Re: [PATCH] c++: CWG 2789 and reversed operator candidates

[Jason Merrill]

On 9/20/24 8:48 PM, Patrick Palka wrote:

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
OK for trunk?

OK.

-- >8 --

As a follow-up to r15-3741-gee3efe06c9c49c, which was only concerned
with usings, it seems we should also compare contexts of a reversed
vs non-reversed (member) candidate during operator overload
resolution.

        DR 2789

gcc/cp/ChangeLog:

        * call.cc (cand_parms_match): Check for matching class contexts
        even in the reversed case.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/concepts-memfun4.C: Adjust expected result
        involving reversed candidate.
---
  gcc/cp/call.cc                                | 11 +++++++----
  gcc/testsuite/g++.dg/cpp2a/concepts-memfun4.C |  6 +++---
  2 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index f2ce50857ec..70783baac24 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -12865,10 +12865,6 @@ cand_parms_match (z_candidate *c1, z_candidate *c2, 
pmatch match_kind)
        }
      }

- else if (reversed)

-    return (reversed_match (c1, c2)
-           && reversed_match (c2, c1));
-
    tree parms1 = TYPE_ARG_TYPES (TREE_TYPE (fn1));
    tree parms2 = TYPE_ARG_TYPES (TREE_TYPE (fn2));

@@ -12880,6 +12876,10 @@ cand_parms_match (z_candidate *c1, z_candidate *c2, pmatch match_kind)

        if (base1 != base2)
        return false;

+ if (reversed)

+       return (reversed_match (c1, c2)
+               && reversed_match (c2, c1));
+
        /* Use object_parms_correspond to simplify comparing iobj/xobj/static
         member functions.  */
        if (!object_parms_correspond (fn1, fn2, base1))
@@ -12897,6 +12897,9 @@ cand_parms_match (z_candidate *c1, z_candidate *c2, 
pmatch match_kind)
        parms1 = skip_parms (fn1, parms1);
        parms2 = skip_parms (fn2, parms2);
      }
+  else if (reversed)
+    return (reversed_match (c1, c2)
+           && reversed_match (c2, c1));
    return compparms (parms1, parms2);
  }

diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-memfun4.C b/gcc/testsuite/g++.dg/cpp2a/concepts-memfun4.C

index cf7f13c007d..d849e9129a3 100644
--- a/gcc/testsuite/g++.dg/cpp2a/concepts-memfun4.C
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-memfun4.C
@@ -91,7 +91,7 @@ namespace N1 {

A<> a;

    B<> b;
-  // when comparing the A op== to the reversed B op==, we compare them in
-  // reverse order, so they match, and we choose the more constrained.
-  static_assert (a == b);
+  // A op== and B op== are defined in different classes so constraints
+  // aren't considered, and the tie is broken via reversedness.
+  static_assert (!(a == b));
  }