Re-land "[Sema] Make getCurFunction() return null outside function parsing"

This relands r326965.

There was a null dereference in typo correction that was triggered in
Sema/diagnose_if.c. We are not always in a function scope when doing
typo correction. The fix is to add a null check.

LLVM's optimizer made it hard to find this bug. I wrote it up in a
not-very-well-editted blog post here:
http://qinsb.blogspot.com/2018/03/ub-will-delete-your-null-checks.html

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@327334 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 2 insertions, 1 deletions

diff --git a/lib/Sema/SemaLookup.cpp b/lib/Sema/SemaLookup.cpp
index dd54fb4ad9..692ce111aa 100644
--- a/lib/Sema/SemaLookup.cpp
+++ b/lib/Sema/SemaLookup.cpp
@@ -4529,7 +4529,8 @@ static void AddKeywordsToConsumer(Sema &SemaRef,
       if (S && S->getContinueParent())
         Consumer.addKeywordResult("continue");
 
-      if (!SemaRef.getCurFunction()->SwitchStack.empty()) {
+      if (SemaRef.getCurFunction() &&
+          !SemaRef.getCurFunction()->SwitchStack.empty()) {
         Consumer.addKeywordResult("case");
         Consumer.addKeywordResult("default");
       }