[libc++] Addresses LWG3782.

[Mordante]

Summary

3782. Should <math.h> declare ::lerp?

Libc++ doesn't declare ::lerp, adds tests to validate the requirement.

[philnik]

LGTM % comment.

libcxx/test/std/language.support/support.c.headers/support.c.headers.other/math.lerp.verify.cpp

27 ↗

(On Diff #493023)

<math.h> is still allowed to include <cmath> (I think libstdc++ might do that), so this should be inside test/libcxx if we want to test it at all. Removing this should also fix the modules build.

[Mordante]

Thanks for the review!

libcxx/docs/Status/Cxx2bIssues.csv

 "`3765 <https://wg21.link/LWG3765>`__","``const_sentinel`` should be constrained", "November 2022","","","|ranges|"
 "`3766 <https://wg21.link/LWG3766>`__","``view_interface::cbegin`` is underconstrained", "November 2022","","","|ranges|"
 "`3770 <https://wg21.link/LWG3770>`__","``const_sentinel_t`` is missing", "November 2022","","","|ranges|"
 "`3773 <https://wg21.link/LWG3773>`__","``views::zip_transform`` still requires ``F`` to be ``copy_constructible`` when empty pack", "November 2022","","","|ranges|"
 "`3774 <https://wg21.link/LWG3774>`__","``<flat_set>`` should include ``<compare>``", "November 2022","","","|flat_containers|"
 "`3775 <https://wg21.link/LWG3775>`__","Broken dependencies in the ``Cpp17Allocator`` requirements", "November 2022","","",""
 "`3778 <https://wg21.link/LWG3778>`__","``vector<bool>`` missing exception specifications", "November 2022","","",""
 "`3781 <https://wg21.link/LWG3781>`__","The exposition-only alias templates ``cont-key-type`` and ``cont-mapped-type`` should be removed", "November 2022","|Nothing to do|","",""
-"`3782 <https://wg21.link/LWG3782>`__","Should ``<math.h>`` declare ``::lerp``?", "November 2022","","",""
+"`3782 <https://wg21.link/LWG3782>`__","Should ``<math.h>`` declare ``::lerp``?", "November 2022","|Complete|","17.0",""
 "`3784 <https://wg21.link/LWG3784>`__","std.compat should not provide ``::byte`` and its friends", "November 2022","","",""
 "`3785 <https://wg21.link/LWG3785>`__","``ranges::to`` is over-constrained on the destination type being a range", "November 2022","","","|ranges|"
 "`3788 <https://wg21.link/LWG3788>`__","``jthread::operator=(jthread&&)`` postconditions are unimplementable under self-assignment", "November 2022","","",""
 "`3792 <https://wg21.link/LWG3792>`__","``__cpp_lib_constexpr_algorithms`` should also be defined in ``<utility>``", "November 2022","|Complete|","16.0",""
 "`3795 <https://wg21.link/LWG3795>`__","Self-move-assignment of ``std::future`` and ``std::shared_future`` have unimplementable postconditions", "November 2022","","",""
 "`3796 <https://wg21.link/LWG3796>`__","``movable-box`` as member should use ``default-initialization`` instead of ``copy-initialization``", "November 2022","","","|ranges|"
 "`3798 <https://wg21.link/LWG3798>`__","Rvalue reference and ``iterator_category``", "November 2022","|Partial| [#note-LWG3798]_","","|ranges|"
 "`3801 <https://wg21.link/LWG3801>`__","``cartesian_product_view::iterator::distance-from`` ignores the size of last underlying range", "November 2022","","","|ranges|"

libcxx/test/libcxx/language.support/support.c.headers/support.c.headers.other/math.lerp.verify.cpp

+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// When built with modules, this test gives diagnostics like
+//  declaration of 'lerp' must be imported from module 'std.compat.cmath'
+//  before it is required
+// therefore disable the test in this configuration.
+// UNSUPPORTED: modules-build
+
+// <math.h>
+
+// [support.c.headers.other]/1
+//   ... except for the functions described in [sf.cmath], the
+//   std::lerp function overloads ([c.math.lerp]) ...
+
+#include <math.h>
+
+void f() {
+  {
+    float f;
+    ::lerp(f, f, f);    // expected-error {{no member named 'lerp' in the global namespace}}
+    std::lerp(f, f, f); // expected-error {{no member named 'lerp' in namespace 'std'}}
+  }
+  {
+    double d;
+    ::lerp(d, d, d);    // expected-error {{no member named 'lerp' in the global namespace}}
+    std::lerp(d, d, d); // expected-error {{no member named 'lerp' in namespace 'std'}}
+  }
+  {
+    long double l;
+    ::lerp(l, l, l);    // expected-error {{no member named 'lerp' in the global namespace}}
+    std::lerp(l, l, l); // expected-error {{no member named 'lerp' in namespace 'std'}}
+  }
+}