[PATCH] c++: consider built-in operator candidates first

[Patrick Palka via Gcc-patches]

During operator overload resolution, we currently consider non-member
candidates before built-in candidates.  This didn't make a difference
before r12-3346, but after this change add_candidates will avoid
computing excess argument conversions if we've already seen a strictly
viable candidate, so it's better to consider built-in candidates first.

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

gcc/cp/ChangeLog:

        * call.c (add_operator_candidates): Consider built-in operator
        candidates before considering non-member candidates.

gcc/testsuite/ChangeLog:

        * g++.dg/template/conv17.C: Extend test.
---
 gcc/cp/call.c                          | 13 +++++++------
 gcc/testsuite/g++.dg/template/conv17.C |  7 +++++++
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index c5601d96ab8..c0da083758f 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -6321,7 +6321,6 @@ add_operator_candidates (z_candidate **candidates,
                         vec<tree, va_gc> *arglist,
                         int flags, tsubst_flags_t complain)
 {
-  z_candidate *start_candidates = *candidates;
   bool ismodop = code2 != ERROR_MARK;
   tree fnname = ovl_op_identifier (ismodop, ismodop ? code2 : code);
 
@@ -6333,6 +6332,12 @@ add_operator_candidates (z_candidate **candidates,
   if (rewritten && code != EQ_EXPR && code != SPACESHIP_EXPR)
     flags &= ~LOOKUP_REWRITTEN;
 
+  /* Add built-in candidates to the candidate set.  The standard says to
+     rewrite built-in candidates, too, but there's no point.  */
+  if (!rewritten)
+    add_builtin_candidates (candidates, code, code2, fnname, arglist,
+                           flags, complain);
+
   bool memonly = false;
   switch (code)
     {
@@ -6352,6 +6357,7 @@ add_operator_candidates (z_candidate **candidates,
 
   /* Add namespace-scope operators to the list of functions to
      consider.  */
+  z_candidate *start_candidates = *candidates;
   if (!memonly)
     {
       tree fns = lookup_name (fnname, LOOK_where::BLOCK_NAMESPACE);
@@ -6423,11 +6429,6 @@ add_operator_candidates (z_candidate **candidates,
 
   if (!rewritten)
     {
-      /* The standard says to rewrite built-in candidates, too,
-        but there's no point.  */
-      add_builtin_candidates (candidates, code, code2, fnname, arglist,
-                             flags, complain);
-
       /* Maybe add C++20 rewritten comparison candidates.  */
       tree_code rewrite_code = ERROR_MARK;
       if (cxx_dialect >= cxx20
diff --git a/gcc/testsuite/g++.dg/template/conv17.C 
b/gcc/testsuite/g++.dg/template/conv17.C
index f0f10f2ef4f..87ecefb8de3 100644
--- a/gcc/testsuite/g++.dg/template/conv17.C
+++ b/gcc/testsuite/g++.dg/template/conv17.C
@@ -61,3 +61,10 @@ concept E = requires { T().h(nullptr); };
 
 static_assert(!E<C>);
 #endif
+
+// Verify that the strictly viable built-in operator+ candidate precludes
+// us from computing all argument conversions for the below non-strictly
+// viable non-member candidate.
+enum N { n };
+int operator+(N&, B);
+int f = n + 42;
-- 
2.33.0.363.g4c719308ce