[analyzer] Fix false negative on post-increment of uninitialized variable.

Summary:
Currently clang static analyzer does warn on:
```
int x;
x+=1;
x-=1;
x=x+1;
x=x-1;
```
But does warn on:
```
int x;
x++;
x--;
--x;
++x;
```

This differential should fix that.
Fixes https://bugs.llvm.org/show_bug.cgi?id=35419

Reviewers: dcoughlin, NoQ

Reviewed By: dcoughlin

Subscribers: NoQ, xazax.hun, szepet, cfe-commits, a.sidorin

Tags: #clang

Differential Revision: https://reviews.llvm.org/D40463

llvm-svn: 319411

1 files changed, 8 insertions, 0 deletions

diff --git a/clang/lib/StaticAnalyzer/Checkers/UndefinedAssignmentChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/UndefinedAssignmentChecker.cpp
index 7a31efc8cef..c3dcf1fac19 100644
--- a/clang/lib/StaticAnalyzer/Checkers/UndefinedAssignmentChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/UndefinedAssignmentChecker.cpp
@@ -60,6 +60,14 @@ void UndefinedAssignmentChecker::checkBind(SVal location, SVal val,
   const Expr *ex = nullptr;
 
   while (StoreE) {
+    if (const UnaryOperator *U = dyn_cast<UnaryOperator>(StoreE)) {
+      str = "The expression is an uninitialized value. "
+            "The computed value will also be garbage";
+
+      ex = U->getSubExpr();
+      break;
+    }
+
     if (const BinaryOperator *B = dyn_cast<BinaryOperator>(StoreE)) {
       if (B->isCompoundAssignmentOp()) {
         ProgramStateRef state = C.getState();