[libc++] [test] Detect an improperly noexcept'ed __decay_copy.

[• Quuxplusone]

Summary

`__decay_copy` is used by `std::thread`'s constructor to copy its arguments
into the new thread. If `__decay_copy` claims to be noexcept, but then
copying the argument does actually throw, we'd call std::terminate instead
of passing this test. (And I've verified that adding an unconditional `noexcept`
to `__decay_copy` does indeed fail this test.)

(This came out of review comments on D100255.)

[cjdb]

Maybe it's worth cleaning up __decay_copy in this patch too? It's probably best if we commit separately to D100225.

[• Quuxplusone]

@cjdb wrote: "Maybe it's worth cleaning up __decay_copy in this patch too?"

__decay_copy is an internal implementation detail of libc++; making it conditionally-noexcept would just slow down the compile for no visible benefit. Therefore I don't propose to make __decay_copy conditionally-noexcept in this patch. The point of this patch is just to add a test that will fail if anyone puts a (too blatantly) wrong conditional-noexcept on it.

Relevant: https://quuxplusone.github.io/blog/2018/06/12/attribute-noexcept-verify/

[• Quuxplusone]

Fix C++03 — in that mode we support std::thread t(f); but not std::thread t(f, args...).

libcxx/test/std/thread/thread.threads/thread.thread.class/thread.thread.constr/F.pass.cpp

 
 // UNSUPPORTED: sanitizer-new-delete
 
 #include <thread>
 #include <new>
 #include <atomic>
 #include <cstdlib>
 #include <cassert>
+#include <vector>
 
 #include "test_macros.h"
 
 std::atomic<unsigned> throw_one(0xFFFF);
 std::atomic<unsigned> outstanding_new(0);
 
 
 void* operator new(std::size_t s) TEST_THROW_SPEC(std::bad_alloc)

 {
     if (!p) return;
     --outstanding_new;
     std::free(p);
 }
 
 bool f_run = false;
 
-void f()
-{
-    f_run = true;
-}
+struct F {
+    std::vector<int> v_;  // so f's copy-ctor calls operator new
+    explicit F() : v_(10) {}
+    void operator()() const { f_run = true; }
+};
+F f;
 
 class G
 {
     int alive_;
 public:
     static int n_alive;
     static bool op_run;