llvm-project

9cfb138e - [Clang][Sema] Defer instantiation of exception specification until after partial ordering when determining primary template (#82417)

[Clang][Sema] Defer instantiation of exception specification until after partial ordering when determining primary template (#82417) Consider the following: ``` struct A { static constexpr bool x = true; }; template<typename T, typename U> void f(T, U) noexcept(T::y); // #1, error: no member named 'y' in 'A' template<typename T, typename U> void f(T, U*) noexcept(T::x); // #2 template<> void f(A, int*) noexcept; // explicit specialization of #2 ``` We currently instantiate the exception specification of all candidate function template specializations when deducting template arguments for an explicit specialization, which results in a error despite `#1` not being selected by partial ordering as the most specialized template. According to [except.spec] p13: > An exception specification is considered to be needed when: > - [...] > - the exception specification is compared to that of another declaration (e.g., an explicit specialization or an overriding virtual function); Assuming that "comparing declarations" means "determining whether the declarations correspond and declare the same entity" (per [basic.scope.scope] p4 and [basic.link] p11.1, respectively), the exception specification does _not_ need to be instantiated until _after_ partial ordering, at which point we determine whether the implicitly instantiated specialization and the explicit specialization declare the same entity (the determination of whether two functions/function templates correspond does not consider the exception specifications). This patch defers the instantiation of the exception specification until a single function template specialization is selected via partial ordering, matching the behavior of GCC, EDG, and MSVC: see https://godbolt.org/z/Ebb6GTcWE.