Use of uninitialized variable is not reported when followed by initialization in loop

[Aaron1011]

Bugzilla Link	49029
Version	11.0
OS	Linux
CC	@DougGregor,@zygoloid

Extended Description

The following code does not produce any warnings when compiled with 'clang++ weird.cpp -Wall'

int main() {
    int x;
    for (int i = 0; i < 10; i++) {
        x += 1;
        x = 0;
    }
}

However, 'x' is uninitialized during the initial iteration through the loop. If the 'x = 0;' line is removed:

int main() {
    int x;
    for (int i = 0; i < 10; i++) {
        x += 1;
    }
}

then a warning is produced when compiled with 'clang++ weird.cpp -Wall'

weird.cpp:4:9: warning: variable 'x' is uninitialized when used here [-Wuninitialized]
        x += 1;
        ^
weird.cpp:2:10: note: initialize the variable 'x' to silence this warning
    int x;
         ^
          = 0
1 warning generated.

The statement 'x = 0;' occurs after the uninitialized use of x, so it should not cause the warning to be suppressed.

[wheatman]

The same behavior is still seen on post 16 trunk (c3b27c236d6a495acbe2ba95c43faf9a98e81a46) with the warning appearing only when the x=0 line is removed

A slightly different warning does appear with -Weverything

When the code is

int main() {
    int x;
    for (int i = 0; i < 10; i++) {
        x += 1;
    }
    return x;
}

and compiled with -Wall then the warning is warning: variable 'x' is uninitialized when used here [-Wuninitialized]

When you put back in the x=0 giving

int main() {
    int x;
    for (int i = 0; i < 10; i++) {
        x += 1;
        x = 0;
    }
    return x;
}

There is no warnings with -Wall but if you instead use -Weverything then you get the warning

warning: variable 'x' may be uninitialized when used here [-Wconditional-uninitialized]

https://godbolt.org/z/rfnMbY683