[libc++] Avoid unnecessary instantiations for __copy_cvref_t

[philnik777]

This changes the implementation of __copy_cvref_t to only template the implementation class on the _From parameter, avoiding instantiations for every combination of _From and _To.

[llvmbot]

@llvm/pr-subscribers-libcxx

Author: Nikolas Klauser (philnik777)

Changes

This changes the implementation of __copy_cvref_t to only template the implementation class on the _From parameter, avoiding instantiations for every combination of _From and _To.

---

Full diff: https://github.com/llvm/llvm-project/pull/123718.diff

1 Files Affected:

• (modified) libcxx/include/__type_traits/copy_cvref.h (+12-9)

diff --git a/libcxx/include/__type_traits/copy_cvref.h b/libcxx/include/__type_traits/copy_cvref.h
index 511d4e0776d609..158e5a5d78bb35 100644
--- a/libcxx/include/__type_traits/copy_cvref.h
+++ b/libcxx/include/__type_traits/copy_cvref.h
@@ -20,23 +20,26 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template <class _From, class _To>
+template <class _From>
 struct __copy_cvref {
-  using type = __copy_cv_t<_From, _To>;
+  template <class _To>
+  using __apply _LIBCPP_NODEBUG = __copy_cv_t<_From, _To>;
 };
 
-template <class _From, class _To>
-struct __copy_cvref<_From&, _To> {
-  using type = __add_lvalue_reference_t<__copy_cv_t<_From, _To> >;
+template <class _From>
+struct __copy_cvref<_From&> {
+  template <class _To>
+  using __apply _LIBCPP_NODEBUG = __add_lvalue_reference_t<__copy_cv_t<_From, _To> >;
 };
 
-template <class _From, class _To>
-struct __copy_cvref<_From&&, _To> {
-  using type = __add_rvalue_reference_t<__copy_cv_t<_From, _To> >;
+template <class _From>
+struct __copy_cvref<_From&&> {
+  template <class _To>
+  using __apply _LIBCPP_NODEBUG = __add_rvalue_reference_t<__copy_cv_t<_From, _To> >;
 };
 
 template <class _From, class _To>
-using __copy_cvref_t _LIBCPP_NODEBUG = typename __copy_cvref<_From, _To>::type;
+using __copy_cvref_t _LIBCPP_NODEBUG = typename __copy_cvref<_From>::template __apply<_To>;
 
 _LIBCPP_END_NAMESPACE_STD