[Sema] Allow static_cast<T&&>(e) to check explicit conversions for non-reference-related types.

Summary:
[expr.cast.static] states:
> 3. A glvalue of type “cv1 T1” can be cast to type “rvalue reference to cv2 T2” if “cv2 T2” is reference-compatible
> with “cv1 T1”. The result refers to the object or the specified base class subobject thereof. If T2 is
> an inaccessible or ambiguous base class of T1, a program that necessitates such a cast is
> ill-formed.
> 
> 4. Otherwise, an expression e can be explicitly converted to a type T using a static_cast of the form static_-
> cast<T>(e) if the declaration T t(e); is well-formed, for some invented temporary variable t. [...]

Currently when checking p3 Clang will diagnose `static_cast<T&&>(e)` as invalid if the argument is not reference compatible with `T`. However I believe the correct behavior is to also check p4 in those cases.  For example:

```
double y = 42;
static_cast<int&&>(y); // this should be OK.  'int&& t(y)' is well formed
```

Note that we still don't check p4 for non-reference-compatible types which are reference-related since  `T&& t(e);` should never be well formed in those cases.


Reviewers: rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D26231

llvm-svn: 285872

1 files changed, 17 insertions, 14 deletions

diff --git a/clang/lib/Sema/SemaCast.cpp b/clang/lib/Sema/SemaCast.cpp
index 43fdf590424..1821009346b 100644
--- a/clang/lib/Sema/SemaCast.cpp
+++ b/clang/lib/Sema/SemaCast.cpp
@@ -983,7 +983,7 @@ static TryCastResult TryStaticCast(Sema &Self, ExprResult &SrcExpr,
   // C++11 [expr.static.cast]p3: 
   //   A glvalue of type "cv1 T1" can be cast to type "rvalue reference to cv2
   //   T2" if "cv2 T2" is reference-compatible with "cv1 T1".
-  tcr = TryLValueToRValueCast(Self, SrcExpr.get(), DestType, CStyle, Kind, 
+  tcr = TryLValueToRValueCast(Self, SrcExpr.get(), DestType, CStyle, Kind,
                               BasePath, msg);
   if (tcr != TC_NotApplicable)
     return tcr;
@@ -1134,12 +1134,12 @@ static TryCastResult TryStaticCast(Sema &Self, ExprResult &SrcExpr,
 }
 
 /// Tests whether a conversion according to N2844 is valid.
-TryCastResult
-TryLValueToRValueCast(Sema &Self, Expr *SrcExpr, QualType DestType,
-                      bool CStyle, CastKind &Kind, CXXCastPath &BasePath, 
-                      unsigned &msg) {
+TryCastResult TryLValueToRValueCast(Sema &Self, Expr *SrcExpr,
+                                    QualType DestType, bool CStyle,
+                                    CastKind &Kind, CXXCastPath &BasePath,
+                                    unsigned &msg) {
   // C++11 [expr.static.cast]p3:
-  //   A glvalue of type "cv1 T1" can be cast to type "rvalue reference to 
+  //   A glvalue of type "cv1 T1" can be cast to type "rvalue reference to
   //   cv2 T2" if "cv2 T2" is reference-compatible with "cv1 T1".
   const RValueReferenceType *R = DestType->getAs<RValueReferenceType>();
   if (!R)
@@ -1160,15 +1160,18 @@ TryLValueToRValueCast(Sema &Self, Expr *SrcExpr, QualType DestType,
     FromType = FromType.getUnqualifiedType();
     ToType = ToType.getUnqualifiedType();
   }
-  
-  if (Self.CompareReferenceRelationship(SrcExpr->getLocStart(),
-                                        ToType, FromType,
-                                        DerivedToBase, ObjCConversion,
-                                        ObjCLifetimeConversion) 
-        != Sema::Ref_Compatible) {
-    if (CStyle)
+
+  Sema::ReferenceCompareResult RefResult = Self.CompareReferenceRelationship(
+      SrcExpr->getLocStart(), ToType, FromType, DerivedToBase, ObjCConversion,
+      ObjCLifetimeConversion);
+  if (RefResult != Sema::Ref_Compatible) {
+    if (CStyle || RefResult == Sema::Ref_Incompatible)
       return TC_NotApplicable;
-    msg = diag::err_bad_lvalue_to_rvalue_cast;
+    // Diagnose types which are reference-related but not compatible here since
+    // we can provide better diagnostics. In these cases forwarding to
+    // [expr.static.cast]p4 should never result in a well-formed cast.
+    msg = SrcExpr->isLValue() ? diag::err_bad_lvalue_to_rvalue_cast
+                              : diag::err_bad_rvalue_to_rvalue_cast;
     return TC_Failed;
   }