On Thu, Mar 15, 2018 at 7:49 AM, Marek Polacek <pola...@redhat.com> wrote:
> On Wed, Mar 14, 2018 at 02:06:36PM -0400, Jason Merrill wrote:
>> On Wed, Mar 14, 2018 at 11:59 AM, Marek Polacek <pola...@redhat.com> wrote:
>> > cxx_constant_value doesn't understand template codes, and neither it
>> > understands OVERLOADs, so if we pass an OVERLOAD to it, we crash. Here
>> > instantiate_non_dependent_expr got an OVERLOAD, but since it calls
>> > is_nondependent_constant_expression which checks type_unknown_p, it left
>> > the
>> > expression as it was. We can't use is_nondependent_constant_expression in
>> > finish_if_stmt_cond because i_n_c_e checks is_constant_expression and that
>> > is
>> > not suitable here; we'd miss diagnostics. So I did the following; I think
>> > we
>> > should reject the testcase with an error.
>> >
>> > Bootstrapped/regtested on x86_64-linux, ok for trunk?
>> >
>> > 2018-03-14 Marek Polacek <pola...@redhat.com>
>> >
>> > PR c++/84854
>> > * semantics.c (finish_if_stmt_cond): Give error if the condition
>> > is an overloaded function with no contextual type information.
>> >
>> > * g++.dg/cpp1z/constexpr-if15.C: New test.
>> >
>> > diff --git gcc/cp/semantics.c gcc/cp/semantics.c
>> > index fdf37bea770..a056e9445e9 100644
>> > --- gcc/cp/semantics.c
>> > +++ gcc/cp/semantics.c
>> > @@ -735,8 +735,16 @@ finish_if_stmt_cond (tree cond, tree if_stmt)
>> > && require_constant_expression (cond)
>> > && !value_dependent_expression_p (cond))
>> > {
>> > - cond = instantiate_non_dependent_expr (cond);
>> > - cond = cxx_constant_value (cond, NULL_TREE);
>> > + if (type_unknown_p (cond))
>> > + {
>> > + cxx_incomplete_type_error (cond, TREE_TYPE (cond));
>> > + cond = error_mark_node;
>>
>> I think I'd prefer to skip this block when type_unknown_p, and leave
>> error handling up to the code shared with regular if.
>
> Like this?
Yes, thanks.
> It was my first version, but I thought we wanted the error;
Getting an error is an improvement, but it should apply to
non-constexpr if as well, so checking in maybe_convert_cond would be
better. Actually, if we deal with unknown type there, we shouldn't
need this patch at all.
...except I also notice that since maybe_convert_cond doesn't do any
conversion in a template, we're trying to extract the constant value
without first converting to bool, which breaks this testcase:
struct A
{
constexpr operator bool () { return true; }
int i;
};
A a;
template <class T> void f()
{
constexpr bool b = a; // works
if constexpr (a) { } // breaks
}
int main()
{
f<int>();
}
A simple fix would be to replace your type_unknown_p check here with a
comparison to boolean_type_node, so we wait until instantiation time
to require a constant value.
Better would be to actually do the conversion. Perhaps this could
share code (for converting and getting a constant value) with
finish_static_assert.
Jason
