Statement expressions allow jumping past initialization

[AaronBallman]

In C++, both Clang and GCC diagnose when a jump statement will jump over an initialization. However, statement expressions seem to be allowed to do so:

void f(void) {          

 for (int i=({ goto ouch; int x = 10; x;});i<0;++i)

ouch:
  ;
}

void g(void) {          
  ({ 
    goto ouch;  
    int i = 12;
  });

ouch:
  ;
}

https://godbolt.org/z/Eda59KqPT

It's possible that this is intentional given that statement expressions tend to be hidden behind macros, but this seems inconsistent and somewhat dangerous to allow.

[llvmbot]

@llvm/issue-subscribers-c-1

Author: Aaron Ballman (AaronBallman)

In C++, both Clang and GCC diagnose when a jump statement will jump over an initialization. However, statement expressions seem to be allowed to do so: ``` void f(void) {

for (int i=({ goto ouch; int x = 10; x;});i<0;++i)

ouch:
;
}

void g(void) {
({
goto ouch;
int i = 12;
});

ouch:
;
}

https://godbolt.org/z/Eda59KqPT

It's possible that this is intentional given that statement expressions tend to be hidden behind macros, but this seems inconsistent and somewhat dangerous to allow.
</details>

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Aaron Ballman (AaronBallman)

In C++, both Clang and GCC diagnose when a jump statement will jump over an initialization. However, statement expressions seem to be allowed to do so: ``` void f(void) {

for (int i=({ goto ouch; int x = 10; x;});i<0;++i)

ouch:
;
}

void g(void) {
({
goto ouch;
int i = 12;
});

ouch:
;
}

https://godbolt.org/z/Eda59KqPT

It's possible that this is intentional given that statement expressions tend to be hidden behind macros, but this seems inconsistent and somewhat dangerous to allow.
</details>

[efriedma-quic]

I'm not sure what you think is wrong with g(): it's jumping "past" the initialization, but it's jumping out of the relevant scope.

Jumping out of the initialization of a variable, into the scope of that variable, should be forbidden in the same cases we generally forbid jumps. Probably just need to adjust the scopes to make sure the jump-scope of a variable starts after its initializer.

[pinskia]

I think Aaron's second example should have been:

void g(void) {          
  int i1 = ({ 
    goto ouch;  
    12;
  });
ouch:
  ;
}

Which leaves i1 uninitialized.

[pinskia]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120076