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>

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

@@ -1421,7 +1421,9 @@ func Shapify(t *types.Type, index int) *types.Type {
 
 	// All pointers have the same shape.
 	// TODO: Make unsafe.Pointer the same shape as normal pointers.
-	if u.Kind() == types.TPTR {
+	// Note: pointers to arrays are special because of slice-to-array-pointer
+	// conversions. See issue 49295.
+	if u.Kind() == types.TPTR && u.Elem().Kind() != types.TARRAY {
 		u = types.Types[types.TUINT8].PtrTo()
 	}
 

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
+}