This shouldn’t be valid. The point of allowed lambdas into unevaluated contexts wasn’t to suddenly allow SFINAE on statements. We do have some wording in [temp.deduct]/9 that makes this clear: A lambda-expression appearing in a function type or a template parameter is not considered part of the immediate context for the purposes of template argument … Read more
As you correctly surmised, the error novel is telling you that there is no index map. That is required for the default color map: UTIL/OUT: color_map(ColorMap color) This is used by the algorithm to keep track of its progress through the graph. The type ColorMap must be a model of Read/Write Property Map and its … Read more
First, let’s start with requires expressions. The section on the behavior of a requires expression has nothing special to say about the access controls of its component expressions. Similarly, the section on access controls says nothing in particular about requires expressions. In particular, there is [class.access]/4, which says: Access control is applied uniformly to all … Read more
This is a common problem of asking the wrong question of a concept, where you’re trying to use them like you would a base class interface. With base classes, you’re declaring the exact, specific functions that the derived classes are to implement. You want the user to implement exactly the function you say they must. … Read more
Yes. Only concepts can be subsumed. The call to foo<int> is ambiguous because neither of the declarations is “at least as constrained as” the other. If, however, C1 and C2 were both concepts instead of inline constexpr bools, then the declaration of the foo() that returns 0 would be at least as constrained as the … Read more
It is because the grammar requires it. It does. A requires constraint does not have to use a requires expression. It can use any more-or-less arbitrary boolean constant expression. Therefore, requires (foo) must be a legitimate requires constraint. A requires expression (that thing that tests whether certain things follow certain constraints) is a distinct construct; … Read more
Yes, concepts lite basically dresses up SFINAE. Plus it allows deeper introspection to allow for better overloading. However that only works if the concept predicates are defined as concept bool. The improved overloading does not work with the current concept predicates, but conditional overloading can be used. Lets look how we can define predicates, constrain … Read more