2 ms·

Since the compiler must verify the expression is constexpr-valid, it could annotate that automatically instead. However, I believe that generally the standard

Since the compiler must verify the expression is constexpr-valid, it could annotate that automatically instead. However, I believe that generally the standard requires that the keyword be explicit so that compilers agree on what are legal programs, and don’t just declare it to be implementation-defined for what is legal in const-initializer contexts. But in clang, you’d need an frontend-optimization framework to do that reliably (due to recursion), and it doesn’t have one. In llvm, the Attributer framework is still relatively new, but is progressing towards that.

dataflow 5y ago
> However, I believe that generally the standard requires that the keyword be explicit so that compilers agree on what are legal programs, and don’t just declare it to be implementation-defined for what is legal in const-initializer contexts. The standard could just declare that anything that could be constexpr-evaluable must be evaluated at compile time up to a depth of at least N, or something like that, after which it is permitted (but not obligated) to be evaluated at runtime. Just like with templates, except over there the implementation just errors out if the template gets too complicated or impossible to evaluate. The standard already has minimum criteria like these for a ton of other cases so this wouldn't be any different to justify a keyword for it.