cmd/compile/internal/types2: slice-to-array-pointer conversion requires go1.17

Add missing version check. Even though this is a new types2 error
we separate between the compiler and the types2 error message: we
have the compiler error message to match the compiler style, and
we have a types2-specific error message to match the types2 style
for these kinds of errors (for now).

Eventually we need to decide which style we like better and clean
this up.

Follow-up on https://golang.org/cl/301650.

Updates #395.

Change-Id: I5b779f345994c66b1f4a4db466466f98b7d3c491
Reviewed-on: https://go-review.googlesource.com/c/go/+/315169
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

Author: griesemer

src/cmd/compile/internal/types2/conversions.go

@@ -38,8 +38,10 @@ func (check *Checker) conversion(x *operand, T Type) {
 	}
 
 	if !ok {
-		check.errorf(x, "cannot convert %s to %s", x, T)
-		x.mode = invalid
+		if x.mode != invalid {
+			check.errorf(x, "cannot convert %s to %s", x, T)
+			x.mode = invalid
+		}
 		return
 	}
 
@@ -141,7 +143,16 @@ func (x *operand) convertibleTo(check *Checker, T Type) bool {
 		if p := asPointer(T); p != nil {
 			if a := asArray(p.Elem()); a != nil {
 				if check.identical(s.Elem(), a.Elem()) {
-					return true
+					if check == nil || check.allowVersion(check.pkg, 1, 17) {
+						return true
+					}
+					// check != nil
+					if check.conf.CompilerErrorMessages {
+						check.error(x, "conversion of slices to array pointers only supported as of -lang=go1.17")
+					} else {
+						check.error(x, "conversion of slices to array pointers requires go1.17 or later")
+					}
+					x.mode = invalid // avoid follow-up error
 				}
 			}
 		}

src/cmd/compile/internal/types2/decl.go

@@ -601,7 +601,7 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *syntax.TypeDecl, def *Named
 
 	if alias {
 		// type alias declaration
-		if !check.allowVersion(obj.pkg, 1, 9) {
+		if !check.allowVersion(check.pkg, 1, 9) {
 			if check.conf.CompilerErrorMessages {
 				check.error(tdecl, "type aliases only supported as of -lang=go1.9")
 			} else {

src/cmd/compile/internal/types2/testdata/check/go1_16.src

@@ -0,0 +1,13 @@
+// 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.
+
+// Check Go language version-specific errors.
+
+package go1_16 // go1.16
+
+type Slice []byte
+type Array [8]byte
+
+var s Slice
+var p = (*Array)(s /* ERROR requires go1.17 or later */ )