Constrained template macros

Overridable: All of the following macros are overridable, define before inclusion.

Header: <outcome/config.hpp>

These macros expand into either the syntax for directly specifying constrained templates in C++ 20, or into a SFINAE based emulation for earlier C++ versions. Form of usage looks as follows:

OUTCOME_TEMPLATE(class ErrorCondEnum)
  OUTCOME_TREQUIRES(
    // If this is a valid expression
    OUTCOME_TEXPR(error_type(make_error_code(ErrorCondEnum()))),
    // If this predicate is true
    OUTCOME_TPRED(predicate::template enable_error_condition_converting_constructor<ErrorCondEnum>)
    // Any additional requirements follow here ...
  )
  constexpr basic_result(ErrorCondEnum &&t, error_condition_converting_constructor_tag /*unused*/ = {});

Be aware that slightly different semantics occur for real C++ 20 constrained templates than for the SFINAE emulation.

• OUTCOME_TEMPLATE(template args ...)

  Begins a constrained template declaration.

• OUTCOME_TREQUIRES(requirements ...)

  Specifies the requirements for the constrained template to be available for selection by the compiler.

• OUTCOME_TEXPR(expression)

  A requirement that the given expression is valid.

• OUTCOME_TPRED(boolean)

  A requirement that the given constant time expression is true.