On Sun, Aug 23, 2020 at 9:40 AM Manu agarwal <manu.agar...@live.in> wrote:
> Hi David, > > Thanks for the reply. I missed to mention the environment and what i am > trying to do. > > > 1. Environment is ClangCL (version 10.0), Microsoft Visual Studio. > > 2. I assumed that since the 'constexpr if' evaluates to false, the > external function's presence would not be checked. So, if my application is > not of type 'client' ClientMain would not be called. My actual code inside > the constexpr if is the commented line of code that does not compile. To > experiment i simply invoked ClientMain directly which compiled fine. > > Here is the assembly generated from the successful compilation (with the > first line inside 'constexpr if' enabled). > > "??$*MyAppInit*@$02@MyAppTemplate@@SQ_NXZ": # @"??$MyAppInit@ > $02@MyAppTemplate@@SQ_NXZ" > .Lfunc_begin2: > .cv_func_id 2 > .cv_loc 2 1 32 0 # Main.cpp:32:0 > # %bb.0: > .cv_loc 2 1 39 0 # Main.cpp:39:0 > *xor eax, eax* > # kill: def $al killed $al killed > $eax > .Ltmp6: > *ret* > .Ltmp7: > .Lfunc_end2: > > Here is how the compiler is invoked: > > *E:\LLVM\bin\clang-cl.exe /c /Z7 /nologo /W3 /WX- /diagnostics:column /Od > /D _DEBUG /D _CONSOLE /D _UNICODE /D UNICODE /EHsc /MTd /GS /fp:precise > /permissive- /std:c++17 /FA /Fa"x64\Debug\\" /Fo"x64\Debug\\" /Gv /TP -m64 > Main.cpp* > *1> Done executing task "CL".* > > Is my understanding of 'constexpr if' wrong ? won't the body get discarded > in case it gets evaluated to false? > Yeah, that's not quite how constexpr works - I think you may've gained that model partly by the combination of MSVC-compatible lazy-parsed templates and constexpr. The spec basically says something /very roughly/ like "dependent things in a non-taken constexpr if branch remain dependent even after template instantiation". But that means code that's invalid no matter what (like "static_assert(false)") still trigger errors. See the latter parts of the constexpr if section here: https://en.cppreference.com/w/cpp/language/if > > Regards, > Manu > > > > ------------------------------ > *From:* David Blaikie <dblai...@gmail.com> > *Sent:* Saturday, August 22, 2020 2:14 AM > *To:* Manu agarwal <manu.agar...@live.in> > *Cc:* cfe-users@lists.llvm.org <cfe-users@lists.llvm.org> > *Subject:* Re: [cfe-users] inconsistent compilation error inside > constexpr if > > From what I could test on godbolt, using LLVM evrsions back to 5.0, > Clang does reject the "return ClientMain();" call you aren't seeing an > error on. So I'm not sure what compiler/version/situation you're > running, but at least at first blush it doesn't look like clang. > > On Fri, Aug 21, 2020 at 1:29 PM Manu agarwal via cfe-users > <cfe-users@lists.llvm.org> wrote: > > > > Hello, > > > > In the below code the compiler throws "undeclared identifier" when the > commented line is uncommented. Whereas the line just before compiles fine. > > > > Regards, > > Manu > > > > typedef bool (* DummyFunc) (); > > > > > > bool ExecDummy (DummyFunc fptr) { > > > > if (fptr) > > return fptr (); > > > > return false; > > } > > > > constexpr unsigned int IsMyAppClient = 0; > > > > constexpr bool CheckForTypeClient (unsigned int pAppType) > > { > > return ((pAppType & IsMyAppClient) != 0); > > } > > > > > > class MyAppTemplate > > { > > public: > > template <unsigned int T> > > static bool MyAppInit (); > > }; > > > > template <unsigned int T> > > bool > > MyAppTemplate::MyAppInit () > > { > > if constexpr (CheckForTypeClient(T)) { > > > > return ClientMain (); // no error > > //return ExecDummy(ClientMain); // error: use of > undeclared identifier 'ClientMain' > > } > > > > return false; > > } > > > > int __cdecl > > main (int pArgc, char* pArgv[]) > > { > > constexpr int TVal = 3; > > > > MyAppTemplate::MyAppInit<TVal> (); > > > > return 0; > > } > > > > > > _______________________________________________ > > cfe-users mailing list > > cfe-users@lists.llvm.org > > https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users >
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