PR 17456

More helpful diagnostic on casts between unrelated class hierarchies.

llvm-svn: 227371

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -5267,6 +5267,11 @@ def err_bad_cstyle_cast_overload : Error<
 def err_bad_cxx_cast_generic : Error<
   "%select{const_cast|static_cast|reinterpret_cast|dynamic_cast|C-style cast|"
   "functional-style cast}0 from %1 to %2 is not allowed">;
+def err_bad_cxx_cast_unrelated_class : Error<
+  "%select{const_cast|static_cast|reinterpret_cast|dynamic_cast|C-style cast|"
+  "functional-style cast}0 from %1 to %2, which are not related by "
+  "inheritance, is not allowed">;
+def note_type_incomplete : Note<"%0 is incomplete">;
 def err_bad_cxx_cast_rvalue : Error<
   "%select{const_cast|static_cast|reinterpret_cast|dynamic_cast|C-style cast|"
   "functional-style cast}0 from rvalue to reference type %2">;

clang/lib/Sema/SemaCast.cpp

@@ -389,6 +389,33 @@ static void diagnoseBadCast(Sema &S, unsigned msg, CastType castType,
 
   S.Diag(opRange.getBegin(), msg) << castType
     << src->getType() << destType << opRange << src->getSourceRange();
+
+  // Detect if both types are (ptr to) class, and note any incompleteness.
+  int DifferentPtrness = 0;
+  QualType From = destType;
+  if (auto Ptr = From->getAs<PointerType>()) {
+    From = Ptr->getPointeeType();
+    DifferentPtrness++;
+  }
+  QualType To = src->getType();
+  if (auto Ptr = To->getAs<PointerType>()) {
+    To = Ptr->getPointeeType();
+    DifferentPtrness--;
+  }
+  if (!DifferentPtrness) {
+    auto RecFrom = From->getAs<RecordType>();
+    auto RecTo = To->getAs<RecordType>();
+    if (RecFrom && RecTo) {
+      auto DeclFrom = RecFrom->getAsCXXRecordDecl();
+      if (!DeclFrom->isCompleteDefinition())
+        S.Diag(DeclFrom->getLocation(), diag::note_type_incomplete)
+          << DeclFrom->getDeclName();
+      auto DeclTo = RecTo->getAsCXXRecordDecl();
+      if (!DeclTo->isCompleteDefinition())
+        S.Diag(DeclTo->getLocation(), diag::note_type_incomplete)
+          << DeclTo->getDeclName();
+    }
+  }
 }
 
 /// UnwrapDissimilarPointerTypes - Like Sema::UnwrapSimilarPointerTypes,
@@ -1080,6 +1107,14 @@ static TryCastResult TryStaticCast(Sema &Self, ExprResult &SrcExpr,
       Self.CheckTollFreeBridgeStaticCast(DestType, SrcExpr.get(), Kind))
     return TC_Success;
 
+  // See if it looks like the user is trying to convert between
+  // related record types, and select a better diagnostic if so.
+  if (auto SrcPointer = SrcType->getAs<PointerType>())
+    if (auto DestPointer = DestType->getAs<PointerType>())
+      if (SrcPointer->getPointeeType()->getAs<RecordType>() &&
+          DestPointer->getPointeeType()->getAs<RecordType>())
+       msg = diag::err_bad_cxx_cast_unrelated_class;
+  
   // We tried everything. Everything! Nothing works! :-(
   return TC_NotApplicable;
 }

clang/test/CXX/temp/temp.param/p15-cxx0x.cpp

@@ -1,5 +1,5 @@
 // RUN: %clang_cc1 -fsyntax-only -std=c++11 -verify %s
-template<typename T> struct X;
+template<typename T> struct X; // expected-note {{'X' is incomplete}}
 template<int I> struct Y;
 
 X<X<int>> *x1;

clang/test/SemaCXX/static-cast.cpp

@@ -9,8 +9,8 @@ struct F : public C1 {};            // Single path to B with virtual.
 struct G1 : public B {};
 struct G2 : public B {};
 struct H : public G1, public G2 {}; // Ambiguous path to B.
-struct I;                           // Incomplete.
-struct J;                           // Incomplete.
+struct I;                           // Incomplete.  expected-note {{'I' is incomplete}}
+struct J;                           // Incomplete.  expected-note {{'J' is incomplete}}
 
 enum Enum { En1, En2 };
 enum Onom { On1, On2 };
@@ -92,9 +92,13 @@ void t_529_5_8()
   (void)static_cast<E&>(*((A*)0)); // expected-error {{cannot cast private base class 'A' to 'E'}}
   (void)static_cast<H*>((A*)0); // expected-error {{ambiguous cast from base 'A' to derived 'H':\n    struct A -> struct B -> struct G1 -> struct H\n    struct A -> struct B -> struct G2 -> struct H}}
   (void)static_cast<H&>(*((A*)0)); // expected-error {{ambiguous cast from base 'A' to derived 'H':\n    struct A -> struct B -> struct G1 -> struct H\n    struct A -> struct B -> struct G2 -> struct H}}
-  (void)static_cast<E*>((B*)0); // expected-error {{static_cast from 'B *' to 'E *' is not allowed}}
+  (void)static_cast<E*>((B*)0); // expected-error {{static_cast from 'B *' to 'E *', which are not related by inheritance, is not allowed}}
   (void)static_cast<E&>(*((B*)0)); // expected-error {{non-const lvalue reference to type 'E' cannot bind to a value of unrelated type 'B'}}
 
+
+  (void)static_cast<E*>((J*)0); // expected-error {{static_cast from 'J *' to 'E *', which are not related by inheritance, is not allowed}}
+  (void)static_cast<I*>((B*)0); // expected-error {{static_cast from 'B *' to 'I *', which are not related by inheritance, is not allowed}}
+
   // TODO: Test inaccessible base in context where it's accessible, i.e.
   // member function and friend.