llvm-project

34ae2265 - [Clang][Sema] Improve support for explicit specializations of constrained member functions & member function templates (#88963)

[Clang][Sema] Improve support for explicit specializations of constrained member functions & member function templates (#88963) Consider the following snippet from the discussion of CWG2847 on the core reflector: ``` template<typename T> concept C = sizeof(T) <= sizeof(long); template<typename T> struct A { template<typename U> void f(U) requires C<U>; // #1, declares a function template void g() requires C<T>; // #2, declares a function template<> void f(char); // #3, an explicit specialization of a function template that declares a function }; template<> template<typename U> void A<short>::f(U) requires C<U>; // #4, an explicit specialization of a function template that declares a function template template<> template<> void A<int>::f(int); // #5, an explicit specialization of a function template that declares a function template<> void A<long>::g(); // #6, an explicit specialization of a function that declares a function ``` A number of problems exist: - Clang rejects `#4` because the trailing _requires-clause_ has `U` substituted with the wrong template parameter depth when `Sema::AreConstraintExpressionsEqual` is called to determine whether it matches the trailing _requires-clause_ of the implicitly instantiated function template. - Clang rejects `#5` because the function template specialization instantiated from `A<int>::f` has a trailing _requires-clause_, but `#5` does not (nor can it have one as it isn't a templated function). - Clang rejects `#6` for the same reasons it rejects `#5`. This patch resolves these issues by making the following changes: - To fix `#4`, `Sema::AreConstraintExpressionsEqual` is passed `FunctionTemplateDecl`s when comparing the trailing _requires-clauses_ of `#4` and the function template instantiated from `#1`. - To fix `#5` and `#6`, the trailing _requires-clauses_ are not compared for explicit specializations that declare functions. In addition to these changes, `CheckMemberSpecialization` now considers constraint satisfaction/constraint partial ordering when determining which member function is specialized by an explicit specialization of a member function for an implicit instantiation of a class template (we previously would select the first function that has the same type as the explicit specialization). With constraints taken under consideration, we match EDG's behavior for these declarations.