[C++ PATCH] C++2A P0704R1 - fixing const-qualified pointers to members

[Jakub Jelinek]

Hi!

(Sorry for the resent, forgot to CC gcc-patches).

Another easy C++2A patch.  Here I wonder if we don't want
a -Wc++2a-compat warning and whether & (const | volatile)) == const
is best (for the case if there are other quals in the future),
or if we just want == const.

For the compat warning, the behavior will be that for c++17 and below
we'll error out or fail SFINAE, for c++2a we'll accept it, so not sure
if it falls into what we want to warn or not.

Regtested on x86_64-linux and i686-linux with make check-c++-all.

2017-09-19  Jakub Jelinek  <ja...@redhat.com>

        P0704R1 - fixing const-qualified pointers to members
        * typeck2.c (build_m_component_ref): For -std=c++2a allow
        pointer to const & qualified method on rvalue.

        * g++.dg/cpp2a/ptrmem1.C: New test.

--- gcc/cp/typeck2.c.jj 2017-09-19 12:06:30.635052397 +0200
+++ gcc/cp/typeck2.c    2017-09-19 12:12:50.725235890 +0200
@@ -1908,9 +1908,10 @@ build_m_component_ref (tree datum, tree
     {
       /* 5.5/6: In a .* expression whose object expression is an rvalue, the
         program is ill-formed if the second operand is a pointer to member
-        function with ref-qualifier &. In a .* expression whose object
-        expression is an lvalue, the program is ill-formed if the second
-        operand is a pointer to member function with ref-qualifier &&.  */
+        function with ref-qualifier & (for C++2A: unless its cv-qualifier-seq
+        is const). In a .* expression whose object expression is an lvalue,
+        the program is ill-formed if the second operand is a pointer to member
+        function with ref-qualifier &&.  */
       if (FUNCTION_REF_QUALIFIED (type))
        {
          bool lval = lvalue_p (datum);
@@ -1921,7 +1922,12 @@ build_m_component_ref (tree datum, tree
                       ptrmem_type);
              return error_mark_node;
            }
-         else if (!lval && !FUNCTION_RVALUE_QUALIFIED (type))
+         else if (!lval
+                  && !FUNCTION_RVALUE_QUALIFIED (type)
+                  && (cxx_dialect < cxx2a
+                      || ((type_memfn_quals (type)
+                           & (TYPE_QUAL_CONST | TYPE_QUAL_VOLATILE))
+                          != TYPE_QUAL_CONST)))
            {
              if (complain & tf_error)
                error ("pointer-to-member-function type %qT requires an lvalue",
--- gcc/testsuite/g++.dg/cpp2a/ptrmem1.C.jj     2017-09-19 12:30:49.344545579 
+0200
+++ gcc/testsuite/g++.dg/cpp2a/ptrmem1.C        2017-09-19 12:33:40.831363371 
+0200
@@ -0,0 +1,23 @@
+// P0704R1
+// { dg-do compile { target c++11 } }
+
+struct S {
+  void ref() & {}
+  void cref() const& {}
+  void vref() volatile & {}
+  void cvref() const volatile & {}
+};
+
+void
+foo ()
+{
+  S{}.ref();           // { dg-error "argument discards qualifiers" }
+  S{}.cref();
+  S{}.vref();          // { dg-error "argument discards qualifiers" }
+  S{}.cvref();         // { dg-error "argument discards qualifiers" }
+
+  (S{}.*&S::ref)();    // { dg-error "pointer-to-member-function type 'void 
\\(S::\\*\\)\\(\\) &' requires an lvalue" }
+  (S{}.*&S::cref)();   // { dg-error "pointer-to-member-function type 'void 
\\(S::\\*\\)\\(\\) const &' requires an lvalue" "" { target c++17_down } }
+  (S{}.*&S::vref)();   // { dg-error "pointer-to-member-function type 'void 
\\(S::\\*\\)\\(\\) volatile &' requires an lvalue" }
+  (S{}.*&S::cvref)();  // { dg-error "pointer-to-member-function type 'void 
\\(S::\\*\\)\\(\\) const volatile &' requires an lvalue" }
+}

        Jakub