cmd/compile: make pointers to arrays their own shape

Pointers to arrays can be used to cast from a slice. We need
the shape of such type params to be different so we can compile
those casts correctly.

This is kind of a big hammer to fix #49295. It would be nice to
only do this when we know there's a []T->*[N]T conversion.

Fixes #49295

Change-Id: Ibda33057fab2dd28162537aab0f1244211d68e3f
Reviewed-on: https://go-review.googlesource.com/c/go/+/361135
Trust: Keith Randall <khr@golang.org>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>

1 files changed, 30 insertions, 0 deletions

diff --git a/test/typeparam/issue49295.go b/test/typeparam/issue49295.go
new file mode 100644
index 0000000000..435b44d10c
--- /dev/null
+++ b/test/typeparam/issue49295.go
@@ -0,0 +1,30 @@
+// 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.
+
+package main
+
+import "io"
+
+type Reader struct {
+	buf []byte
+}
+type Token *[16]byte
+
+func Read[T interface{ ~*[16]byte }](r *Reader) (t T, err error) {
+	if n := len(t); len(r.buf) >= n {
+		t = T(r.buf[:n])
+		r.buf = r.buf[n:]
+		return
+	}
+	err = io.EOF
+	return
+}
+
+func main() {
+	r := &Reader{buf: []byte("0123456789abcdef")}
+	token, err := Read[Token](r)
+	_, _ = token, err
+}