Don't warn about an unreachable fallthrough annotation in a template function

[ahmedasadi]

Summary

Clang currently doesn't warn about unreachable code in template functions, as it may not be unreachable in all instantiations.

Do the same for unreachable fallthrough annotations.

[rsmith]

This needs a test case, but the change itself looks fine to me.

[ahmedasadi]

Added a test case.

[rsmith]

LGTM, do you need someone to commit for you?

[ahmedasadi]

Thanks for reviewing. Yes, I need someone to commit for me.

lib/Sema/AnalysisBasedWarnings.cpp

       }
     }
 
-    bool checkFallThroughIntoBlock(const CFGBlock &B, int &AnnotatedCnt) {
+    bool checkFallThroughIntoBlock(const CFGBlock &B, int &AnnotatedCnt, bool isTemplateInstantiation) {
       assert(!ReachableBlocks.empty() && "ReachableBlocks empty");
 
       int UnannotatedCnt = 0;

                ElemIt != ElemEnd; ++ElemIt) {
             if (Optional<CFGStmt> CS = ElemIt->getAs<CFGStmt>()) {
               if (const AttributedStmt *AS = asFallThroughAttr(CS->getStmt())) {
-                S.Diag(AS->getLocStart(),
-                       diag::warn_fallthrough_attr_unreachable);
+                // Don't issue a warning for an unreachable fallthrough
+                // attribute in template instantiations as it may not be
+                // unreachable in all instantiations of the template
+                if (!isTemplateInstantiation)
+                    S.Diag(AS->getLocStart(),
+                           diag::warn_fallthrough_attr_unreachable);
                 markFallthroughVisited(AS);
                 ++AnnotatedCnt;
                 break;

 
     int AnnotatedCnt;
 
-    if (!FM.checkFallThroughIntoBlock(*B, AnnotatedCnt))
+    bool isTemplateInstantiation = false;
+    if (const FunctionDecl *Function = dyn_cast<FunctionDecl>(AC.getDecl()))
+        isTemplateInstantiation = Function->isTemplateInstantiation();
+    if (!FM.checkFallThroughIntoBlock(*B, AnnotatedCnt, isTemplateInstantiation))
       continue;
 
     S.Diag(Label->getLocStart(),