c++: shortcut constexpr vector ctor [PR113835]

Since std::vector became usable in constant evaluation in C++20, a vector
variable with static storage duration might be manifestly
constant-evaluated, so we properly try to constant-evaluate its initializer.
But it can never succeed since the result will always refer to the result of
operator new, so trying is a waste of time.  Potentially a large waste of
time for a large vector, as in the testcase in the PR.

So, let's recognize this case and skip trying constant-evaluation.  I do
this only for the case of an integer argument, as that's the case that's
easy to write but slow to (fail to) evaluate.

In the test, I use dg-timeout-factor to lower the default timeout from 300
seconds to 15; on my laptop, compilation without the patch takes about 20
seconds versus about 2 with the patch.

	PR c++/113835

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_outermost_constant_expr): Bail out early
	for std::vector(N).

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/constexpr-vector1.C: New test.

0 files changed, 0 insertions, 0 deletions