test: add a test for parameterized embedded field

Make sure that an embedded field like "MyStruct[T]" works and can be
referenced via the name MyStruct.

Change-Id: I8be1f1184dd42c4e54e4144aff2fd85e30af722f
Reviewed-on: https://go-review.googlesource.com/c/go/+/356312
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/typecheck/subr.go

@@ -1324,9 +1324,6 @@ func (ts *Tsubster) tstruct(t *types.Type, force bool) *types.Type {
 			}
 		}
 		if newfields != nil {
-			// TODO(danscales): make sure this works for the field
-			// names of embedded types (which should keep the name of
-			// the type param, not the instantiated type).
 			newfields[i] = types.NewField(f.Pos, f.Sym, t2)
 			newfields[i].Embedded = f.Embedded
 			newfields[i].Note = f.Note

test/typeparam/genembed2.go

@@ -0,0 +1,46 @@
+// 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.
+
+// Test for declaration and use of a parameterized embedded field.
+
+package main
+
+import (
+	"fmt"
+	"sync"
+)
+
+type MyStruct[T any] struct {
+	val T
+}
+
+type Lockable[T any] struct {
+	MyStruct[T]
+	mu sync.Mutex
+}
+
+// Get returns the value stored in a Lockable.
+func (l *Lockable[T]) Get() T {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	return l.MyStruct.val
+}
+
+// Set sets the value in a Lockable.
+func (l *Lockable[T]) Set(v T) {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	l.MyStruct = MyStruct[T]{v}
+}
+
+func main() {
+	var li Lockable[int]
+
+	li.Set(5)
+	if got, want := li.Get(), 5; got != want {
+		panic(fmt.Sprintf("got %d, want %d", got, want))
+	}
+}