A constrained function with an auto return type would have it's definition
instantiated in order to deduce the auto return type before the constraints
are checked.
Move the constraints check after the return type deduction.
Now with concepts support merged and mostly complete, we do not need -fconcepts-ts
(which was also misleading as we were not implementing the TS) and can enable
concepts features under C++2a. A warning will be generated if users still attempt
to use -fconcepts-ts.
Function trailing requires clauses now parsed, supported in overload resolution and when calling, referencing and taking the address of functions or function templates.
Differential Revision: https://reviews.llvm.org/D43357
Saar Raz