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>

diff --git a/src/go/types/api.go b/src/go/types/api.go
index 436c23099b4c17959eb0ca80403e89d2dee15c4f..da23c8c52aa9739e9e09bd5b875ede929f33f2f4 100644 (file)
--- 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 e977d0db1f516c63d824de417db4451e6e30df49..ad6d3eef1027a839d0c0dea3e87172cde7ef8c02 100644 (file)
--- 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 b5b9d35b243a7b7efa6657c2814b600d1158b15e..5f38a346cee82600da867b1f968026f0bf196fad 100644 (file)
--- 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 (file)
index 0000000..fdf5c99
--- /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 */ )