c++: variable template and targ deduction [PR108550]
In this test, we get a bogus error because we failed to deduce the auto in
constexpr auto is_pointer_v = is_pointer<Tp>::value;
to bool. Then ensure_literal_type_for_constexpr_object thinks the object
isn't literal and an error is reported.
This is another case of the interaction between tf_partial and 'auto',
where the auto was not reduced so the deduction failed. In more detail:
we have
Wrap1<int>()
in the code and we need to perform OR -> fn_type_unification. The targ
list is incomplete, so we do
tsubst_flags_t ecomplain = complain | tf_partial | tf_fndecl_type;
fntype = tsubst (TREE_TYPE (fn), explicit_targs, ecomplain, NULL_TREE);
where TREE_TYPE (fn) is struct integral_constant <T402> (void). Then
we substitute the return type, which results in tsubsting is_pointer_v<int>.
is_pointer_v is a variable template with a placeholder type:
template <class Tp>
constexpr auto is_pointer_v = is_pointer<Tp>::value;
so we find ourselves in lookup_and_finish_template_variable. tf_partial is
still set, so finish_template_variable -> instantiate_template -> tsubst
won't reduce the level of auto. But then we do mark_used which eventually
calls do_auto_deduction which clears tf_partial, because we want to replace
the auto now. But we hadn't reduced auto's level so this fails. And
since we're not in an immediate context, we emit a hard error.
I suppose that when we reach lookup_and_finish_template_variable it's
probably time to clear tf_partial. (I added an assert and our testsuite
doesn't have a test whereby we get to lookup_and_finish_template_variable
while tf_partial is still active.)