r349201 - Add extension to always default-initialize nullptr_t.

[Erich Keane via cfe-commits]

Author: erichkeane
Date: Fri Dec 14 14:22:29 2018
New Revision: 349201

URL: http://llvm.org/viewvc/llvm-project?rev=349201&view=rev
Log:
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

Added:
    cfe/trunk/test/SemaCXX/nullptr_t-init.cpp   (with props)
Modified:
    cfe/trunk/lib/Sema/SemaInit.cpp
    cfe/trunk/test/Analysis/nullptr.cpp
    cfe/trunk/test/SemaCXX/ast-print-crash.cpp

Modified: cfe/trunk/lib/Sema/SemaInit.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Sema/SemaInit.cpp?rev=349201&r1=349200&r2=349201&view=diff
==============================================================================
--- cfe/trunk/lib/Sema/SemaInit.cpp (original)
+++ cfe/trunk/lib/Sema/SemaInit.cpp Fri Dec 14 14:22:29 2018
@@ -4881,6 +4881,13 @@ static void TryDefaultInitialization(Sem
     return;
   }
 
+  // As an extension, and to fix Core issue 1013, zero initialize nullptr_t.
+  // Since there is only 1 valid value of nullptr_t, we can just use that.
+  if (DestType->isNullPtrType()) {
+    Sequence.AddZeroInitializationStep(Entity.getType());
+    return;
+  }
+
   //     - otherwise, no initialization is performed.
 
   //   If a program calls for the default initialization of an object of

Modified: cfe/trunk/test/Analysis/nullptr.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/Analysis/nullptr.cpp?rev=349201&r1=349200&r2=349201&view=diff
==============================================================================
--- cfe/trunk/test/Analysis/nullptr.cpp (original)
+++ cfe/trunk/test/Analysis/nullptr.cpp Fri Dec 14 14:22:29 2018
@@ -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 at -1{{Taking false branch}}
-                   // expected-note at -2{{Assuming the condition is false}}
-                   // expected-note at -3{{Taking false branch}}
-                   // expected-note at -4{{Assuming the condition is true}}
-                   // expected-note at -5{{Taking true branch}}
-    invokeF(p); // expected-warning{{1st function call argument is an uninitialized value}}
-                // expected-note at -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 at -1{{Taking false branch}}
                    // expected-note at -2{{Assuming the condition is true}}
                    // expected-note at -3{{Taking true branch}}
-    invokeF(nullptr); // expected-note   {{Calling 'invokeF'}}
-                      // expected-note at -1{{Passing null pointer value via 1st parameter 'x'}}
+    invokeF(p); // expected-note{{Passing null pointer value via 1st parameter 'x'}}
+                // expected-note at -1{{Calling 'invokeF'}}
+  if (getSymbol()) // expected-note   {{Assuming the condition is false}}
+                   // expected-note at -1{{Taking false branch}}
+    invokeF(nullptr);
   if (getSymbol()) {  // expected-note  {{Assuming the condition is true}}
                       // expected-note at -1{{Taking true branch}}
     X *xx = Type().x; // expected-note   {{Null pointer value stored to field 'x'}}

Modified: cfe/trunk/test/SemaCXX/ast-print-crash.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/SemaCXX/ast-print-crash.cpp?rev=349201&r1=349200&r2=349201&view=diff
==============================================================================
--- cfe/trunk/test/SemaCXX/ast-print-crash.cpp (original)
+++ cfe/trunk/test/SemaCXX/ast-print-crash.cpp Fri Dec 14 14:22:29 2018
@@ -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;

Added: cfe/trunk/test/SemaCXX/nullptr_t-init.cpp
URL: http://llvm.org/viewvc/llvm-project/cfe/trunk/test/SemaCXX/nullptr_t-init.cpp?rev=349201&view=auto
==============================================================================
--- cfe/trunk/test/SemaCXX/nullptr_t-init.cpp (added)
+++ cfe/trunk/test/SemaCXX/nullptr_t-init.cpp Fri Dec 14 14:22:29 2018
@@ -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;
+}

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