is_function is currently implemented with a zillion partial specializations. This allows us to inspect the result and argument types (needed for deprecated/removed machinery), but is costly for finding the true/false answer. A simpler approach, explained by Walter Brown at CppCon 2019, is to take advantage of the fact that const T isn't const for function types and reference types (and detecting reference types is easy).
This doesn't actually matter for MSVC's compiler front-end due to how it optimizes well-known type trait structs and variable templates (is_function is one of them). However, it matters for Clang/LLVM (which does not have such an optimization to my knowledge) and general code clarity.
is_functionis currently implemented with a zillion partial specializations. This allows us to inspect the result and argument types (needed for deprecated/removed machinery), but is costly for finding the true/false answer. A simpler approach, explained by Walter Brown at CppCon 2019, is to take advantage of the fact thatconst Tisn'tconstfor function types and reference types (and detecting reference types is easy).This doesn't actually matter for MSVC's compiler front-end due to how it optimizes well-known type trait structs and variable templates (
is_functionis one of them). However, it matters for Clang/LLVM (which does not have such an optimization to my knowledge) and general code clarity.