CODEWITHSUNDEEP
c++templatesc++20c++-conceptsrequires-expression
con ko
con ko

Reputation: 1388

Why the using of a noneffective concept regarded as ill-formed

The newly published draft mentions in [expr.prim.req]/6:

If the substitution of template arguments into a requirement would always result in a substitution failure, the program is ill-formed; no diagnostic required. [ Example:

template<typename T> concept C =
requires {
  new int[-(int)sizeof(T)];     // ill-formed, no diagnostic required
};

— end example ]

But why can't we guarantee the diagnostic to always fail, rather than skip the diagnostic?

Upvotes: 2

Views: 165

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473397

Requirement expressions can do pretty much anything. They can provoke further template substitutions, cascading outwardly through an arbitrary amount of code. And recall that template substitutions constitute a Turning complete language.

So you're asking the compiler to, given a Turing complete program, prove whether there is some input which causes that program to be well-formed. This is just a restatement of the Halting Problem. Just like the Halting Problem, there are simple cases where it's obvious the program halts/doesn't halt. But when you're dealing with a Turing-complete language, it can get arbitrarily complex.

The standard isn't going to force compilers to solve the Halting Problem.

Upvotes: 6

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