Index: include/llvm/Analysis/AliasAnalysis.h
===================================================================
--- include/llvm/Analysis/AliasAnalysis.h
+++ include/llvm/Analysis/AliasAnalysis.h
@@ -963,7 +963,7 @@
   /// top-level \c AAResults aggregation if one is registered, or to the
   /// current result if none are registered.
   class AAResultsProxy {
-    AAResults *AAR;
+    AAResults *AAR = nullptr;
     DerivedT &CurrentResult;
 
   public:
Index: unittests/Analysis/BasicAliasAnalysisTest.cpp
===================================================================
--- unittests/Analysis/BasicAliasAnalysisTest.cpp
+++ unittests/Analysis/BasicAliasAnalysisTest.cpp
@@ -64,13 +64,10 @@
       : M("BasicAATest", C), B(C), DL(DLString), TLI(TLII), F(nullptr) {}
 };
 
-// FIXME: Both of these are disabled at the moment due to strange buildbot
-// failures. Please see https://bugs.llvm.org/show_bug.cgi?id=42719
-
 // Check that a function arg can't trivially alias a global when we're accessing
 // >sizeof(global) bytes through that arg, unless the access size is just an
 // upper-bound.
-TEST_F(BasicAATest, DISABLED_AliasInstWithObjectOfImpreciseSize) {
+TEST_F(BasicAATest, AliasInstWithObjectOfImpreciseSize) {
   F = Function::Create(
       FunctionType::get(B.getVoidTy(), {B.getInt32Ty()->getPointerTo()}, false),
       GlobalValue::ExternalLinkage, "F", &M);
@@ -104,7 +101,7 @@
 
 // Check that we fall back to MayAlias if we see an access of an entire object
 // that's just an upper-bound.
-TEST_F(BasicAATest, DISABLED_AliasInstWithFullObjectOfImpreciseSize) {
+TEST_F(BasicAATest, AliasInstWithFullObjectOfImpreciseSize) {
   F = Function::Create(
       FunctionType::get(B.getVoidTy(), {B.getInt64Ty()}, false),
       GlobalValue::ExternalLinkage, "F", &M);