[committed] libstdc++: Add missing test for std::optional::transform(F&&)

[Jonathan Wakely <jwakely@redhat.com>]

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On Wed, 20 Oct 2021 at 09:42, Jonathan Wakely wrote:
>
> On Wed, 20 Oct 2021 at 06:48, Tim Song wrote:
> >
> > On Tue, Oct 19, 2021 at 9:05 AM Jonathan Wakely via Gcc-patches
> > <gcc-patches@gcc.gnu.org> wrote:
> > >
> > > +constexpr bool
> > > +test_copy_elision()
> > > +{
> > > +  return true;
> > > +}
> > > +
> > > +static_assert( test_copy_elision() );
> > > +
> >
> > This isn't much of a test :)
>
> The ultimate copy elision. The copies even get removed from the source code.
>
> Thanks for spotting it. I meant to check we're constructing directly
> into the union member without materializing the temporary early. I'll
> add that today.

Fixed by the attached patch. Tested x86_64-linux, pushed to trunk.

Thanks again!

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commit 4ba4b053151a20262d4b61eb4501aa1c48337abb
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Oct 20 20:12:28 2021

    libstdc++: Add missing test for std::optional::transform(F&&)
    
    The test_copy_elision() function was supposed to ensure that the result
    is constructed directly in the std::optional, without early temporary
    materialization. But I forgot to write the test.
    
    libstdc++-v3/ChangeLog:
    
            * testsuite/20_util/optional/monadic/transform.cc: Check that
            an rvalue result is not materialized too soon.

diff --git a/libstdc++-v3/testsuite/20_util/optional/monadic/transform.cc b/libstdc++-v3/testsuite/20_util/optional/monadic/transform.cc
index d01ccb2e0f2..13977b8ba8d 100644
--- a/libstdc++-v3/testsuite/20_util/optional/monadic/transform.cc
+++ b/libstdc++-v3/testsuite/20_util/optional/monadic/transform.cc
@@ -110,6 +110,23 @@ static_assert( test_forwarding() );
 constexpr bool
 test_copy_elision()
 {
+  struct immovable
+  {
+    constexpr immovable(int p) : power_level(p) { }
+    immovable(immovable&&) = delete;
+
+    int power_level;
+  };
+
+  struct Force
+  {
+    constexpr immovable operator()(int i) const { return {i+1}; }
+  };
+
+  std::optional<int> irresistible(9000);
+  std::optional<immovable> object = irresistible.transform(Force{});
+  VERIFY( object->power_level > 9000 );
+
   return true;
 }