[Bug c++/116646] New: Compilation of code inside if constexpr with failed condition.

[Bug c++/116646] New: Compilation of code inside if constexpr with failed condition.

[cas43 at cs dot stanford.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116646

            Bug ID: 116646
           Summary: Compilation of code inside if constexpr with failed
                    condition.
           Product: gcc
           Version: 12.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: cas43 at cs dot stanford.edu
  Target Milestone: ---

When compiling the following code (g++ -std=c++20 file.cpp):

static const int y=0;
void f() {if constexpr(y<0){static_assert(y<0);}}

I expect no errors, but I get the following diagnostic with gcc version 12.3.0
(Ubuntu 12.3.0-1ubuntu1~22.04):

a.cpp: In function ‘void f()’:
a.cpp:2:44: error: static assertion failed
    2 | void f() {if constexpr(y<0){static_assert(y<0);}}
      |                                           ~^~
a.cpp:2:44: note: the comparison reduces to ‘(0 < 0)’

I get similar diagnostics with every compiler I have ready access to:

gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04) 
gcc version 10.5.0 (Ubuntu 10.5.0-1ubuntu1~22.04) 
Ubuntu clang version 15.0.7
Ubuntu clang version 14.0.0-1ubuntu1.1
Ubuntu clang version 13.0.1-2ubuntu2.2
Ubuntu clang version 12.0.1-19ubuntu3
Ubuntu clang version 11.1.0-6

This suggests a mistake in my own code, but I don't see it.  My understanding
is that the "if constexpr" should fail and prevent compilation within its body.
 Declaring y as "static constexpr" still fails.  The static_assert can be
replaced with other types of errors, such as a call to a function call that
fails to match.

[Bug c++/116646] Compilation of code inside if constexpr with failed condition.

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116646

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
It works when y is dependent though:
```
template<int y>
void f() {
        if constexpr(y<0)
        {
                static_assert(y<0);
        }
}

auto t = &f<0>;
```

I think since it is not dependent, it gets resolved right away.

[Bug c++/116646] Compilation of code inside if constexpr with failed condition.

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116646

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
https://developercommunity.visualstudio.com/t/non-dependent-static-assertfalse-in-constexpr-if-s/1267343

[Bug c++/116646] Compilation of code inside if constexpr with failed condition.

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116646

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> I think since it is not dependent, it gets resolved right away.

Right. A discarded statement (i.e. an untaken if-constexpr branch) is not
instantiated in a template. But in a normal function, it's always compiled
normally.

[Bug c++/116646] Compilation of code inside if constexpr with failed condition.

[cas43 at cs dot stanford.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116646

--- Comment #4 from Craig Schroeder <cas43 at cs dot stanford.edu> ---
Thank you for taking the time to explain this.  I understand why the code
snippet does not work now.