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

[Marek Polacek <polacek@redhat.com>]

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.  Alternatively, as in this patch, we can create a new
GC root to hold the expression list.  Since I'm adding a new global, I
felt compelled to get rid of one, and I actually did find an unused one!
So, 86 it.  But I'm also happy to just go with bumping function_depth.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/10?

gcc/cp/ChangeLog:

	PR c++/99831
	* parser.c (cp_parser_functional_cast): Make the expression list
	a GC root.
	* pt.c: Remove the saved_trees global.

gcc/testsuite/ChangeLog:

	PR c++/99831
	* g++.dg/other/gc6.C: New test.
---
 gcc/cp/parser.c                  | 20 ++++++++++++--------
 gcc/cp/pt.c                      |  1 -
 gcc/testsuite/g++.dg/other/gc6.C | 16 ++++++++++++++++
 3 files changed, 28 insertions(+), 9 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/other/gc6.C

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index d0477c42afe..28d7a8be258 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -30569,13 +30569,17 @@ cp_parser_simple_cast_expression (cp_parser *parser)
 }
 
 /* Parse a functional cast to TYPE.  Returns an expression
-   representing the cast.  */
+   representing the cast.  Preserve the expression list across
+   a garbage collect (by making it a GC root), which can occur
+   in finish_compound_literal -> complete_type, when we synthesize
+   a constexpr comparison function.  */
+
+static GTY(()) tree fncast_expression_list;
 
 static cp_expr
 cp_parser_functional_cast (cp_parser* parser, tree type)
 {
   vec<tree, va_gc> *vec;
-  tree expression_list;
   cp_expr cast;
   bool nonconst_p;
 
@@ -30588,12 +30592,12 @@ cp_parser_functional_cast (cp_parser* parser, tree type)
     {
       cp_lexer_set_source_position (parser->lexer);
       maybe_warn_cpp0x (CPP0X_INITIALIZER_LISTS);
-      expression_list = cp_parser_braced_list (parser, &nonconst_p);
-      CONSTRUCTOR_IS_DIRECT_INIT (expression_list) = 1;
+      fncast_expression_list = cp_parser_braced_list (parser, &nonconst_p);
+      CONSTRUCTOR_IS_DIRECT_INIT (fncast_expression_list) = 1;
       if (TREE_CODE (type) == TYPE_DECL)
 	type = TREE_TYPE (type);
 
-      cast = finish_compound_literal (type, expression_list,
+      cast = finish_compound_literal (type, fncast_expression_list,
 				      tf_warning_or_error, fcl_functional);
       /* Create a location of the form:
 	    type_name{i, f}
@@ -30612,10 +30616,10 @@ cp_parser_functional_cast (cp_parser* parser, tree type)
 						 /*allow_expansion_p=*/true,
 						 /*non_constant_p=*/NULL);
   if (vec == NULL)
-    expression_list = error_mark_node;
+    fncast_expression_list = error_mark_node;
   else
     {
-      expression_list = build_tree_list_vec (vec);
+      fncast_expression_list = build_tree_list_vec (vec);
       release_tree_vector (vec);
     }
 
@@ -30626,7 +30630,7 @@ cp_parser_functional_cast (cp_parser* parser, tree type)
      finishing at the closing paren.  */
   location_t combined_loc = make_location (start_loc, start_loc,
 					   parser->lexer);
-  cast = build_functional_cast (combined_loc, type, expression_list,
+  cast = build_functional_cast (combined_loc, type, fncast_expression_list,
                                 tf_warning_or_error);
   
   /* [expr.const]/1: In an integral constant expression "only type
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index dc6f2f37f9b..7956e83c1de 100644
--- a/gcc/cp/pt.c
+++ b/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;
diff --git a/gcc/testsuite/g++.dg/other/gc6.C b/gcc/testsuite/g++.dg/other/gc6.C
new file mode 100644
index 00000000000..ff45dd313d6
--- /dev/null
+++ b/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">{});

base-commit: 5f00df5925082c7b66da91270f2ed29bf4818c93
-- 
2.30.2