Yes, Clang allows it. Most importantly though: you don’t have to be in a constexpr function for `if constexpr` to be useful. e.g. the Algorithms aren’t constexpr until C++20, but a lot of them have overloads for improving the algorithm based on the iterator category. Having `if constexpr` there instead of enable_ifs seems like quite a nice improvement to me. The main problem is probably that libc++ back-ports a lot of the C++11 stuff to C++03, so in these cases `if constexpr` won’t help.
> On Aug 8, 2023, at 9:10 AM, Jonathan Wakely <jwakely....@gmail.com> wrote: > > On Tue, 8 Aug 2023 at 17:07, Jonathan Wakely wrote: >> >> On Tue, 8 Aug 2023 at 17:04, Nikolas Klauser wrote: >>> >>> Luckily most of these aren’t problems for libc++. We only support the >>> latest GCC. We can only use `if constexpr` in C++11, but that is already a >>> win I think. >> >> Can you use it in C++11 though? The body of a constexpr function must >> be a single return statement, so if-constexpr isn't allowed. > > Clang allows it with multiple warnings: > > ifc.cc:3:6: warning: constexpr if is a C++17 extension [-Wc++17-extensions] > if constexpr (sizeof(i) >= 4) > ^ > ifc.cc:3:3: warning: use of this statement in a constexpr function is > a C++14 extension [-Wc++14-extensions] > if constexpr (sizeof(i) >= 4) > ^ > ifc.cc:8:5: warning: multiple return statements in constexpr function > is a C++14 extension [-Wc++14-extensions] > return 0; > ^ > ifc.cc:5:5: note: previous return statement is here > return i << 3; > ^ > > But GCC gives a warning for if-constexpr and then an error for the > invalid function body: > > ifc.cc: In function 'constexpr int f(int)': > ifc.cc:3:6: warning: 'if constexpr' only available with '-std=c++17' > or '-std=gnu++17' [-Wc++17-extensions] > 3 | if constexpr (sizeof(i) >= 4) > | ^~~~~~~~~ > ifc.cc:9:1: error: body of 'constexpr' function 'constexpr int f(int)' > not a return-statement > 9 | } > | ^
