On the non-uniform semantics of return-type-requirements

Introduction

What we have as a return-type-requirement today is a misnomer in both the syntactic and semantic sense. The Concepts TS introduced two kinds of semantic constraints that syntactically took the form of a trailing-return-type: the deduction constraint and the implicit conversion constraint. In the time since, deduction constraints were first changed by the lack of constrained placeholders until the adoption of P1141R1 at the 2018 San Diego meeting (becoming syntactically different from a trailing-return-type), and then—with P1084R2—they ceased to deduce in the manner associated with return type deduction. The syntactic space occupied by deduction constraints in the Concepts TS is now occupied by something else altogether; however, the implicit conversion constraint lives on. Yet the two are both return-type-requirements, using the -> token.

The inconsistency between the uses of the -> token in the same context seems unfortunate; and the trailing-return-type form of return-type-requirement, with its current semantics, appears to be easily replaceable. Its presence in the language seems to be an unnecessary complication that may frustrate future extensions.

-> Type versus -> Concept

The -> Type form of return-type-requirement behaves consistently with the behaviour of return E; for a function that is declared to return Type except that placeholder types are allowed for trailing-return-types in function declarations and not for return-type-requirements. That is, the semantics of the various compound-requirements in

requires {
  { E } -> Type;
  { [](Type) {}(E) };
  { E } -> ConvertibleTo<Type>;
};

are roughly the same. The last form is less aware of the context in terms of access checking, null pointer conversion, and other cases where perfect forwarding is less-than-perfect. This last form is also the form used by the library in N4791, the post-San Diego working draft. Note, however, that Same is more often used than ConvertibleTo in N4791.

In contrast, the -> Concept form of return-type-requirement most notably does not use a type after the -> token. It also deduces in a manner different from either of Concept auto or Concept decltype(auto) would for a function whose declared return type contains such a placeholder type, and it does not involve a check for convertibility.

Extending the current -> Type case to allow placeholder types would leave us with a problem where the extension of the current semantics would lead to a difference in the meaning of the first two compound-requirements in

requires {
  { E } -> Concept;
  { E } -> Concept auto;
  { [](Concept auto) {}(E) };
};

However, we note that the third compound-requirement already expresses a deduction constraint (except that the TS wording causes access checking, etc. to be ignored). It would seem that extending from the current semantics of -> Type is not very profitable.

Possibility for a more powerful -> Type

Today’s placeholder types are limited in context; however, the Concepts TS allowed deduction constraints like

-> std::vector<Boolean>

We note that this sort of “type pattern” is expressed in the language as a type. Thus, if we retain -> Type with its current semantics, we will not gain the benefit of applying P1084 to such cases if we were to adopt them into the language.

We also note that, if we performed the template argument deduction upon decltype((E)) (while requiring that the deduced A is identical to A) in the style of partial specialization matching instead of applying template argument deduction upon the expression in the style of a call to a function template, then -> Type would have semantics matching that of -> Same<Type>. Additional benefits are that -> Concept auto would work like -> Concept does in N4791, and that replacing a concrete type in a return-type-requirement with a concept that the type models would not cause the compound-requirement to reject previously accepted cases.

Proposal for C++20: remove -> Type

The -> Type form of return-type-requirement is underpowered and does not bring additional expressiveness to the language. It is on the wrong side of the split between ConvertibleTo and Same as used with compound-requirements in N4791, and it introduces complications for future extensions; therefore, it is proposed that the trailing-return-type form of return-type-requirement be removed. At the same time, it is proposed that trailing-type-requirement is a more appropriate name.