Partial fix for #15048 Inconsistent error severity for invalidFunctionArg - #8962

AI review

I found no new problems. With this change, invalidFunctionArg drops to warning when the value is conditional, the same as zerodiv/zerodivcond, negativeIndex, negativeMemoryAllocationSize and the STL out-of-bounds check. Values that are known and unconditional still give error.

The downgrade also applies when the invalid value reaches the call on every path, and the "condition" is only an early return before it. For example, int n = -1; if (b) return; memset(buf, 0, n); is now a warning. The other checks above already behave this way (int n = 0; if (b) return 0; return 1 / n; gives zerodivcond warning), so this is consistent and not a regression.

Testing

• Built cppcheck and testrunner in pr/, and cppcheck in base/.
• ./testrunner TestFunctions: 61 tests, 0 failed, 7 todos.
• Compared base and PR on scratch cases with cppcheck -q --enable=warning --template='{line}:{severity}:{id}:{message}':
  • double d=-1.0; if (b) return d; return std::log(d); (the PR's test case): base error, PR warning.
  • double d=-1.0; return std::log(d);: error in both, so unconditional known values are unchanged.
  • if (x == -1) {} return std::sqrt(x);: warning in both.
  • int n=-1; if (b) return; memset(buf,0,n);: base error, PR warning.
  • int c=300; if (b) c=1; return isalpha(c);: warning in both.
• For consistency I ran the PR binary on int n=0; if (b) return 0; return 1/n;, which gives zerodivcond warning, and on int n=-1; if (b) return; buf[n]=0;, which gives negativeIndex warning. The new invalidFunctionArg severity matches both.
• No crashes or other differences seen.

Automated review of a718c2e by Claude. The pull request was built and tested locally.