[Sema] Disallow returning a __block variable via a move.

r274291 made changes to prefer calling a move constructor to calling a
copy constructor when returning from a function. This caused programs to
crash when a __block variable in the heap was moved out and used later.

This commit fixes the bug by disallowing moving out of __block variables
implicitly.

rdar://problem/28181080

Differential Revision: https://reviews.llvm.org/D29908

llvm-svn: 295150

2 files changed, 20 insertions, 4 deletions

diff --git a/clang/lib/Sema/SemaStmt.cpp b/clang/lib/Sema/SemaStmt.cpp
index a8832e9a1c5..390e1b52c8e 100644
--- a/clang/lib/Sema/SemaStmt.cpp
+++ b/clang/lib/Sema/SemaStmt.cpp
@@ -2743,15 +2743,17 @@ bool Sema::isCopyElisionCandidate(QualType ReturnType, const VarDecl *VD,
   // ...automatic...
   if (!VD->hasLocalStorage()) return false;
 
+  // Return false if VD is a __block variable. We don't want to implicitly move
+  // out of a __block variable during a return because we cannot assume the
+  // variable will no longer be used.
+  if (VD->hasAttr<BlocksAttr>()) return false;
+
   if (AllowParamOrMoveConstructible)
     return true;
 
   // ...non-volatile...
   if (VD->getType().isVolatileQualified()) return false;
 
-  // __block variables can't be allocated in a way that permits NRVO.
-  if (VD->hasAttr<BlocksAttr>()) return false;
-
   // Variables with higher required alignment than their type's ABI
   // alignment cannot use NRVO.
   if (!VD->getType()->isDependentType() && VD->hasAttr<AlignedAttr>() &&
diff --git a/clang/test/SemaObjCXX/blocks.mm b/clang/test/SemaObjCXX/blocks.mm
index 09d614d3728..3f901cc0a84 100644
--- a/clang/test/SemaObjCXX/blocks.mm
+++ b/clang/test/SemaObjCXX/blocks.mm
@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify -fblocks -Wno-objc-root-class %s
+// RUN: %clang_cc1 -fsyntax-only -verify -fblocks -Wno-objc-root-class -std=c++11 %s
 @protocol NSObject;
 
 void bar(id(^)(void));
@@ -144,3 +144,17 @@ namespace DependentReturn {
 
   template void f<X>(X);
 }
+
+namespace MoveBlockVariable {
+struct B0 {
+};
+
+struct B1 { // expected-note 2 {{candidate constructor (the implicit}}
+  B1(B0&&); // expected-note {{candidate constructor not viable}}
+};
+
+B1 test_move() {
+  __block B0 b;
+  return b; // expected-error {{no viable conversion from returned value of type 'MoveBlockVariable::B0' to function return type 'MoveBlockVariable::B1'}}
+}
+}