
+
+package main
+
+func init() {
+	allowTypeParams = true
+}
diff --git a/src/cmd/gofmt/gofmt_test.go b/src/cmd/gofmt/gofmt_test.go
index 60e4f2e03d..9e2239a692 100644
--- a/src/cmd/gofmt/gofmt_test.go
+++ b/src/cmd/gofmt/gofmt_test.go
@@ -78,7 +78,8 @@ func runTest(t *testing.T, in, out string) {
 			// fake flag - pretend input is from stdin
 			stdin = true
 		case "-G":
-			*allowTypeParams = true
+			// fake flag - allow parsing type parameters
+			allowTypeParams = true
 		default:
 			t.Errorf("unrecognized flag name: %s", name)
 		}