Fix __split_buffer_pointer_layout compatibility with swift's C++ interop

[junov-google]

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++.

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[llvmbot]

@llvm/pr-subscribers-libcxx

Author: None (junov-google)

Changes

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 --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) {}