Use ExprResult& instead of Expr *& in Sema

This patch authored by Eric Niebler.

Many methods on the Sema class (e.g. ConvertPropertyForRValue) take Expr
pointers as in/out parameters (Expr *&).  This is especially true for the
routines that apply implicit conversions to nodes in-place.  This design is
workable only as long as those conversions cannot fail.  If they are allowed
to fail, they need a way to report their failures.  The typical way of doing
this in clang is to use an ExprResult, which has an extra bit to signal a
valid/invalid state.  Returning ExprResult is de riguour elsewhere in the Sema
interface.  We suggest changing the Expr *& parameters in the Sema interface
to ExprResult &.  This increases interface consistency and maintainability.

This interface change is important for work supporting MS-style C++
properties.  For reasons explained here
<http://lists.cs.uiuc.edu/pipermail/cfe-dev/2011-February/013180.html>,
seemingly trivial operations like rvalue/lvalue conversions that formerly
could not fail now can.  (The reason is that given the semantics of the
feature, getter/setter method lookup cannot happen until the point of use, at
which point it may be found that the method does not exist, or it may have the
wrong type, or overload resolution may fail, or it may be inaccessible.)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@129143 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 10 insertions, 10 deletions

diff --git a/lib/Sema/Sema.cpp b/lib/Sema/Sema.cpp
index 0e783018ec..a83722d824 100644
--- a/lib/Sema/Sema.cpp
+++ b/lib/Sema/Sema.cpp
@@ -203,36 +203,36 @@ Sema::~Sema() {
 /// ImpCastExprToType - If Expr is not of type 'Type', insert an implicit cast.
 /// If there is already an implicit cast, merge into the existing one.
 /// The result is of the given category.
-void Sema::ImpCastExprToType(Expr *&Expr, QualType Ty,
-                             CastKind Kind, ExprValueKind VK,
-                             const CXXCastPath *BasePath) {
-  QualType ExprTy = Context.getCanonicalType(Expr->getType());
+ExprResult Sema::ImpCastExprToType(Expr *E, QualType Ty,
+                                   CastKind Kind, ExprValueKind VK,
+                                   const CXXCastPath *BasePath) {
+  QualType ExprTy = Context.getCanonicalType(E->getType());
   QualType TypeTy = Context.getCanonicalType(Ty);
 
   if (ExprTy == TypeTy)
-    return;
+    return Owned(E);
 
   // If this is a derived-to-base cast to a through a virtual base, we
   // need a vtable.
   if (Kind == CK_DerivedToBase && 
       BasePathInvolvesVirtualBase(*BasePath)) {
-    QualType T = Expr->getType();
+    QualType T = E->getType();
     if (const PointerType *Pointer = T->getAs<PointerType>())
       T = Pointer->getPointeeType();
     if (const RecordType *RecordTy = T->getAs<RecordType>())
-      MarkVTableUsed(Expr->getLocStart(), 
+      MarkVTableUsed(E->getLocStart(), 
                      cast<CXXRecordDecl>(RecordTy->getDecl()));
   }
 
-  if (ImplicitCastExpr *ImpCast = dyn_cast<ImplicitCastExpr>(Expr)) {
+  if (ImplicitCastExpr *ImpCast = dyn_cast<ImplicitCastExpr>(E)) {
     if (ImpCast->getCastKind() == Kind && (!BasePath || BasePath->empty())) {
       ImpCast->setType(Ty);
       ImpCast->setValueKind(VK);
-      return;
+      return Owned(E);
     }
   }
 
-  Expr = ImplicitCastExpr::Create(Context, Ty, Kind, Expr, BasePath, VK);
+  return Owned(ImplicitCastExpr::Create(Context, Ty, Kind, E, BasePath, VK));
 }
 
 /// ScalarTypeToBooleanCastKind - Returns the cast kind corresponding