Problem:
- On Apple clang 11.0.3 (clang-1103.0.32.62), pedantic mode compilation
generates the following error:
test/std-format-test.cc:114:22: error: implicit conversion changes
signedness: 'int' to 'size_t' (aka 'unsigned long')
[-Werror,-Wsign-conversion]
width_arg_id = c - '0';
~ ~~^~~~~
Solution:
- Use a `to_unsigned` to make the conversion explicit. This is
guaranteed to be safe due to the check before the ASCII-to-int
conversion.