[Sema, NFC] Reset HasFallthroughStmt when clearing FunctionScopeInfo

[erik.pilkington]

Summary

The FunctionScopeInfo stack in Sema uses an optimization where the memory for the top-level functions is reused. The function FunctionScopeInfo::Clear() is used to reset this memory to the original, marking all the flags as false. The flag HasFallthroughStmt was omitted from this, meaning that once it set, it remained so for the rest of the TU. This meant we ran DiagnoseSwitchLabelsFallthrough() for every top level function in a TU where one function used [[fallthrough]].

Thanks!

[arphaman]

LGTM, Thanks.

[aaron.ballman]

I don't suppose there's a testcase we could generate for this fix?

[arphaman]

I looked at the way HasFallthroughStmt is used, and it didn't seem so, no. It's used in the following manner in AnalysisBasedWarnings.cpp:

bool FallThroughDiagFull =
    !Diags.isIgnored(diag::warn_unannotated_fallthrough, D->getLocStart());
bool FallThroughDiagPerFunction = !Diags.isIgnored(
    diag::warn_unannotated_fallthrough_per_function, D->getLocStart());
if (FallThroughDiagFull || FallThroughDiagPerFunction ||
    fscope->HasFallthroughStmt) {
  DiagnoseSwitchLabelsFallthrough(S, AC, !FallThroughDiagFull);
}

So it seems to me that even if HasFallthroughStmt isn't cleared, the analysis won't show up anything different for a function that re-used the scope info, because when diagnostics are ignored and the flag actually leads to incorrect call to DiagnoseSwitchLabelsFallthrough , the analysis won't show any warnings since the diagnostics are ignored. And likewise if one of the diagnostics is enabled then it doesn't matter if HasFallthroughStmt is true, so we won't be able to observe a difference there.

lib/Sema/ScopeInfo.cpp

 using namespace sema;
 
 void FunctionScopeInfo::Clear() {
   HasBranchProtectedScope = false;
   HasBranchIntoScope = false;
   HasIndirectGoto = false;
   HasDroppedStmt = false;
   HasOMPDeclareReductionCombiner = false;
+  HasFallthroughStmt = false;
   ObjCShouldCallSuper = false;
   ObjCIsDesignatedInit = false;
   ObjCWarnForNoDesignatedInitChain = false;
   ObjCIsSecondaryInit = false;
   ObjCWarnForNoInitDelegation = false;
   FirstReturnLoc = SourceLocation();
   FirstCXXTryLoc = SourceLocation();
   FirstSEHTryLoc = SourceLocation();