go/parser: add resolution tests for type params

For #45104
For #45221

Change-Id: I8966555f4e8844d5b6766d00d48f7a81868ccf40
Reviewed-on: https://go-review.googlesource.com/c/go/+/304453
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>

src/go/parser/resolver_test.go

@@ -39,7 +39,11 @@ func TestResolution(t *testing.T) {
 			fset := token.NewFileSet()
 			path := filepath.Join(dir, fi.Name())
 			src := readFile(path) // panics on failure
-			file, err := ParseFile(fset, path, src, 0)
+			var mode Mode
+			if strings.HasSuffix(path, ".go2") {
+				mode = parseTypeParams
+			}
+			file, err := ParseFile(fset, path, src, mode)
 			if err != nil {
 				t.Fatal(err)
 			}

src/go/parser/testdata/resolution/resolution.src

@@ -19,7 +19,9 @@ const (
 	labelOk // =@labelOk
 )
 
-func _ /* =@blankFunc */ () {
+type T /* =@T */ int
+
+func _ /* =@blankFunc */ (count /* =@count */ T /* @T */) {
 	x /* =@x1 */ := c /* @cdecl */{}
 	switch x /* =@x2 */ := x /* @x1 */; x /* =@x3 */ := x /* @x2 */.(type) {
 	case c /* @cdecl */:

src/go/parser/testdata/resolution/typeparams.go2

@@ -0,0 +1,24 @@
+// 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 resolution
+
+type List /* =@List */ [E /* =@E */ any] []E // @E
+
+type Pair /* =@Pair */ [L /* =@L */, R /* =@R */ any] struct {
+	Left /* =@Left */ L // @L
+	Right /* =@Right */ R // @R
+}
+
+var _ /* =@blank */ = Pair /* @Pair */ [int, string]{}
+
+type Addable /* =@Addable */ interface {
+	type int64, float64
+}
+
+// TODO (#45221): resolve references to T in the signature below.
+func Add /* =@AddDecl */[T /* =@T */ Addable /* @Addable */](l /* =@l */, r /* =@r */ T) T {
+	var t /* =@t */ T /* @T */
+	return l /* @l */ + r /* @r */ + t /* @t */
+}