DR1213: element access on an array xvalue or prvalue produces an xvalue. In the

latter case, a temporary array object is materialized, and can be
lifetime-extended by binding a reference to the member access. Likewise, in an
array-to-pointer decay, an rvalue array is materialized before being converted
into a pointer.

This caused IR generation to stop treating file-scope array compound literals
as having static storage duration in some cases in C++; that has been rectified
by modeling such a compound literal as an lvalue. This also improves clang's
compatibility with GCC for those cases.

llvm-svn: 288654

1 files changed, 33 insertions, 5 deletions

diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 4415100ab45..5182e164538 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -4376,6 +4376,16 @@ Sema::CreateBuiltinArraySubscriptExpr(Expr *Base, SourceLocation LLoc,
   Expr *LHSExp = Base;
   Expr *RHSExp = Idx;
 
+  ExprValueKind VK = VK_LValue;
+  ExprObjectKind OK = OK_Ordinary;
+
+  // Per C++ core issue 1213, the result is an xvalue if either operand is
+  // a non-lvalue array, and an lvalue otherwise.
+  if (getLangOpts().CPlusPlus11 &&
+      ((LHSExp->getType()->isArrayType() && !LHSExp->isLValue()) ||
+       (RHSExp->getType()->isArrayType() && !RHSExp->isLValue())))
+    VK = VK_XValue;
+
   // Perform default conversions.
   if (!LHSExp->getType()->getAs<VectorType>()) {
     ExprResult Result = DefaultFunctionArrayLvalueConversion(LHSExp);
@@ -4389,8 +4399,6 @@ Sema::CreateBuiltinArraySubscriptExpr(Expr *Base, SourceLocation LLoc,
   RHSExp = Result.get();
 
   QualType LHSTy = LHSExp->getType(), RHSTy = RHSExp->getType();
-  ExprValueKind VK = VK_LValue;
-  ExprObjectKind OK = OK_Ordinary;
 
   // C99 6.5.2.1p2: the expression e1[e2] is by definition precisely equivalent
   // to the expression *((e1)+(e2)). This means the array "Base" may actually be
@@ -5587,11 +5595,31 @@ Sema::BuildCompoundLiteralExpr(SourceLocation LParenLoc, TypeSourceInfo *TInfo,
   }
 
   // In C, compound literals are l-values for some reason.
-  ExprValueKind VK = getLangOpts().CPlusPlus ? VK_RValue : VK_LValue;
+  // For GCC compatibility, in C++, file-scope array compound literals with
+  // constant initializers are also l-values, and compound literals are
+  // otherwise prvalues.
+  //
+  // (GCC also treats C++ list-initialized file-scope array prvalues with
+  // constant initializers as l-values, but that's non-conforming, so we don't
+  // follow it there.)
+  //
+  // FIXME: It would be better to handle the lvalue cases as materializing and
+  // lifetime-extending a temporary object, but our materialized temporaries
+  // representation only supports lifetime extension from a variable, not "out
+  // of thin air".
+  // FIXME: For C++, we might want to instead lifetime-extend only if a pointer
+  // is bound to the result of applying array-to-pointer decay to the compound
+  // literal.
+  // FIXME: GCC supports compound literals of reference type, which should
+  // obviously have a value kind derived from the kind of reference involved.
+  ExprValueKind VK =
+      (getLangOpts().CPlusPlus && !(isFileScope && literalType->isArrayType()))
+          ? VK_RValue
+          : VK_LValue;
 
   return MaybeBindToTemporary(
-           new (Context) CompoundLiteralExpr(LParenLoc, TInfo, literalType,
-                                             VK, LiteralExpr, isFileScope));
+      new (Context) CompoundLiteralExpr(LParenLoc, TInfo, literalType,
+                                        VK, LiteralExpr, isFileScope));
 }
 
 ExprResult