Add extension to always default-initialize nullptr_t.

Core issue 1013 suggests that having an uninitialied std::nullptr_t be
UB is a bit foolish, since there is only a single valid value. This DR
reports that DR616 fixes it, which does so by making lvalue-to-rvalue
conversions from nullptr_t be equal to nullptr.

However, just implementing that results in warnings/etc in many places.
In order to fix all situations where nullptr_t would seem uninitialized,
this patch instead (as an otherwise transparent extension) default
initializes uninitialized VarDecls of nullptr_t.

Differential Revision: https://reviews.llvm.org/D53713

Change-Id: I84d72a9290054fa55341e8cbdac43c8e7f25b885

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@349201 91177308-0d34-0410-b5e6-96231b3b80d8

3 files changed, 17 insertions, 12 deletions

diff --git a/test/Analysis/nullptr.cpp b/test/Analysis/nullptr.cpp
index 38e099b7fb..1d913c11d3 100644
--- a/test/Analysis/nullptr.cpp
+++ b/test/Analysis/nullptr.cpp
@@ -125,21 +125,16 @@ struct Type {
 };
 
 void shouldNotCrash() {
-  decltype(nullptr) p; // expected-note{{'p' declared without an initial value}}
-  if (getSymbol()) // expected-note   {{Assuming the condition is false}}
-                   // expected-note@-1{{Taking false branch}}
-                   // expected-note@-2{{Assuming the condition is false}}
-                   // expected-note@-3{{Taking false branch}}
-                   // expected-note@-4{{Assuming the condition is true}}
-                   // expected-note@-5{{Taking true branch}}
-    invokeF(p); // expected-warning{{1st function call argument is an uninitialized value}}
-                // expected-note@-1{{1st function call argument is an uninitialized value}}
+  decltype(nullptr) p; // expected-note{{'p' initialized to a null pointer value}}
   if (getSymbol()) // expected-note   {{Assuming the condition is false}}
                    // expected-note@-1{{Taking false branch}}
                    // expected-note@-2{{Assuming the condition is true}}
                    // expected-note@-3{{Taking true branch}}
-    invokeF(nullptr); // expected-note   {{Calling 'invokeF'}}
-                      // expected-note@-1{{Passing null pointer value via 1st parameter 'x'}}
+    invokeF(p); // expected-note{{Passing null pointer value via 1st parameter 'x'}}
+                // expected-note@-1{{Calling 'invokeF'}}
+  if (getSymbol()) // expected-note   {{Assuming the condition is false}}
+                   // expected-note@-1{{Taking false branch}}
+    invokeF(nullptr);
   if (getSymbol()) {  // expected-note  {{Assuming the condition is true}}
                       // expected-note@-1{{Taking true branch}}
     X *xx = Type().x; // expected-note   {{Null pointer value stored to field 'x'}}
diff --git a/test/SemaCXX/ast-print-crash.cpp b/test/SemaCXX/ast-print-crash.cpp
index c108f666d7..33edc34823 100644
--- a/test/SemaCXX/ast-print-crash.cpp
+++ b/test/SemaCXX/ast-print-crash.cpp
@@ -7,6 +7,6 @@
 
 // CHECK:      struct {
 // CHECK-NEXT: } dont_crash_on_syntax_error;
-// CHECK-NEXT: decltype(nullptr) p;
+// CHECK-NEXT: decltype(nullptr) p(/*implicit*/(decltype(nullptr))0);
 struct {
 } dont_crash_on_syntax_error /* missing ; */ decltype(nullptr) p;
diff --git a/test/SemaCXX/nullptr_t-init.cpp b/test/SemaCXX/nullptr_t-init.cpp
new file mode 100644
index 0000000000..f7843de1bc
--- /dev/null
+++ b/test/SemaCXX/nullptr_t-init.cpp
@@ -0,0 +1,10 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 -ffreestanding -Wuninitialized %s
+// expected-no-diagnostics
+typedef decltype(nullptr) nullptr_t;
+
+// Ensure no 'uninitialized when used here' warnings (Wuninitialized), for 
+// nullptr_t always-initialized extension.
+nullptr_t default_init() {
+  nullptr_t a;
+  return a;
+}