c++: Allow template rvalue-ref conv to bind to lvalue ref.

When I implemented the [over.match.ref] rule that a reference conversion
function needs to match l/rvalue of the target reference type it changed our
handling of this testcase.  It seems to me that our current behavior is what
the standard says, but it doesn't seem desirable, and all the other
compilers have our old behavior.  So let's limit the change to non-templates
until there's some clarification from the committee.

	PR c++/90546
	* call.c (build_user_type_conversion_1): Allow a template conversion
	returning an rvalue reference to bind directly to an lvalue.

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 259b0c717b9f4c8773a973a48706244c744759d9..a110efe90c02e9a91740328d1a262287fddd854e 100644 (file)
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,5 +1,9 @@
 2020-01-28  Jason Merrill  <jason@redhat.com>
 
+       PR c++/90546
+       * call.c (build_user_type_conversion_1): Allow a template conversion
+       returning an rvalue reference to bind directly to an lvalue.
+
        PR c++/90731
        * decl.c (grokdeclarator): Propagate eh spec from typedef.
 

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index 009cb85ad60db76fa1c2b50682e48e4d7414b5f9..fde29f8f6318feb7f6e92b1d239d4188baa963e6 100644 (file)
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -4115,6 +4115,8 @@ build_user_type_conversion_1 (tree totype, tree expr, int flags,
                                                       EXPR_LOCATION (expr));
            }
          else if (TYPE_REF_P (totype) && !ics->rvaluedness_matches_p
+                  /* Limit this to non-templates for now (PR90546).  */
+                  && !cand->template_decl
                   && TREE_CODE (TREE_TYPE (totype)) != FUNCTION_TYPE)
            {
              /* If we are called to convert to a reference type, we are trying

diff --git a/gcc/testsuite/g++.dg/cpp0x/rv-conv3.C b/gcc/testsuite/g++.dg/cpp0x/rv-conv3.C
new file mode 100644 (file)
index 0000000..5f727fc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/rv-conv3.C
@@ -0,0 +1,15 @@
+// PR c++/90546
+// { dg-do link { target c++11 } }
+
+struct Foo { };
+void test(const Foo&) {}
+Foo f;
+struct Bar {
+  template <class T> operator T&&();
+};
+template<> Bar::operator const Foo&&() {
+    return static_cast<Foo&&>(f);
+}
+int main() {
+  test(Bar{});
+}