On Thu, Mar 19, 2020 at 01:06:35PM -0400, Patrick Palka via Gcc-patches wrote:
> On Thu, 19 Mar 2020, Patrick Palka wrote:
>
> > This patch adds a check to detect changing the active union member during
> > initialization of the union. It uses the CONSTRUCTOR_NO_CLEARING flag as a
> > proxy for whether the non-empty CONSTRUCTOR of UNION_TYPE we're assigning
> > to in
> > cxx_eval_store_expression is in the process of being initialized, which
> > seems to
> > work well.
>
> If we can't rely on CONSTRUCTOR_NO_CLEARING to be set iff a CONSTRUCTOR
> is in the process of being initialized, then here's an alternative patch
> for consideration, that detects this UB in an indirect way and after the
> fact.
Yeah, I'm not sure if that would work well, especially in C++20 where we
sometimes don't clear it:
/* The result of a constexpr function must be completely initialized.
However, in C++20, a constexpr constructor doesn't necessarily have
to initialize all the fields, so we don't clear CONSTRUCTOR_NO_CLEARING
in order to detect reading an unitialized object in constexpr instead
of value-initializing it. (reduced_constant_expression_p is expected to
take care of clearing the flag.) */
if (TREE_CODE (result) == CONSTRUCTOR
&& (cxx_dialect < cxx2a
|| !DECL_CONSTRUCTOR_P (fun)))
clear_no_implicit_zero (result);
and rely on reduced_constant_expression_p to clear it.
Marek
