[PATCH] D26231: [Sema] Allow static_cast<T&&>(e) to check explicit conversions for non-reference-related types.

[Eric Fiselier via cfe-commits]

EricWF created this revision.
EricWF added a reviewer: rsmith.
EricWF added a subscriber: cfe-commits.

[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.


https://reviews.llvm.org/D26231

Files:
  include/clang/Basic/DiagnosticSemaKinds.td
  lib/Sema/SemaCast.cpp
  test/SemaCXX/rval-references.cpp


Index: test/SemaCXX/rval-references.cpp
===================================================================
--- test/SemaCXX/rval-references.cpp
+++ 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 @@
   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 reference 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);
Index: lib/Sema/SemaCast.cpp
===================================================================
--- lib/Sema/SemaCast.cpp
+++ lib/Sema/SemaCast.cpp
@@ -983,7 +983,7 @@
   // 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;
@@ -1134,10 +1134,10 @@
 }
 
 /// 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 
   //   cv2 T2" if "cv2 T2" is reference-compatible with "cv1 T1".
@@ -1160,15 +1160,15 @@
     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;
+    msg = SrcExpr->isLValue() ? diag::err_bad_lvalue_to_rvalue_cast
+                              : diag::err_bad_rvalue_to_rvalue_cast;
     return TC_Failed;
   }
 
Index: include/clang/Basic/DiagnosticSemaKinds.td
===================================================================
--- include/clang/Basic/DiagnosticSemaKinds.td
+++ include/clang/Basic/DiagnosticSemaKinds.td
@@ -5938,6 +5938,9 @@
 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 reference 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<


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D26231.76667.patch
Type: text/x-patch
Size: 4409 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/cfe-commits/attachments/20161102/a00b3d03/attachment-0001.bin>