This patch implements P1141R2 "Yet another approach for constrained declarations", and is the last in the (initial) series for implementing C++2a concepts.
General strategy for this patch was:
- Expand AutoType to include optional type-constraint, reflecting the wording and easing the integration of constraints.
- Recognize AutoTypes used in functions parameters in ActOnFunctionDeclarator (the first place where we have the required information to fetch the correct template parameter list to append the invented parameters to) and fix the function and parameter types there.
Based on D60939.