Partial fix for #15048 Inconsistent error severity for uninitvar - #8966

AI review

The change makes the severity less consistent and turns definite bugs into warnings. Whether Value::conditional is set depends on the code after the if, not on whether the read is guarded by a condition. Two examples:

• if (b) return i; return 0; is still reported as an error.
• if (b) return i; i = 1; return i; is now reported as a warning.

In both functions the read inside the branch is uninitialized every time it runs, so the second case should still be an error. Calls such as if (b) g(i); i = 1; g(i); are also demoted to a warning, although if (b) g(i); alone stays an error.

For if (b) return i; return i;, the read at line 17 is now a warning, while the unconditional return i; on the next line is reported only by legacyUninitvar (as an error). The new test makes the demotion of a definite bug the expected result. I found no crashes.

Testing

I built cppcheck and testrunner in both pr/ and base/.

• ./testrunner TestUninitVar: all passed.
• Full ./testrunner: all passed.

Test file b.cpp:

int f1(bool b) {
    int i;
    if (b)
        return i;
    i = 1;
    return i;
}
int f3(bool b) {
    int i;
    if (b)
        return i;
    return 0;
}
int f4(bool b) {
    int i;
    if (b)
        return i;
    return i;
}
int f5(bool b) {
    int i;
    if (b)
        return i + 1;
    i = 2;
    return i;
}
void g(int);
void f6(bool b) {
    int i;
    if (b)
        g(i);
    i = 1;
    g(i);
}

Command: cppcheck -q --template='{line}:{severity}:{id}:{message}' b.cpp

base:

4:error:uninitvar:Uninitialized variable: i
11:error:uninitvar:Uninitialized variable: i
17:error:uninitvar:Uninitialized variable: i
23:error:uninitvar:Uninitialized variable: i
31:error:uninitvar:Uninitialized variable: i
18:error:legacyUninitvar:Uninitialized variable: i

PR:

4:warning:uninitvar:Uninitialized variable: i
11:error:uninitvar:Uninitialized variable: i
17:warning:uninitvar:Uninitialized variable: i
23:warning:uninitvar:Uninitialized variable: i
31:warning:uninitvar:Uninitialized variable: i
18:error:legacyUninitvar:Uninitialized variable: i

Lines 4 and 11 are the same kind of read, but they now get different severities. Lines 4, 17, 23 and 31 are reads that are always uninitialized whenever they run, yet they were demoted from error to warning.

A second set of 14 one-line variants (if (b) g(i);, loops, switch, ?:, &&, nested if, pointer dereference) gave the same results in base and PR, apart from the case above.

Automated review of fe389d3 by Claude. The pull request was built and tested locally.