Bug#1032317: libc++-15-dev: after installing libc++-15-dev, wasi-wasm32 stddef.h stops declaring size_t

[Simon McVittie]

Package: libc++-15-dev
Version: 1:15.0.7-1
Severity: important

This is a simplification of the compilation failure in #1029010.

To reproduce:

$ podman run --rm -it debian:sid-slim
# cat > stddef.cpp <<EOF
#include <stddef.h>
int main () { return sizeof (size_t); }
EOF
# apt-get update
# apt-get upgrade
# apt-get install --no-install-recommends make wasi-libc clang-15 libc++-15-dev-wasm32
# clang++-15 -c --target=wasi-wasm32 -ostddef-cpp.o stddef.cpp
# apt-get install --no-install-recommends libc++-15-dev
# clang++-15 -c --target=wasi-wasm32 -ostddef-cpp.o stddef.cpp

Expected result: both clang++-15 calls succeed (stddef.h declares size_t)

Actual result: the second clang++-15 call fails:

> stddef.cpp:2:30: error: use of undeclared identifier 'size_t'; did you mean 'sizeof'?
> int main () { return sizeof (size_t); }                                           
>                              ^~~~~~
>                              sizeof
> stddef.cpp:2:36: error: expected expression
> int main () { return sizeof (size_t); }                                           
>                                    ^
> 2 errors generated.

If I just preprocess (clang++-15 -E -dI --target=wasi-wasm32 stddef.cpp),
I get this (after removing blank lines):

# 1 "stddef.cpp"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 368 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "stddef.cpp" 2
#include <stddef.h> /* clang -E -dI */
# 1 "stddef.cpp"
# 1 "/usr/include/wasm32-wasi/c++/v1/stddef.h" 1 3
# 39 "/usr/include/wasm32-wasi/c++/v1/stddef.h" 3
#include <__config> /* clang -E -dI */
# 39 "/usr/include/wasm32-wasi/c++/v1/stddef.h" 3
# 1 "/usr/include/wasm32-wasi/c++/v1/__config" 1 3
# 13 "/usr/include/wasm32-wasi/c++/v1/__config" 3
#include <__config_site> /* clang -E -dI */
# 13 "/usr/include/wasm32-wasi/c++/v1/__config" 3
# 1 "/usr/include/wasm32-wasi/c++/v1/__config_site" 1 3
# 37 "/usr/include/wasm32-wasi/c++/v1/__config_site" 3
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wmacro-redefined"
#pragma clang diagnostic pop
# 14 "/usr/include/wasm32-wasi/c++/v1/__config" 2 3
# 23 "/usr/include/wasm32-wasi/c++/v1/__config" 3
# 660 "/usr/include/wasm32-wasi/c++/v1/__config" 3
namespace std { inline namespace __2 { }}
# 40 "/usr/include/wasm32-wasi/c++/v1/stddef.h" 2 3
# 43 "/usr/include/wasm32-wasi/c++/v1/stddef.h" 3
#include_next <stddef.h> /* clang -E -dI */
# 45 "/usr/include/wasm32-wasi/c++/v1/stddef.h" 3
# 1 "/usr/include/c++/v1/stddef.h" 1 3
# 46 "/usr/include/wasm32-wasi/c++/v1/stddef.h" 2 3
    typedef decltype(nullptr) nullptr_t;
# 2 "stddef.cpp" 2
int main () { return sizeof (size_t); }

which you will notice doesn't declare size_t as you might expect.

If I remove libc++-15-dev and re-run preprocessing with the same
command-line, I get this, which looks better in some ways but worse in
others (it defines ptrdiff_t, size_t and max_align_t, but not nullptr_t):

# 1 "stddef.cpp"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 368 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "stddef.cpp" 2
#include <stddef.h> /* clang -E -dI */
# 1 "stddef.cpp"
# 1 "/usr/lib/llvm-15/lib/clang/15.0.7/include/stddef.h" 1 3
# 35 "/usr/lib/llvm-15/lib/clang/15.0.7/include/stddef.h" 3
typedef long int ptrdiff_t;
# 46 "/usr/lib/llvm-15/lib/clang/15.0.7/include/stddef.h" 3
typedef long unsigned int size_t;
# 102 "/usr/lib/llvm-15/lib/clang/15.0.7/include/stddef.h" 3
#include "__stddef_max_align_t.h" /* clang -E -dI */
# 102 "/usr/lib/llvm-15/lib/clang/15.0.7/include/stddef.h" 3
# 1 "/usr/lib/llvm-15/lib/clang/15.0.7/include/__stddef_max_align_t.h" 1 3
# 19 "/usr/lib/llvm-15/lib/clang/15.0.7/include/__stddef_max_align_t.h" 3
typedef struct {
  long long __clang_max_align_nonce1
      __attribute__((__aligned__(__alignof__(long long))));
  long double __clang_max_align_nonce2
      __attribute__((__aligned__(__alignof__(long double))));
} max_align_t;
# 103 "/usr/lib/llvm-15/lib/clang/15.0.7/include/stddef.h" 2 3
# 2 "stddef.cpp" 2
int main () { return sizeof (size_t); }

I think this is leading to the autopkgtest failure seen in #1029010.
The autopkgtest uses "Depends: @" which installs all binary packages built
by the llvm-toolchain-15 source package, and as a result of this bug,
/usr/include/wasm32-wasi/c++/v1/cstddef doesn't define std::size_t because
there is no declaration for ::size_t, which should have come from stddef.h.

Sorry, I don't know enough about the internals of libc++ to suggest
a solution.

    smcv