Fixed pointer to const& member function on rvalues, P0704r1

[Rakete1111]

Summary

This patch allows pointer to const& member functions on rvalues for C++20, and as an extension for prior versions. This is P0704r1.

[rsmith]

Thanks!

include/clang/Basic/DiagnosticSemaKinds.td

4091

I would be inclined to use an Extension rather than an ExtWarn for this. (I really don't see why we didn't move this as a DR -- it seems to be obviously a defect rather than an extension to me.)

4092

Hmm, this seems hard to express clearly and concisely. How about something like:

"invoking a pointer to a 'const &' member function on an rvalue is a C++2a extension"

4093

Please mark this as SFINAEFailure too; if we're going to reject under -pedantic-errors, we should also treat it as SFINAEable.

[rsmith]

Oh, and you'll need an accompanying test case before this can be committed (look at the existing tests in test/SemaCXX for ideas).

[Rakete1111]

Thanks! Agreed, your wording is better than mine :)

[rsmith]

Thanks, will you need someone to commit this for you?

include/clang/Basic/DiagnosticGroups.td

794 ↗

(On Diff #111602)

We've historically avoided using precise year numbers until a standard is finalized. I would call this CXX2a for now, and only provide the c++2a-extensions spelling, just in case the next standard ships a year late (or early).

include/clang/Basic/DiagnosticSemaKinds.td

4095–4096

Please also update this to match the prior diagnostic's text.

4097

This should now be in a CXXPre2aCompatPedantic group, because the corresponding extension is a pedantic warning.

[Rakete1111]

Yes please.

[Rakete1111]

Rebased because another commit already added the definitions of the C++20 groups.

[Rakete1111]

Update entry on the C++ status page.

[rsmith]

Sorry for the delay, this looks good to me.

[rsmith]

Committed as r311744.

include/clang/Basic/DiagnosticSemaKinds.td

 def err_pointer_to_member_oper_value_classify: Error<
   "pointer-to-member function type %0 can only be called on an "
   "%select{rvalue|lvalue}1">;
+def ext_pointer_to_const_ref_member_on_rvalue : Extension<
+  "invoking a pointer to a 'const &' member function on an rvalue is a C++2a extension">,
+  InGroup<CXX2a>, SFINAEFailure;
+def warn_cxx17_compat_pointer_to_const_ref_member_on_rvalue : Warning<
+  "invoking a pointer to a 'const &' member function on an rvalue is "
+  "incompatible with C++ standards before C++2a">,
+  InGroup<CXXPre2aCompatPedantic>, DefaultIgnore;
 def ext_ms_deref_template_argument: ExtWarn<
   "non-type template argument containing a dereference operation is a "
   "Microsoft extension">, InGroup<MicrosoftTemplate>;

lib/Sema/SemaExprCXX.cpp

       break;
 
     case RQ_LValue:
-      if (!isIndirect && !LHS.get()->Classify(Context).isLValue())
-        Diag(Loc, diag::err_pointer_to_member_oper_value_classify)
-          << RHSType << 1 << LHS.get()->getSourceRange();
+      if (!isIndirect && !LHS.get()->Classify(Context).isLValue()) {
+        // C++2a allows functions with ref-qualifier & if they are also 'const'.
+        if (Proto->isConst())
+          Diag(Loc, getLangOpts().CPlusPlus2a
+                        ? diag::warn_cxx17_compat_pointer_to_const_ref_member_on_rvalue
+                        : diag::ext_pointer_to_const_ref_member_on_rvalue);
+        else
+          Diag(Loc, diag::err_pointer_to_member_oper_value_classify)
+              << RHSType << 1 << LHS.get()->getSourceRange();
+      }
       break;
 
     case RQ_RValue:

test/SemaCXX/cxx2a-pointer-to-const-ref-member.cpp

+// RUN: %clang_cc1 -std=c++2a %s -verify
+
+struct X {
+  void ref() & {}
+  void cref() const& {}
+};
+
+void test() {
+  X{}.ref(); // expected-error{{cannot initialize object parameter of type 'X' with an expression of type 'X'}}
+  X{}.cref(); // expected-no-error
+
+  (X{}.*&X::ref)(); // expected-error{{pointer-to-member function type 'void (X::*)() &' can only be called on an lvalue}}
+  (X{}.*&X::cref)(); // expected-no-error
+}

www/cxx_status.html

     <tr>
       <td><tt>const&amp;</tt>-qualified pointers to members</td>
       <td><a href="http://wg21.link/p0704r1">P0704R1</a></td>
-      <td class="none" align="center">No</td>
+      <td class="svn" align="center">SVN</td>
     </tr>
     <tr>
       <td>Allow <i>lambda-capture</i> <tt>[=, this]</tt></td>