Ranges and lambdas weirdness about stl

[cpplearner]

filter requires a indirect_unary_predicate, which requires the predicate to be callable with different types of arguments, and return a boolean-testable type in each case.

As you correctly identified, calling the predicate with the iterator's value_type is checked by the concept, but is not executed. Call with a dereferenced iterator is checked and executed.

With -> bool, the lambda body is not instantiated when checking the return type, so the validity of std::get<A&>(el) is not checked by the concept.

Blame the authors of ranges-v3 for the design of indirect_unary_predicate :)

[StephanTLavavej]

On a related note, I just filed #4575 because the diagnostic for bogus get-by-type is very confusing.

[samsinsane]

Thank you for the responses, really appreciate the time you both spent on this.

@StephanTLavavej @CaseyCarter If there's nothing else to be actioned on your end, I'm happy for this to be closed off.

[StephanTLavavej]

Sounds good to me! Casey offered to provide additional clarification if necessary but if you're happy then we're happy.