Flow analysis and _Defer

[thradams]

Consider:

void f2(int i) {}
void f(int k) {
    int i;
    _Defer f2(i);

    if (k > 1) return;
    
    i = 1;

    if (k > 2) return;

    i = 2;
}
int main() {}

We have two warnings:

<source>:4:15: warning: variable 'i' is uninitialized when used here [-Wuninitialized]
    4 |     _Defer f2(i);
      |               ^
<source>:3:10: note: initialize the variable 'i' to silence this warning
    3 |     int i;
      |          ^
      |           = 0
1 warning generated.
ASM generation compiler returned: 0
<source>:4:15: warning: variable 'i' is uninitialized when used here [-Wuninitialized]
    4 |     _Defer f2(i);
      |               ^
<source>:3:10: note: initialize the variable 'i' to silence this warning
    3 |     int i;
      |          ^
      |           = 0
1 warning generated.
Execution build compiler returned: 0
Program returned: 0

https://godbolt.org/z/3sz7cT8Wv

But the uninitialized warning should be only at if (k > 1) return;.

Also, any warning captured by flow analysis should print the line were defer is executed.

[efriedma-quic]

Yes, it looks like the CFG handling in clang/lib/Analysis/CFG.cpp is completely missing; we construct the CFG as if the user omitted the _Defer keyword.

I think fixing that should naturally improve the diagnostics, although some additional followup might be necessary for some cases.

[efriedma-quic]

CC @Sirraide @cor3ntin @AaronBallman