[C++20] [Modules] Cannot export type_info/operator new/operator delete

[huangqinjin]

test.cppm

module;

#include <stddef.h>

export module test;

extern "C++"
{
    export class type_info;                                 // 0

    export void* operator new(size_t);                      // 1
    export void* operator new[](size_t);                    // 2
    export void operator delete(void*) noexcept;            // 3
    export void operator delete[](void*) noexcept;          // 4

    export void operator delete(void*, size_t) noexcept;    // 5
    export void operator delete[](void*, size_t) noexcept;  // 6
}

targeting x86_64-linux-gnu

✗ clang --target=x86_64-linux-gnu -std=c++20 --precompile test.cppm
test.cppm:11:18: error: cannot export redeclaration 'operator new' here since the previous declaration is not exported
   11 |     export void* operator new(size_t);                      // 1
      |                  ^
note: previous declaration is here
test.cppm:12:18: error: cannot export redeclaration 'operator new[]' here since the previous declaration is not exported
   12 |     export void* operator new[](size_t);                    // 2
      |                  ^
note: previous declaration is here
test.cppm:13:17: error: cannot export redeclaration 'operator delete' here since the previous declaration is not exported
   13 |     export void operator delete(void*) noexcept;            // 3
      |                 ^
note: previous declaration is here
test.cppm:14:17: error: cannot export redeclaration 'operator delete[]' here since the previous declaration is not exported
   14 |     export void operator delete[](void*) noexcept;          // 4
      |                 ^
note: previous declaration is here
4 errors generated.

targeting x86_64-windows-msvc

✗ clang --target=x86_64-windows-msvc -std=c++20 --precompile test.cppm
test.cppm:9:18: error: cannot export redeclaration 'type_info' here since the previous declaration is not exported
    9 |     export class type_info;                                 // 0
      |                  ^
note: previous declaration is here
test.cppm:11:18: error: cannot export redeclaration 'operator new' here since the previous declaration is not exported
   11 |     export void* operator new(size_t);                      // 1
      |                  ^
note: previous declaration is here
test.cppm:12:18: error: cannot export redeclaration 'operator new[]' here since the previous declaration is not exported
   12 |     export void* operator new[](size_t);                    // 2
      |                  ^
note: previous declaration is here
test.cppm:13:17: error: cannot export redeclaration 'operator delete' here since the previous declaration is not exported
   13 |     export void operator delete(void*) noexcept;            // 3
      |                 ^
note: previous declaration is here
test.cppm:14:17: error: cannot export redeclaration 'operator delete[]' here since the previous declaration is not exported
   14 |     export void operator delete[](void*) noexcept;          // 4
      |                 ^
note: previous declaration is here
5 errors generated.

The strange thing is that other overloads of operator new and operator delete can be exported without error.
Edit: Clang does not enable sized deallocation by default, adding -fsized-deallocation to the command line will cause 5 & 6 failed also.

[llvmbot]

@llvm/issue-subscribers-clang-modules

Author: None (huangqinjin)

#### `test.cppm`

module;

#include <stddef.h>

export module test;

extern "C++"
{
    export class type_info;                                 // 0

    export void* operator new(size_t);                      // 1
    export void* operator new[](size_t);                    // 2
    export void operator delete(void*) noexcept;            // 3
    export void operator delete[](void*) noexcept;          // 4

    export void operator delete(void*, size_t) noexcept;    // 5
    export void operator delete[](void*, size_t) noexcept;  // 6
}

targeting x86_64-linux-gnu

✗ clang --target=x86_64-linux-gnu -std=c++20 --precompile test.cppm
test.cppm:11:18: error: cannot export redeclaration 'operator new' here since the previous declaration is not exported
   11 |     export void* operator new(size_t);                      // 1
      |                  ^
note: previous declaration is here
test.cppm:12:18: error: cannot export redeclaration 'operator new[]' here since the previous declaration is not exported
   12 |     export void* operator new[](size_t);                    // 2
      |                  ^
note: previous declaration is here
test.cppm:13:17: error: cannot export redeclaration 'operator delete' here since the previous declaration is not exported
   13 |     export void operator delete(void*) noexcept;            // 3
      |                 ^
note: previous declaration is here
test.cppm:14:17: error: cannot export redeclaration 'operator delete[]' here since the previous declaration is not exported
   14 |     export void operator delete[](void*) noexcept;          // 4
      |                 ^
note: previous declaration is here
4 errors generated.

targeting x86_64-windows-msvc

✗ clang --target=x86_64-windows-msvc -std=c++20 --precompile test.cppm
test.cppm:9:18: error: cannot export redeclaration 'type_info' here since the previous declaration is not exported
    9 |     export class type_info;                                 // 0
      |                  ^
note: previous declaration is here
test.cppm:11:18: error: cannot export redeclaration 'operator new' here since the previous declaration is not exported
   11 |     export void* operator new(size_t);                      // 1
      |                  ^
note: previous declaration is here
test.cppm:12:18: error: cannot export redeclaration 'operator new[]' here since the previous declaration is not exported
   12 |     export void* operator new[](size_t);                    // 2
      |                  ^
note: previous declaration is here
test.cppm:13:17: error: cannot export redeclaration 'operator delete' here since the previous declaration is not exported
   13 |     export void operator delete(void*) noexcept;            // 3
      |                 ^
note: previous declaration is here
test.cppm:14:17: error: cannot export redeclaration 'operator delete[]' here since the previous declaration is not exported
   14 |     export void operator delete[](void*) noexcept;          // 4
      |                 ^
note: previous declaration is here
5 errors generated.

The strange thing is that other overloads of operator new and operator delete can be exported without error.

[ChuanqiXu9]

I am not sure about typeinfo but I roughtly remember these operator new and deletes are special and implicitly declarared for every module units. So the behavior about operator new and delete is more or less correct if I remember correctly.

For typeinfo, I am not sure if this is the problme of headers in windows.

[ChuanqiXu9]

Yeah, the replaceable operator new and delete are implicit declared: https://eel.is/c++draft/basic.stc.dynamic and replace it doesn't require to export it. So I think the behavior is correct.

If we can find typeinfo in the header's of windows, we can close the issue.

[huangqinjin]

Yeah, the replaceable operator new and delete are implicit declared: https://eel.is/c++draft/basic.stc.dynamic and replace it doesn't require to export it. So I think the behavior is correct.

But below two overloads can be exported.

export void operator delete(void*, size_t) noexcept;    // 5
export void operator delete[](void*, size_t) noexcept;  // 6

[huangqinjin]

If we can find typeinfo in the header's of windows, we can close the issue.

I don't understand this, does it mean type_info is also implicit declared?

[huangqinjin]

Yeah, the replaceable operator new and delete are implicit declared: https://eel.is/c++draft/basic.stc.dynamic and replace it doesn't require to export it. So I think the behavior is correct.

But below two overloads can be exported.

export void operator delete(void*, size_t) noexcept;    // 5
export void operator delete[](void*, size_t) noexcept;  // 6

Ok, it is because clang does not enable sized deallocation by default, adding -fsized-deallocation to the command line will cause them failed also.

[ChuanqiXu9]

If we can find typeinfo in the header's of windows, we can close the issue.

I don't understand this, does it mean type_info is also implicit declared?

I mean, if we can find typeinfo in stddef.h from windows, we can close this.

[huangqinjin]

stddef.h is a C header, it unlikely contains class type_info.

Pointed out by microsoft/STL#4643, the declaration is ineh.h.

[ChuanqiXu9]

Fine. Then it looks like the behavior of clang is good. I'll close the issue. Feel free to reopen this if you have different opinions.

[huangqinjin]

It doesn't explain where is the previous declaration. Does clang include eh.h implicitly?

[ChuanqiXu9]

hmm, I am not sure. It might be windows specific.

[huangqinjin]

@ChuanqiXu9 This issue is no longer repeatable with Clang 19, could you please confirm and point out which change fixed this issue, thanks.

[ChuanqiXu9]

Probably e0d66c2?

And I'll close this. Thanks for updating.