[PATCH] Add -C when using -Wimplicit-fallthrough and --save-temps (PR preprocessor/78497).

[Martin Liška]

Hi.

This helps the warning with --save-temps. Doing that one needs to preserve 
comments
in preprocessed source file.

Ready for trunk?
Martin

gcc/ChangeLog:

2018-04-03  Martin Liska  <mli...@suse.cz>

        PR preprocessor/78497
        * gcc.c: Add -C when using -Wimplicit-fallthrough and --save-temps.

gcc/testsuite/ChangeLog:

2018-04-03  Martin Liska  <mli...@suse.cz>

        PR preprocessor/78497
        * c-c++-common/Wimplicit-fallthrough-37.c: New test.
---
 gcc/gcc.c                                          |  3 ++-
 .../c-c++-common/Wimplicit-fallthrough-37.c        | 22 ++++++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/c-c++-common/Wimplicit-fallthrough-37.c



diff --git a/gcc/gcc.c b/gcc/gcc.c
index a716f708259..f641c249e27 100644
--- a/gcc/gcc.c
+++ b/gcc/gcc.c
@@ -1131,7 +1131,8 @@ static const char *cpp_options =
 "%(cpp_unique_options) %1 %{m*} %{std*&ansi&trigraphs} %{W*&pedantic*} %{w}\
  %{f*} %{g*:%{%:debug-level-gt(0):%{g*}\
  %{!fno-working-directory:-fworking-directory}}} %{O*}\
- %{undef} %{save-temps*:-fpch-preprocess}";
+ %{undef} %{save-temps*:-fpch-preprocess}\
+ %{Wimplicit-fallthrough*|Werror=implicit-fallthrough*:%{save-temps*:-C}}";
 
 /* This contains cpp options which are not passed when the preprocessor
    output will be used by another program.  */
diff --git a/gcc/testsuite/c-c++-common/Wimplicit-fallthrough-37.c b/gcc/testsuite/c-c++-common/Wimplicit-fallthrough-37.c
new file mode 100644
index 00000000000..ca9d21fc70e
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/Wimplicit-fallthrough-37.c
@@ -0,0 +1,22 @@
+/* PR preprocessor/78497 */
+/* { dg-do compile } */
+/* { dg-options "-Wimplicit-fallthrough --save-temps" } */
+
+int main (int argc, char **argv)
+{
+  int a;
+  switch (argc)
+    {
+    case 1:
+      a = 1;
+      break;
+    case 2:
+      a = 2;
+      /* FALLTHROUGH */
+    case 3:
+      a = 3;
+      break;
+    }
+
+  return a;
+}