c++: GC collects live data when synthesizing operator== [PR99831]

Here we crash in reshape_init because we're accessing ggc_freed
& poisoned data: since r277865 in defaulted_late_check we call
synthesize_method here:

  if (kind == sfk_comparison)
    {
      /* If the function was declared constexpr, check that the definition
         qualifies.  Otherwise we can define the function lazily.  */
      if (DECL_DECLARED_CONSTEXPR_P (fn) && !DECL_INITIAL (fn))
        synthesize_method (fn);
      return;
    }

which in this test triggers when we're processing the string<"a">{} in
the static_assert.  First, we create a CONSTRUCTOR for the "{}" in
cp_parser_functional_cast, then we call finish_compound_literal which
calls complete_type and that results in garbage collection, which then
frees the CONSTRUCTOR {} we created when parsing the braced-list in
string<"a">{} -- at this point, it's not referenced by anything.
(That's not the case for 'type' in finish_compound_literal: the symbol
table contains a node for operator==, so ggc_mark_roots goes and marks
the fn decl, its type, its arguments etc., as used, so we don't collect
it.)

We could just bump function_depth around the new call to synthesize_method
to prevent GC.

gcc/cp/ChangeLog:

	PR c++/99831
	* method.c (defaulted_late_check): ++ and -- function_depth around
	the call to synthesize_method.
	* pt.c: Remove the saved_trees global.

gcc/testsuite/ChangeLog:

	PR c++/99831
	* g++.dg/other/gc6.C: New test.

gcc/cp/method.c

@@ -3131,7 +3131,12 @@ defaulted_late_check (tree fn)
       /* If the function was declared constexpr, check that the definition
 	 qualifies.  Otherwise we can define the function lazily.  */
       if (DECL_DECLARED_CONSTEXPR_P (fn) && !DECL_INITIAL (fn))
-	synthesize_method (fn);
+	{
+	  /* Prevent GC.  */
+	  function_depth++;
+	  synthesize_method (fn);
+	  function_depth--;
+	}
       return;
     }
 

gcc/cp/pt.c

@@ -65,7 +65,6 @@ static GTY(()) struct pending_template *last_pending_template;
 int processing_template_parmlist;
 static int template_header_count;
 
-static GTY(()) tree saved_trees;
 static vec<int> inline_parm_levels;
 
 static GTY(()) struct tinst_level *current_tinst_level;

gcc/testsuite/g++.dg/other/gc6.C

@@ -0,0 +1,16 @@
+// PR c++/99831
+// { dg-do compile { target c++20 } }
+// { dg-options "--param ggc-min-heapsize=0 --param ggc-min-expand=0" }
+
+template <int N> struct S {
+  constexpr S(const char (&str)[N]) : value{} { }
+  char value[N];
+};
+template <S> struct string {
+  constexpr bool operator==(const string &) const = default;
+};
+template <S L2> void operator+(string<L2>) {
+  char value[1];
+  S{value};
+}
+static_assert(string<"a">{} == string<"a">{});