Only reject -f(no-)color-diagnostics for known GCC compiler

61ce8c4 made it so that ccache rejects -f(no-)color-diagnostics early
for non-Clang compilers. This was needed to avoid false cache hits for
the GCC case, but it had the side effect of rejecting
-fcolor-diagnostics for unknown compilers as well, such as for a
compiler named c++ that in reality is clang++ (which actually accepts
-f(no-)color-diagnostics).

Fix this by simply doing the special case for GCC instead of non-Clang.
This is OK since the speculative handling of color diagnostics options
is only done for GCC and Clang, not for other compiler types.

Fixes #806.

src/argprocessing.cpp

@@ -690,13 +690,13 @@ process_arg(Context& ctx,
     return nullopt;
   }
 
-  if (config.compiler_type() != CompilerType::clang
+  if (config.compiler_type() == CompilerType::gcc
       && (args[i] == "-fcolor-diagnostics"
           || args[i] == "-fno-color-diagnostics")) {
-    // Special case: If a non-Clang compiler gets -f(no-)color-diagnostics we'll
-    // bail out and just execute the compiler. The reason is that we don't
-    // include -f(no-)color-diagnostics in the hash so there can be a false
-    // cache hit in the following scenario:
+    // Special case: If a GCC compiler gets -f(no-)color-diagnostics we'll bail
+    // out and just execute the compiler. The reason is that we don't include
+    // -f(no-)color-diagnostics in the hash so there can be a false cache hit in
+    // the following scenario:
     //
     //   1. ccache gcc -c example.c                      # adds a cache entry
     //   2. ccache gcc -c example.c -fcolor-diagnostics  # unexpectedly succeeds

test/suites/color_diagnostics.bash

@@ -122,6 +122,7 @@ color_diagnostics_test() {
         if $CCACHE_COMPILE -fcolor-diagnostics -c test.c >&/dev/null; then
             test_failed "-fcolor-diagnostics unexpectedly accepted by GCC"
         fi
+        expect_stat 'unsupported compiler option' 1
 
         # ---------------------------------------------------------------------
         TEST "-fcolor-diagnostics not accepted for GCC for cached result"
@@ -135,6 +136,21 @@ color_diagnostics_test() {
         if $CCACHE_COMPILE -fcolor-diagnostics -c test.c >&/dev/null; then
             test_failed "-fcolor-diagnostics unexpectedly accepted by GCC"
         fi
+        expect_stat 'unsupported compiler option' 1
+
+        # ---------------------------------------------------------------------
+        TEST "-fcolor-diagnostics passed to underlying compiler for unknown compiler type"
+
+        generate_code 1 test.c
+
+        if CCACHE_COMPILERTYPE=other $CCACHE_COMPILE -fcolor-diagnostics -c test.c >&/dev/null; then
+            test_failed "-fcolor-diagnostics unexpectedly accepted by GCC"
+        fi
+
+        # Verify that -fcolor-diagnostics was passed to the compiler for the
+        # unknown compiler case, i.e. ccache did not exit early with
+        # "unsupported compiler option".
+        expect_stat 'compile failed' 1
     fi
 
     if $COMPILER_TYPE_CLANG; then