[gcc r14-6724] c++: local class memfn synth from uneval context [PR113063]

[Patrick Palka]

https://gcc.gnu.org/g:fced59166f95e9922a72392955e4fed095afd47e

commit r14-6724-gfced59166f95e9922a72392955e4fed095afd47e
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Dec 19 16:33:55 2023 -0500

    c++: local class memfn synth from uneval context [PR113063]
    
    Here we first use and therefore synthesize the local class operator<=>
    from an unevaluated context, which inadvertently affects synthesization
    by preventing functions used within the definition (such as the copy
    constructor of std::strong_ordering) from getting marked as odr-used.
    
    This patch fixes this by using maybe_push_to_top_level in synthesize_method
    which ensures cp_unevaluated_operand gets cleared even in the function-local
    case.
    
            PR c++/113063
    
    gcc/cp/ChangeLog:
    
            * method.cc (synthesize_method): Use maybe_push_to_top_level
            and maybe_pop_from_top_level.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp2a/spaceship-synth16.C: New test.

Diff:
---
 gcc/cp/method.cc                               | 12 ++----------
 gcc/testsuite/g++.dg/cpp2a/spaceship-synth16.C | 13 +++++++++++++
 2 files changed, 15 insertions(+), 10 deletions(-)

diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
index 26e6eb79946..f645066077f 100644
--- a/gcc/cp/method.cc
+++ b/gcc/cp/method.cc
@@ -1770,8 +1770,6 @@ decl_remember_implicit_trigger_p (tree decl)
 void
 synthesize_method (tree fndecl)
 {
-  bool nested = (current_function_decl != NULL_TREE);
-  tree context = decl_function_context (fndecl);
   bool need_body = true;
   tree stmt;
   location_t save_input_location = input_location;
@@ -1795,10 +1793,7 @@ synthesize_method (tree fndecl)
      it now.  */
   push_deferring_access_checks (dk_no_deferred);
 
-  if (! context)
-    push_to_top_level ();
-  else if (nested)
-    push_function_context ();
+  bool push_to_top = maybe_push_to_top_level (fndecl);
 
   input_location = DECL_SOURCE_LOCATION (fndecl);
 
@@ -1843,10 +1838,7 @@ synthesize_method (tree fndecl)
 
   input_location = save_input_location;
 
-  if (! context)
-    pop_from_top_level ();
-  else if (nested)
-    pop_function_context ();
+  maybe_pop_from_top_level (push_to_top);
 
   pop_deferring_access_checks ();
 
diff --git a/gcc/testsuite/g++.dg/cpp2a/spaceship-synth16.C b/gcc/testsuite/g++.dg/cpp2a/spaceship-synth16.C
new file mode 100644
index 00000000000..37a183de0f5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/spaceship-synth16.C
@@ -0,0 +1,13 @@
+// PR c++/113063
+// { dg-do link { target c++20 } }
+
+#include <compare>
+
+int main() {
+  struct X {
+    auto operator<=>(const X&) const = default;
+  };
+  X x;
+  static_assert(noexcept(x <=> x));
+  x <=> x;
+}