[Bug c++/74762] [5/6/7 Regression] missing uninitialized warning (C++, parenthesized expr, TREE_NO_WARNING)

["ppalka at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=74762

--- Comment #6 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to Manuel López-Ibáñez from comment #5)
> (In reply to Patrick Palka from comment #1)
> > Looks like warn_uninit() suppresses the warning on 'i' because the
> > TREE_NO_WARNING flag is set on the ARRAY_REF by the C++ FE (in
> > finish_parenthesized_expr(), for an unrelated purpose).
> 
> I wonder if a quick work-around for this case in particular is to check for:
> 
>   if (TREE_CODE (expr) == MODIFY_EXPR)
> 
> like the C parser does:
> 
> 	  /* A parenthesized expression.  */
> 	  location_t loc_open_paren = c_parser_peek_token (parser)->location;
> 	  c_parser_consume_token (parser);
> 	  expr = c_parser_expression (parser);
> 	  if (TREE_CODE (expr.value) == MODIFY_EXPR)
> 	    TREE_NO_WARNING (expr.value) = 1;
> 	  if (expr.original_code != C_MAYBE_CONST_EXPR)
> 	    expr.original_code = ERROR_MARK;
> 	  /* Don't change EXPR.ORIGINAL_TYPE.  */
> 	  location_t loc_close_paren = c_parser_peek_token (parser)->location;
> 	  set_c_expr_source_range (&expr, loc_open_paren, loc_close_paren);
> 	  c_parser_skip_until_found (parser, CPP_CLOSE_PAREN,
> 				     "expected %<)%>");
> 
> Then the bug will only trigger for code such as ((lhs=rhs)), which is what
> TREE_NO_WARNING is trying to avoid warning for.

Unfortunately it looks like the purpose of setting TREE_NO_WARNING in
finish_parenthesized_expr() is not limited to assignments within conditionals,
it's also important for the -Wparentheses warning about confusing precedence
(e.g. warn for "a + b << c" but not for "a + (b << c)")