go/types: slice-to-array-pointer conversion requires go1.17

This is a port of CL 315169 to go/types. It uses a slightly different
mechanism for evaluating the convertibility error message, to be
consistent with operand.assignableTo.

Change-Id: Iea2e2a9fbb4cf17d472b2b7392786118e079528a
Reviewed-on: https://go-review.googlesource.com/c/go/+/315809
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

4 files changed, 29 insertions, 6 deletions

diff --git a/src/go/types/api.go b/src/go/types/api.go
index 436c23099b..da23c8c52a 100644
--- a/src/go/types/api.go
+++ b/src/go/types/api.go
@@ -406,7 +406,7 @@ func AssignableTo(V, T Type) bool {
 // ConvertibleTo reports whether a value of type V is convertible to a value of type T.
 func ConvertibleTo(V, T Type) bool {
 	x := operand{mode: value, typ: V}
-	return x.convertibleTo(nil, T) // check not needed for non-constant x
+	return x.convertibleTo(nil, T, nil) // check not needed for non-constant x
 }
 
 // Implements reports whether type V implements interface T.
diff --git a/src/go/types/conversions.go b/src/go/types/conversions.go
index e977d0db1f..ad6d3eef10 100644
--- a/src/go/types/conversions.go
+++ b/src/go/types/conversions.go
@@ -17,6 +17,7 @@ func (check *Checker) conversion(x *operand, T Type) {
 	constArg := x.mode == constant_
 
 	var ok bool
+	var reason string
 	switch {
 	case constArg && isConstType(T):
 		// constant conversion
@@ -31,14 +32,18 @@ func (check *Checker) conversion(x *operand, T Type) {
 			x.val = constant.MakeString(string(codepoint))
 			ok = true
 		}
-	case x.convertibleTo(check, T):
+	case x.convertibleTo(check, T, &reason):
 		// non-constant conversion
 		x.mode = value
 		ok = true
 	}
 
 	if !ok {
-		check.errorf(x, _InvalidConversion, "cannot convert %s to %s", x, T)
+		if reason != "" {
+			check.errorf(x, _InvalidConversion, "cannot convert %s to %s (%s)", x, T, reason)
+		} else {
+			check.errorf(x, _InvalidConversion, "cannot convert %s to %s", x, T)
+		}
 		x.mode = invalid
 		return
 	}
@@ -79,7 +84,7 @@ func (check *Checker) conversion(x *operand, T Type) {
 // convertibleTo reports whether T(x) is valid.
 // The check parameter may be nil if convertibleTo is invoked through an
 // exported API call, i.e., when all methods have been type-checked.
-func (x *operand) convertibleTo(check *Checker, T Type) bool {
+func (x *operand) convertibleTo(check *Checker, T Type, reason *string) bool {
 	// "x is assignable to T"
 	if ok, _ := x.assignableTo(check, T, nil); ok {
 		return true
@@ -139,7 +144,12 @@ 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
+					}
+					if reason != nil {
+						*reason = "conversion of slices to array pointers requires go1.17 or later"
+					}
 				}
 			}
 		}
diff --git a/src/go/types/decl.go b/src/go/types/decl.go
index b5b9d35b24..5f38a346ce 100644
--- a/src/go/types/decl.go
+++ b/src/go/types/decl.go
@@ -655,7 +655,7 @@ func (check *Checker) typeDecl(obj *TypeName, tdecl *ast.TypeSpec, def *Named) {
 
 	if alias {
 		// type alias declaration
-		if !check.allowVersion(obj.pkg, 1, 9) {
+		if !check.allowVersion(check.pkg, 1, 9) {
 			check.errorf(atPos(tdecl.Assign), _BadDecl, "type aliases requires go1.9 or later")
 		}
 
diff --git a/src/go/types/testdata/check/go1_16.src b/src/go/types/testdata/check/go1_16.src
new file mode 100644
index 0000000000..fdf5c99d7e
--- /dev/null
+++ b/src/go/types/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 */ )