c++: partial ordering of object parameter [PR53499]

Looks like we implemented option 1 (skip the object parameter) for CWG532
before the issue was resolved, and never updated to the final resolution of
option 2 (model it as a reference).  More recently CWG2445 extended this
handling to static member functions; I think that's wrong, and have
opened CWG2834 to address that and how explicit object member functions
interact with it.

The FIXME comments are to guide how the explicit object member function
support should change the uses of DECL_NONSTATIC_MEMBER_FUNCTION_P.

The library testsuite changes are to make partial ordering work again
between the generic operator- in the testcase and
_Pointer_adapter::operator-.

	DR 532
	PR c++/53499

gcc/cp/ChangeLog:

	* pt.cc (more_specialized_fn): Fix object parameter handling.

gcc/testsuite/ChangeLog:

	* g++.dg/template/partial-order4.C: New test.
	* g++.dg/template/spec26.C: Adjust for CWG532.

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/vector/ext_pointer/types/1.cc
	* testsuite/23_containers/vector/ext_pointer/types/2.cc
	(N::operator-): Make less specialized.

1 files changed, 49 insertions, 15 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 5765982..669d2ad 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -25198,27 +25198,61 @@ more_specialized_fn (tree pat1, tree pat2, int len)
   bool lose1 = false;
   bool lose2 = false;
 
-  /* Remove the this parameter from non-static member functions.  If
-     one is a non-static member function and the other is not a static
-     member function, remove the first parameter from that function
-     also.  This situation occurs for operator functions where we
-     locate both a member function (with this pointer) and non-member
-     operator (with explicit first operand).  */
-  if (DECL_NONSTATIC_MEMBER_FUNCTION_P (decl1))
-    {
-      len--; /* LEN is the number of significant arguments for DECL1 */
-      args1 = TREE_CHAIN (args1);
-      if (!DECL_STATIC_FUNCTION_P (decl2))
+  /* C++17 [temp.func.order]/3 (CWG532)
+
+     If only one of the function templates M is a non-static member of some
+     class A, M is considered to have a new first parameter inserted in its
+     function parameter list. Given cv as the cv-qualifiers of M (if any), the
+     new parameter is of type "rvalue reference to cv A" if the optional
+     ref-qualifier of M is && or if M has no ref-qualifier and the first
+     parameter of the other template has rvalue reference type. Otherwise, the
+     new parameter is of type "lvalue reference to cv A".  */
+
+  if (DECL_STATIC_FUNCTION_P (decl1) || DECL_STATIC_FUNCTION_P (decl2))
+    {
+      /* Note C++20 DR2445 extended the above to static member functions, but
+	 I think think the old G++ behavior of just skipping the object
+	 parameter when comparing to a static member function was better, so
+	 let's stick with that for now.  This is CWG2834.  --jason 2023-12 */
+      if (DECL_NONSTATIC_MEMBER_FUNCTION_P (decl1)) /* FIXME or explicit */
+	{
+	  len--; /* LEN is the number of significant arguments for DECL1 */
+	  args1 = TREE_CHAIN (args1);
+	}
+      else if (DECL_NONSTATIC_MEMBER_FUNCTION_P (decl2)) /* FIXME or explicit */
 	args2 = TREE_CHAIN (args2);
     }
-  else if (DECL_NONSTATIC_MEMBER_FUNCTION_P (decl2))
+  else if (DECL_NONSTATIC_MEMBER_FUNCTION_P (decl1) /* FIXME implicit only */
+	   && DECL_NONSTATIC_MEMBER_FUNCTION_P (decl2))
     {
+      /* Note DR2445 also (IMO wrongly) removed the "only one" above, which
+	 would break e.g.  cpp1y/lambda-generic-variadic5.C.  */
+      len--;
+      args1 = TREE_CHAIN (args1);
       args2 = TREE_CHAIN (args2);
-      if (!DECL_STATIC_FUNCTION_P (decl1))
+    }
+  else if (DECL_NONSTATIC_MEMBER_FUNCTION_P (decl1) /* FIXME implicit only */
+	   || DECL_NONSTATIC_MEMBER_FUNCTION_P (decl2))
+    {
+      /* The other is a non-member or explicit object member function;
+	 rewrite the implicit object parameter to a reference.  */
+      tree ns = DECL_NONSTATIC_MEMBER_FUNCTION_P (decl2) ? decl2 : decl1;
+      tree &nsargs = ns == decl2 ? args2 : args1;
+      tree obtype = TREE_TYPE (TREE_VALUE (nsargs));
+
+      nsargs = TREE_CHAIN (nsargs);
+
+      cp_ref_qualifier rqual = type_memfn_rqual (TREE_TYPE (ns));
+      if (rqual == REF_QUAL_NONE)
 	{
-	  len--;
-	  args1 = TREE_CHAIN (args1);
+	  tree otherfirst = ns == decl1 ? args2 : args1;
+	  otherfirst = TREE_VALUE (otherfirst);
+	  if (TREE_CODE (otherfirst) == REFERENCE_TYPE
+	      && TYPE_REF_IS_RVALUE (otherfirst))
+	    rqual = REF_QUAL_RVALUE;
 	}
+      obtype = cp_build_reference_type (obtype, rqual == REF_QUAL_RVALUE);
+      nsargs = tree_cons (NULL_TREE, obtype, nsargs);
     }
 
   /* If only one is a conversion operator, they are unordered.  */