[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

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -5944,6 +5944,9 @@ def err_bad_cxx_cast_vector_to_vector_different_size : Error<
 def err_bad_lvalue_to_rvalue_cast : Error<
   "cannot cast from lvalue of type %1 to rvalue reference type %2; types are "
   "not compatible">;
+def err_bad_rvalue_to_rvalue_cast : Error<
+  "cannot cast from rvalue of type %1 to rvalue reference type %2; types are "
+  "not compatible">;
 def err_bad_static_cast_pointer_nonpointer : Error<
   "cannot cast from type %1 to pointer type %2">;
 def err_bad_static_cast_member_pointer_nonmp : Error<

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;
   }
 

clang/test/CXX/expr/expr.post/expr.static.cast/p3-p4-0x.cpp

@@ -0,0 +1,43 @@
+// RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify %s
+
+
+// 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" (8.5.3).
+// p4
+// 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 (8.5). [...]
+struct A { };
+struct B : A { };
+
+struct C { explicit operator A&&(); };
+struct D { operator B(); };
+
+template<typename T> T& lvalue();
+template<typename T> T&& xvalue();
+template <typename T> T prvalue();
+
+void test(A &a, B &b) {
+  A &&ar0 = static_cast<A&&>(prvalue<A>());
+  A &&ar1 = static_cast<A&&>(prvalue<B>());
+  A &&ar2 = static_cast<A&&>(lvalue<C>());
+  A &&ar3 = static_cast<A&&>(xvalue<C>());
+  A &&ar4 = static_cast<A&&>(prvalue<C>());
+  A &&ar5 = static_cast<A&&>(lvalue<D>());
+  A &&ar6 = static_cast<A&&>(xvalue<D>());
+  A &&ar7 = static_cast<A&&>(prvalue<D>());
+
+  A &&ar8 = static_cast<A&&>(prvalue<const A>()); // expected-error {{binding value of type 'const A' to reference to type 'A' drops 'const' qualifier}}
+  A &&ar9 = static_cast<A&&>(lvalue<const A>()); // expected-error {{cannot cast from lvalue of type 'const A'}}
+  A &&ar10 = static_cast<A&&>(xvalue<const A>()); // expected-error {{cannot cast from rvalue of type 'const A'}}
+
+  const A &&ar11 = static_cast<const A&&>(prvalue<A>());
+  const A &&ar12 = static_cast<const A&&>(prvalue<B>());
+  const A &&ar13 = static_cast<const A&&>(lvalue<C>());
+  const A &&ar14 = static_cast<const A&&>(xvalue<C>());
+  const A &&ar15 = static_cast<const A&&>(prvalue<C>());
+  const A &&ar16 = static_cast<const A&&>(lvalue<D>());
+
+  const A &&ar17 = static_cast<const A&&>(prvalue<A const volatile>()); // expected-error {{binding value of type 'const volatile A' to reference to type 'const A' drops 'volatile' qualifier}}
+}

clang/test/SemaCXX/rval-references.cpp

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fcxx-exceptions -fexceptions -fsyntax-only -verify -std=c++11 %s
+// RUN: %clang_cc1 -fcxx-exceptions -fexceptions -fsyntax-only  -verify -std=c++11 %s
 
 typedef int&& irr;
 typedef irr& ilr_c1; // Collapses to int&
@@ -30,11 +30,15 @@ void f() {
   int &&virr2 = 0;
   int &&virr3 = virr2; // expected-error {{rvalue reference to type 'int' cannot bind to lvalue of type 'int'}}
   int i1 = 0;
+  const double d1 = 0;
+  const int ci1 = 1;
   int &&virr4 = i1; // expected-error {{rvalue reference to type 'int' cannot bind to lvalue of type 'int'}}
   int &&virr5 = ret_irr();
   int &&virr6 = static_cast<int&&>(i1);
-  (void)static_cast<not_int&&>(i1); // expected-error {{types are not compatible}}
-
+  (void)static_cast<not_int &&>(i1); // expected-error {{reference to type 'not_int' could not bind to an lvalue of type 'int'}}
+  (void)static_cast<int &&>(static_cast<int const&&>(i1)); // expected-error {{cannot cast from rvalue of type 'const int' to rvalue reference type 'int &&'}}
+  (void)static_cast<int &&>(ci1);    // expected-error {{types are not compatible}}
+  (void)static_cast<int &&>(d1);
   int i2 = over(i1);
   not_int ni1 = over(0);
   int i3 = over(virr2);