Comment by lochsh | Hacker News Reader

Comment by lochsh | Hacker News Reader

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This is what I was going for in the blog post, but I got confused after seeing these comments as Clang _does_ compile this when the buffer is of static storage class.

Which I don't think is standard -- but it's not using VLAs. I wondered if it just has constant expression semantics for const variables.

Weirdly, adding a _Static_assert to test this theory proves it for c99 but not c11 :/

https://godbolt.org/z/q-bb-n c99 with clang https://godbolt.org/z/ad14Ah c11 with clang https://godbolt.org/z/xJSDQa c11 with gcc (which is the only one giving the output I'd expect)

>I wondered if it just has constant expression semantics for const variables.

That would be my guess also, for applicable const variables. File scope const variables are quite constexpr-y in C anyway, since C requires all file scope variable initializers to be constant expressions (C++ only requires that for constexpr variables).

Toying around with clang in Godbolt, it seems that there are some quirks regarding this. The following is accepted:

  static const int n = 0;
  int buf[n]; // invalid zero-length array is accepted
              // probably another non-standard extension

But the following is not, despite n having the same zero value:

  static const int n;
  int buf[n]; // complains about a file scope VLA

The C standard permits implementations to support additional forms of constant expressions.

Try "clang -std=c11 -pedantic-errors".

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