ICE during bootstrap with --enable-checking

[Martin v. Loewis]

> Shall I send the preprossed source?  It should be easy to reproduce
> this - otherwise tell me.

I was just thinking about my previous claim that it might be a GC
problem. Still without having reproduced the problem: What in gcc
currently guarantees that tree nodes that appear as semantic values of
bison are marked as GC roots?

Or, put it differently: We have

extension:
	EXTENSION
		{ $$ = build_int_2 (pedantic | (warn_pointer_arith << 1), 0);
		  pedantic = 0;
		  warn_pointer_arith = 0; }
	;

$$ is the only reference to the integer, and that eventually lives in
yyparse:yyvsa. Is there any guarantee it won't be collected?

Martin