[libcxx-commits] [libcxx] Fix __split_buffer_pointer_layout compatibility with swift's C++ interop (PR #170957)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libcxx

Author: None (junov-google)

<details>
<summary>Changes</summary>

When the swift compiler generates a bridging module for importing C++ symbols, it instantiates all methods of the class template instances used by the symbols imported from C++. This is because it cannot determine which methods will be called from swift at that stage. This means that it may attempt to generate invalid instantiations (due to missing type traits) that would not be generated in a normal C++ build.

The default constructor of `__split_buffer_pointer_layout` has this issue because it does not initialize `__alloc_` which may or may not be a reference depending on the `allocator_type` tempalte parameter. The initialization of class members that are references is mandatory. `vector` uses a reference type for the allocator and `deque` does not. Therefore, only `deque` is allowed to use the default constructor of `__split_buffer_pointer_layout`.

When the swift compiler's C++ importer generates an instantiation of `std::vector`, it will also attempt to instantiate the default constructor of `__split_buffer_pointer_layout`, which fails to compile.

This change fixes the issue by adding a `requires` clause to suppress the instantiation of the problematic constructor whenever its instatiation would be invalid.

There are no tests included in this change because the `requires` statement has no observable effect on pure C++ builds. Test coverage will be automatically assured downstream in swift-project once it updates its fork of libc++.

---
Full diff: https://github.com/llvm/llvm-project/pull/170957.diff


1 Files Affected:

- (modified) libcxx/include/__split_buffer (+10-2) 


``````````diff
diff --git a/libcxx/include/__split_buffer b/libcxx/include/__split_buffer
index 1e05e4df8ba0f..5e4e265b10296 100644
--- a/libcxx/include/__split_buffer
+++ b/libcxx/include/__split_buffer
@@ -30,6 +30,9 @@
 #include <__type_traits/integral_constant.h>
 #include <__type_traits/is_nothrow_assignable.h>
 #include <__type_traits/is_nothrow_constructible.h>
+#if _LIBCPP_STD_VER >= 20
+#  include <__type_traits/is_reference.h>
+#endif
 #include <__type_traits/is_swappable.h>
 #include <__type_traits/is_trivially_destructible.h>
 #include <__type_traits/is_trivially_relocatable.h>
@@ -68,8 +71,13 @@ protected:
 
 public:
   // Can't be defaulted due to _LIBCPP_COMPRESSED_PAIR not being an aggregate in C++03 and C++11.
-  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI __split_buffer_pointer_layout() : __back_cap_(nullptr) {}
-
+  _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI __split_buffer_pointer_layout()
+#if _LIBCPP_STD_VER >= 20
+      // Prevents Swift compiler's C++ interop from implicitly instantiating this ctor when it's not supported.
+      requires (!is_reference_v<allocator_type>)
+#endif
+      : __back_cap_(nullptr) {}
+  
   _LIBCPP_CONSTEXPR_SINCE_CXX20
   _LIBCPP_HIDE_FROM_ABI explicit __split_buffer_pointer_layout(const allocator_type& __alloc)
       : __back_cap_(nullptr), __alloc_(__alloc) {}

``````````

</details>


https://github.com/llvm/llvm-project/pull/170957