[PATCH] D27424: Add the diagnose_if attribute to clang.

[Eric Fiselier via Phabricator via cfe-commits]

EricWF added a comment.

Awesome, this is exactly what I was hoping for! Thanks for working on this!

I'm a bit surprised that the attribute fails to evaluate the condition as a constant expression during the evaluation of constexpr functions. 
Ex:

  #define DIAG(...) __attribute__((diagnose_if(__VA_ARGS__, "message", "error")))
  constexpr bool foo(int x) DIAG(x == 0) // fails to evaluate this
  { 
    // assert(x != 0); <-- ironically a simple assert would catch this
    return x == 0;
  }  
  constexpr bool bar(int x) { return foo(x); }
  static_assert(bar(0), "");

Clearly the compiler can evaluate `x == 0` as a constant expression, but the attribute fails to see this. If the condition could be evaluated as-if it were part of the function body (during constexpr evaluation) that would be incredibly powerful. Especially within the STL where everything is `constexpr`.

The second thing that would be amazing if this attribute was late parsed so it  the ability to reference `this` when applied to member functions.
For example:

  struct Foo {
    bool is_good;
    constexpr bool good() const { return is_good; }
    constexpr bool foo() const _diagnose_if(!good(), "message", "error")
    {return true; }
  };
  constexpr Foo f{false};
  static_assert(f.foo(), "");

Thanks again for working on this! I'm very excited to get to use it!


https://reviews.llvm.org/D27424