[cfe-users] inconsistent compilation error inside constexpr if

[David Blaikie via cfe-users]

On Sun, Aug 23, 2020 at 9:40 AM Manu agarwal <manu.agarwal at 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*@$02 at MyAppTemplate@@SQ_NXZ": # @"??$MyAppInit@
> $02 at 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 <dblaikie at gmail.com>
> *Sent:* Saturday, August 22, 2020 2:14 AM
> *To:* Manu agarwal <Manu.Agarwal at live.in>
> *Cc:* cfe-users at lists.llvm.org <cfe-users at 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 at 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 at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-users
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-users/attachments/20200823/b2eccecf/attachment.html>