Index: lib/StaticAnalyzer/Core/RegionStore.cpp
===================================================================
--- lib/StaticAnalyzer/Core/RegionStore.cpp
+++ lib/StaticAnalyzer/Core/RegionStore.cpp
@@ -230,11 +230,6 @@
 }
 
 Optional<SVal> RegionBindingsRef::getDefaultBinding(const MemRegion *R) const {
-  if (R->isBoundable())
-    if (const TypedValueRegion *TR = dyn_cast<TypedValueRegion>(R))
-      if (TR->getValueType()->isUnionType())
-        return UnknownVal();
-
   return Optional<SVal>::create(lookup(R, BindingKey::Default));
 }
 
@@ -1099,7 +1094,7 @@
     return;
   }
 
-  if (T->isStructureOrClassType()) {
+  if (T->isRecordType()) {
     // Invalidate the region by setting its default value to
     // conjured symbol. The type of the symbol is irrelevant.
     DefinedOrUnknownSVal V = svalBuilder.conjureSymbolVal(baseR, Ex, LCtx,
Index: test/Analysis/unions.cpp
===================================================================
--- test/Analysis/unions.cpp
+++ test/Analysis/unions.cpp
@@ -79,8 +79,7 @@
     IntOrString vv;
     vv.i = 5;
     uu = vv;
-    // FIXME: Should be true.
-    clang_analyzer_eval(uu.i == 5); // expected-warning{{UNKNOWN}}
+    clang_analyzer_eval(uu.i == 5); // expected-warning{{TRUE}}
   }
 
   void testInvalidation() {
@@ -106,3 +105,20 @@
     clang_analyzer_eval(uu.s[0] == 'a'); // expected-warning{{UNKNOWN}}
   }
 }
+
+namespace assume_union_contents {
+union U {
+  int x;
+};
+
+U get();
+
+void test() {
+  U u = get();
+  int y = 0;
+  if (u.x)
+    y = 1;
+  if (u.x)
+    y = 1 / y; // no-warning
+}
+} // end namespace assume_union_contents