Why does -Werror=unused-variable not detect some unused variables?

Question

This code

#include <vector>

int main()
{
        std::vector< double > x;
        int k,l;
}

compiled with Apple clang version 11.0.0 as such:

 > clang test.cpp -std=c++14 -Werror=unused-variable

generates only a subset of the expected errors (unused vector is not detected):

test.cpp:6:13: error: unused variable 'k' [-Werror,-Wunused-variable]
        int k,l;
            ^
test.cpp:6:15: error: unused variable 'l' [-Werror,-Wunused-variable]
        int k,l;
              ^
2 errors generated.

GCC 5 also does not abort here.

The GCC doc does not mention any caveats here

Warn whenever a local or static variable is unused aside from its declaration. This option implies -Wunused-const-variable=1 for C, but not for C++. This warning is enabled by -Wall. To suppress this warning use the unused attribute (see Variable Attributes).