Disable -Wimplicit-fallthrough when not using C++.

The rationale is that there is no good workflow to silence the warning
for specific cases, other than using pragmas.  This is because the
attribute to decorate an explicit fall through is only available
in C++11.

By that argument, this should probably also be disabled unless one
is using C++11, but apparently there is an explicit test case for
this warning when using C++98.  This will require further discussion
on cfe-commits.

Fixes:  <rdar://problem/12584746>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@167655 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 13 insertions, 0 deletions

diff --git a/lib/Sema/AnalysisBasedWarnings.cpp b/lib/Sema/AnalysisBasedWarnings.cpp
index a20817f965..5c21ea4867 100644
--- a/lib/Sema/AnalysisBasedWarnings.cpp
+++ b/lib/Sema/AnalysisBasedWarnings.cpp
@@ -818,6 +818,19 @@ namespace {
 
 static void DiagnoseSwitchLabelsFallthrough(Sema &S, AnalysisDeclContext &AC,
                                             bool PerFunction) {
+  // Only perform this analysis when using C++.  There is no good workflow
+  // for this warning when using straight C.  There is no good way to silence
+  // the warning (no attribute is available) unless we are using C++11's support
+  // for generalized attributes.  Once could use pragmas to silence the warning,
+  // but as a general solution that is gross and not in the spirit of this
+  // warning.
+  //
+  // NOTE: this argument also applies to C++ code not using C++11, as the
+  // generalized attributes are not available in earlier C++ dialects.
+  // This will require some discussion.
+  if (!AC.getASTContext().getLangOpts().CPlusPlus)
+    return;
+
   FallthroughMapper FM(S);
   FM.TraverseStmt(AC.getBody());