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.

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 ();
 

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;
+}