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[RFH / Patch] PR 51222


I'm having a look to this PR filed by Daniel, which is again about SFINAE for combined delete / new expressions, eg a simple example could be (Daniel provided tens)

template<class T, class = decltype(::delete ::new T())>
auto g(int) -> char;

auto g(...) -> char(&)[2];

static_assert(sizeof(g<void>(0)) == 2, "Ouch");

We handle incorrectly all such cases, all the static_assert for the testcases provided by Daniel fire, for a simple reason: in finish_decltype_type the check:

   /* FIXME instantiation-dependent  */
  if (type_dependent_expression_p (expr)
      /* In a template, a COMPONENT_REF has an IDENTIFIER_NODE for op1 even
        if it isn't dependent, so that we can check access control at
        instantiation time, so defer the decltype as well (PR 42277).  */
      || (id_expression_or_member_access_p
&& processing_template_decl

is false, thus we don't produce a DECLTYPE_TYPE and we don't tsubst into it. Nothing can possibly work... Now I did the trivial experiment of replacing the above with the usual:

  if (processing_template_decl)

and *the whole* testsuite passes (all the tests provided by Daniel included) with the only exception of nullptr26.C, ie:

template <class T, class U>
void f(T, U);

template <class T>
void f(T, decltype(nullptr));

int main()
  f(1, nullptr);

which we reject as ambiguous (and of course it would be easy to handle it specially, for sure std::nullptr_t it *is* special!).

Thus, I suspect the fix for this issue could turn out to be pretty simple: any help?

In particular about the exact meaning of the FIXME? More generally, is the issue here matter of compile-time optimization? Like we want to save work when we actually *know* the type even in template context? (then looks like type_dependent_expression_p isn't really what we want...)


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