test/typeparam: add test for indexing on typeparams with various constraints

Add test for indexing on variables whose types are constrained to
various kinds of types.

Change-Id: I991eecfe39dba5d817c0fbe259ba558d4881ea84
Reviewed-on: https://go-review.googlesource.com/c/go/+/360867
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

test/typeparam/index2.go

@@ -0,0 +1,67 @@
+// 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.
+
+// Testing various generic uses of indexing, both for reads and writes.
+
+package main
+
+import "fmt"
+
+// Can index an argument (read/write) constrained to be a slice or an array.
+func Index1[T interface{ []int64 | [5]int64 }](x T) int64 {
+	x[2] = 5
+	return x[3]
+}
+
+// Can index an argument (read) constrained to be a byte array or a string.
+func Index2[T interface{ []byte | string }](x T) byte {
+	return x[3]
+}
+
+// Can index an argument (write) constrained to be a byte array, but not a string.
+func Index2a[T interface{ []byte }](x T) byte {
+	x[2] = 'b'
+	return x[3]
+}
+
+// Can index an argument (read/write) constrained to be a map. Maps can't
+// be combined with any other type for indexing purposes.
+func Index3[T interface{ map[int]int64 }](x T) int64 {
+	x[2] = 43
+	return x[3]
+}
+
+// But the type of the map keys or values can be parameterized.
+func Index4[T any](x map[int]T) T {
+	var zero T
+	x[2] = zero
+	return x[3]
+}
+
+func test[T comparable](got, want T) {
+	if got != want {
+		panic(fmt.Sprintf("got %v, want %v", got, want))
+	}
+}
+
+func main() {
+	x := make([]int64, 4)
+	x[3] = 2
+	y := [5]int64{1, 2, 3, 4, 5}
+	z := "abcd"
+	w := make([]byte, 4)
+	w[3] = 5
+	v := make(map[int]int64)
+	v[3] = 18
+
+	test(Index1(x), int64(2))
+	test(Index1(y), int64(4))
+	test(Index2(z), byte(100))
+	test(Index2(w), byte(5))
+	test(Index2a(w), byte(5))
+	test(Index3(v), int64(18))
+	test(Index4(v), int64(18))
+}