Background

ccache currently can't share cache entries between these two cases:

1. ccache clang -c source.c
2. ccache clang -c source.c 2>/dev/null

We can fix it. See also the discussion in #222.

What to do

Here's a sketch of what could be done to fix the problem:

If the compiler is clang:

• Don't include any explicit -f(no-)color-diagnostics options in the hash.
• Strip any explicit -fno-color-diagnostics options from the compiler arguments and add a -fcolor-diagnostics option (if not already present) to force colors.
• On a cache hit: Strip color codes from the stored stderr if -fno-color-diagnostics has been passed OR (stderr does not refer to a TTY AND -fcolor-diagnostics has not been passed).
• Prove that it works by adding a test case that checks that color codes are stored in the cache and that color codes are not produced when stderr is redirected.
• Remove the corresponding caveat from the documentation.

See also #224 which is somewhat similar but for GCC.