[analyzer] Treat fields of unions as having symbolic offsets.

This allows only one field to be active at a time in RegionStore.
This isn't quite the correct behavior for unions, but it at least
would handle the case of "value goes in, value comes out" from the
same field.

RegionStore currently has a number of places where any access to a union
results in UnknownVal being returned. However, it is clearly missing
some cases, or the original issue wouldn't have occurred. It is probably
now safe to remove those changes, but that's a potentially destabilizing
change that should wait for more thorough testing.

Fixes PR14054.

llvm-svn: 165660

1 files changed, 5 insertions, 1 deletions

diff --git a/clang/lib/StaticAnalyzer/Core/MemRegion.cpp b/clang/lib/StaticAnalyzer/Core/MemRegion.cpp
index 7c66739558a..da34a799682 100644
--- a/clang/lib/StaticAnalyzer/Core/MemRegion.cpp
+++ b/clang/lib/StaticAnalyzer/Core/MemRegion.cpp
@@ -1168,8 +1168,12 @@ RegionOffset MemRegion::getAsOffset() const {
       R = FR->getSuperRegion();
 
       const RecordDecl *RD = FR->getDecl()->getParent();
-      if (!RD->isCompleteDefinition()) {
+      if (/*RD->isUnion() || */!RD->isCompleteDefinition()) {
         // We cannot compute offset for incomplete type.
+        // For unions, we could treat everything as offset 0, but we'd rather
+        // treat each field as a symbolic offset so they aren't stored on top
+        // of each other, since we depend on things in typed regions actually
+        // matching their types.
         SymbolicOffsetBase = R;
       }