[Clang][accepts-invalid] Clang accepts capturing const-qualified bit-fields by reference

[zwuis]

Clang accepts this code:

void test1() {
  struct S { int a : 4; };
  const auto [x] = S{};
  [&x] {}(); // #1
  [&] { (void)x; }(); // #2
}

void test2() {
  struct S { const int a : 4; };
  auto [x] = S{};
  [&x] {}(); // #3
  [&] { (void)x; }(); // #4
}

According to [expr.prim.lambda.capture]/12, all of these 4 cases should be ill-formed:

... A bit-field or a member of an anonymous union shall not be captured by reference.

You can see examples in P1381R1 showing that capturing structured bindings refering bit-fields "means" capturing the bit-fields.

Compiler Explorer: https://godbolt.org/z/s8bY75Pqn

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Yanzuo Liu (zwuis)

Clang accepts this code:

void test1() {
  struct S { int a : 4; };
  const auto [x] = S{};
  [&amp;x] {}(); // #<!-- -->1
  [&amp;] { (void)x; }(); // #<!-- -->2
}

void test2() {
  struct S { const int a : 4; };
  auto [x] = S{};
  [&amp;x] {}(); // #<!-- -->3
  [&amp;] { (void)x; }(); // #<!-- -->4
}

According to [[expr.prim.lambda.capture]/12](https://eel.is/c++draft/expr.prim.lambda.capture#12.sentence-4), all of these 4 cases should be ill-formed:

> ... A bit-field or a member of an anonymous union shall not be captured by reference.

You can see examples in P1381R1 showing that capturing structured bindings refering bit-fields "means" capturing the bit-fields.

Compiler Explorer: <https://godbolt.org/z/s8bY75Pqn>

[frederick-vs-ja]

If CWG2737 gets accepted and implemented, IIUC these cases will also be naturally rejected.

[pinskia]

Note GCC has the same bug, filed as https://gcc.gnu.org/PR118667 .