Add a warning for when an array-to-pointer decay is performed on an array

temporary or an array subobject of a class temporary, and the resulting value
is used to initialize a pointer which outlives the temporary. Such a pointer
is always left dangling after the initialization completes and the array's
lifetime ends.

In order to detect this situation, this change also adds an
LValueClassification of LV_ArrayTemporary for temporaries of array type which
aren't subobjects of class temporaries. These occur in C++11 T{...} and GNU C++
(T){...} expressions, when T is an array type. Previously we treated the former
as a generic prvalue and the latter as a class temporary.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@157955 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 49 insertions, 0 deletions

diff --git a/lib/Sema/SemaInit.cpp b/lib/Sema/SemaInit.cpp
index 66cc1e14e5..66a64a9e5c 100644
--- a/lib/Sema/SemaInit.cpp
+++ b/lib/Sema/SemaInit.cpp
@@ -4747,6 +4747,43 @@ PerformConstructorInitialization(Sema &S,
   return move(CurInit);
 }
 
+/// Determine whether the specified InitializedEntity definitely has a lifetime
+/// longer than the current full-expression. Conservatively returns false if
+/// it's unclear.
+static bool
+InitializedEntityOutlivesFullExpression(const InitializedEntity &Entity) {
+  const InitializedEntity *Top = &Entity;
+  while (Top->getParent())
+    Top = Top->getParent();
+
+  switch (Top->getKind()) {
+  case InitializedEntity::EK_Variable:
+  case InitializedEntity::EK_Result:
+  case InitializedEntity::EK_Exception:
+  case InitializedEntity::EK_Member:
+  case InitializedEntity::EK_New:
+  case InitializedEntity::EK_Base:
+  case InitializedEntity::EK_Delegating:
+    return true;
+
+  case InitializedEntity::EK_ArrayElement:
+  case InitializedEntity::EK_VectorElement:
+  case InitializedEntity::EK_BlockElement:
+  case InitializedEntity::EK_ComplexElement:
+    // Could not determine what the full initialization is. Assume it might not
+    // outlive the full-expression.
+    return false;
+
+  case InitializedEntity::EK_Parameter:
+  case InitializedEntity::EK_Temporary:
+  case InitializedEntity::EK_LambdaCapture:
+    // The entity being initialized might not outlive the full-expression.
+    return false;
+  }
+
+  llvm_unreachable("unknown entity kind");
+}
+
 ExprResult
 InitializationSequence::Perform(Sema &S,
                                 const InitializedEntity &Entity,
@@ -4826,6 +4863,18 @@ InitializationSequence::Perform(Sema &S,
       << Init->getSourceRange();
   }
 
+  // Diagnose cases where we initialize a pointer to an array temporary, and the
+  // pointer obviously outlives the temporary.
+  if (Args.size() == 1 && Args.get()[0]->getType()->isArrayType() &&
+      Entity.getType()->isPointerType() &&
+      InitializedEntityOutlivesFullExpression(Entity)) {
+    Expr *Init = Args.get()[0];
+    Expr::LValueClassification Kind = Init->ClassifyLValue(S.Context);
+    if (Kind == Expr::LV_ClassTemporary || Kind == Expr::LV_ArrayTemporary)
+      S.Diag(Init->getLocStart(), diag::warn_temporary_array_to_pointer_decay)
+        << Init->getSourceRange();
+  }
+
   QualType DestType = Entity.getType().getNonReferenceType();
   // FIXME: Ugly hack around the fact that Entity.getType() is not
   // the same as Entity.getDecl()->getType() in cases involving type merging,