User talk:9rnsr/DIP: Template Parameter Constraint
(Page found through random Google search).
I really like the inline constraint feature:
auto foo(R if isInputRange)(R range);
Would love to see this giving cleaner errors.
Similar syntax could make the auto function parameter proposal more useful:
auto foo()(auto range if isInputRange);
Beside being easier to read, this disallows the user from supplying a type for R, and clearly documents the fact the user isn't expected to provide a type.