[Bug c++/91335] New: False positive "unused variable" warning with variable initialized in 'if' condition

[zeratul976 at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91335

            Bug ID: 91335
           Summary: False positive "unused variable" warning with variable
                    initialized in 'if' condition
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zeratul976 at hotmail dot com
  Target Milestone: ---

In the following code:

int* foo();

int main() {
  if (int* f = foo()) {
    return 1;
  }
  return 0;
}

GCC (tested version 9.1.0) issues the following warning with -Wall:

test.cpp: In function ‘int main()’:
test.cpp:4:12: warning: unused variable ‘f’ [-Wunused-variable]
    4 |   if (int* f = foo()) {
      |            ^

I consider this a false positive because testing the variable's value in the
if-condition constitutes a use.

The language does not syntactically permit omitting the variable name here:

int main() {
  if (int* = foo()) {  // error: expected unqualified-id before '=' token
    return 1;
  }
  return 0;
}

As a result, the only way to fix the warning is to lift the variable into the
outer scope, which can be undesirable from a code style point of view:

int* foo();

int main() {
  int* f = foo();
  if (f) {
    return 1;
  }
  // variable 'f' still in scope here, when I don't want it to be
  return 0;
}