https://gcc.gnu.org/g:cacbb4daac3e9a9d86992fef1a7c607b4cee5f22
commit r15-4340-gcacbb4daac3e9a9d86992fef1a7c607b4cee5f22
Author: Jason Merrill <ja...@redhat.com>
Date: Fri Oct 11 14:52:43 2024 -0400
c++: address deduction and concepts [CWG2918]
CWG2918 changes deduction from an overload set for the case where multiple
candidates succeed and have the same type; previously this made the overload
set a non-deduced context, now it succeeds since the result is consistent
between the candidates.
This is needed for cases of overloading based on requirements, where we want
to choose the most constrained overload. I also needed to adjust
resolve_address_of_overloaded_function accordingly; we already handled the
comparison for template candidates in most_specialized_instantiation, but
need to also do the comparison for non-template candidates such as member
functions of a class template.
CWG 2918 (proposed)
gcc/cp/ChangeLog:
* cp-tree.h (most_constrained_function): Declare..
* class.cc (resolve_address_of_overloaded_function): Call it.
* pt.cc (get_template_for_ordering): Handle list from
resolve_address_of_overloaded_function.
(most_constrained_function): No longer static.
(resolve_overloaded_unification): Always compare type rather
than decl.
gcc/testsuite/ChangeLog:
* g++.dg/DRs/dr2918.C: New test.
Diff:
---
gcc/cp/cp-tree.h | 1 +
gcc/cp/class.cc | 13 ++++++++++---
gcc/cp/pt.cc | 18 ++++++++++++++----
gcc/testsuite/g++.dg/DRs/dr2918.C | 11 +++++++++++
4 files changed, 36 insertions(+), 7 deletions(-)
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index dc153a97dc46..94ee550bd9cb 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -7593,6 +7593,7 @@ extern int template_args_equal (tree,
tree);
extern tree maybe_process_partial_specialization (tree);
extern tree most_specialized_instantiation (tree);
extern tree most_specialized_partial_spec (tree, tsubst_flags_t, bool =
false);
+extern tree most_constrained_function (tree);
extern void print_candidates (tree);
extern void instantiate_pending_templates (int);
extern tree tsubst_default_argument (tree, int, tree, tree,
diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
index 646072d4f202..8c39bb4a76b1 100644
--- a/gcc/cp/class.cc
+++ b/gcc/cp/class.cc
@@ -8896,11 +8896,18 @@ resolve_address_of_overloaded_function (tree
target_type,
tree match = most_specialized_instantiation (matches);
if (match != error_mark_node)
- matches = tree_cons (TREE_PURPOSE (match),
- NULL_TREE,
- NULL_TREE);
+ {
+ matches = match;
+ TREE_CHAIN (match) = NULL_TREE;
+ }
}
}
+ else if (flag_concepts && TREE_CHAIN (matches))
+ if (tree match = most_constrained_function (matches))
+ {
+ matches = match;
+ TREE_CHAIN (match) = NULL_TREE;
+ }
/* Now we should have exactly one function in MATCHES. */
if (matches == NULL_TREE)
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 03a1144765b2..c0a37a51cba3 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -2071,6 +2071,10 @@ get_template_for_ordering (tree list)
{
gcc_assert (TREE_CODE (list) == TREE_LIST);
tree f = TREE_VALUE (list);
+ if (f == NULL_TREE)
+ /* Also handle a list from resolve_address_of_overloaded_function with the
+ function in TREE_PURPOSE. */
+ f = TREE_PURPOSE (list);
if (tree ti = DECL_TEMPLATE_INFO (f))
return TI_TEMPLATE (ti);
return f;
@@ -2084,7 +2088,7 @@ get_template_for_ordering (tree list)
Note that we don't compare constraints on the functions
themselves, but rather those of their templates. */
-static tree
+tree
most_constrained_function (tree candidates)
{
// Try to find the best candidate in a first pass.
@@ -23865,11 +23869,14 @@ resolve_overloaded_unification (tree tparms,
tree fn = *iter;
if (flag_noexcept_type)
maybe_instantiate_noexcept (fn, tf_none);
- if (try_one_overload (tparms, targs, tempargs, parm, TREE_TYPE (fn),
+ if (TREE_CODE (fn) == FUNCTION_DECL && !constraints_satisfied_p (fn))
+ continue;
+ tree elem = TREE_TYPE (fn);
+ if (try_one_overload (tparms, targs, tempargs, parm, elem,
strict, sub_strict, addr_p, explain_p)
- && (!goodfn || !decls_match (goodfn, fn)))
+ && (!goodfn || !same_type_p (goodfn, elem)))
{
- goodfn = fn;
+ goodfn = elem;
++good;
}
}
@@ -23879,6 +23886,9 @@ resolve_overloaded_unification (tree tparms,
overloaded functions does not contain function templates and at most
one of a set of overloaded functions provides a unique match.
+ CWG2918 allows multiple functions to match if they all have the same type,
+ so that we can choose the most constrained later.
+
So if we found multiple possibilities, we return success but don't
deduce anything. */
diff --git a/gcc/testsuite/g++.dg/DRs/dr2918.C
b/gcc/testsuite/g++.dg/DRs/dr2918.C
new file mode 100644
index 000000000000..982ced71f1f3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/DRs/dr2918.C
@@ -0,0 +1,11 @@
+// CWG 2918 makes this OK
+// { dg-do compile { target c++20 } }
+
+template<bool B> struct X {
+ static void f(short) requires B; // #1
+ static void f(short); // #2
+};
+void test() {
+ auto x = &X<true>::f; // OK, deduces void(*)(short), selects
#1
+ auto y = &X<false>::f; // OK, deduces void(*)(short), selects #2
+}
