[clang] Fix static_cast bypassing access control

Fix access and ambiguity checks not being performed when converting to
an rvalue reference to a base class type with `static_cast`.

Author: offsetof

clang/docs/ReleaseNotes.rst

@@ -357,6 +357,7 @@ Bug Fixes to C++ Support
 - Fixed a Clang regression in C++20 mode where unresolved dependent call expressions were created inside non-dependent contexts (#GH122892)
 - Clang now emits the ``-Wunused-variable`` warning when some structured bindings are unused
   and the ``[[maybe_unused]]`` attribute is not applied. (#GH125810)
+- Fixed ``static_cast`` not performing access or ambiguity checks when converting to an rvalue reference to a base class. (#GH121429)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaCast.cpp

@@ -221,7 +221,7 @@ static void DiagnoseCastQual(Sema &Self, const ExprResult &SrcExpr,
 // %2: Destination Type
 static TryCastResult TryLValueToRValueCast(Sema &Self, Expr *SrcExpr,
                                            QualType DestType, bool CStyle,
-                                           CastKind &Kind,
+                                           SourceRange OpRange, CastKind &Kind,
                                            CXXCastPath &BasePath,
                                            unsigned &msg);
 static TryCastResult TryStaticReferenceDowncast(Sema &Self, Expr *SrcExpr,
@@ -1357,8 +1357,8 @@ 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,
-                              BasePath, msg);
+  tcr = TryLValueToRValueCast(Self, SrcExpr.get(), DestType, CStyle, OpRange,
+                              Kind, BasePath, msg);
   if (tcr != TC_NotApplicable)
     return tcr;
 
@@ -1534,8 +1534,8 @@ 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) {
+                                    SourceRange OpRange, 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".
@@ -1548,7 +1548,6 @@ TryCastResult TryLValueToRValueCast(Sema &Self, Expr *SrcExpr,
 
   // Because we try the reference downcast before this function, from now on
   // this is the only cast possibility, so we issue an error if we fail now.
-  // FIXME: Should allow casting away constness if CStyle.
   QualType FromType = SrcExpr->getType();
   QualType ToType = R->getPointeeType();
   if (CStyle) {
@@ -1572,13 +1571,12 @@ TryCastResult TryLValueToRValueCast(Sema &Self, Expr *SrcExpr,
 
   if (RefConv & Sema::ReferenceConversions::DerivedToBase) {
     Kind = CK_DerivedToBase;
-    CXXBasePaths Paths(/*FindAmbiguities=*/true, /*RecordPaths=*/true,
-                       /*DetectVirtual=*/true);
-    if (!Self.IsDerivedFrom(SrcExpr->getBeginLoc(), SrcExpr->getType(),
-                            R->getPointeeType(), Paths))
-      return TC_NotApplicable;
-
-    Self.BuildBasePathArray(Paths, BasePath);
+    if (Self.CheckDerivedToBaseConversion(FromType, ToType,
+                                          SrcExpr->getBeginLoc(), OpRange,
+                                          &BasePath, CStyle)) {
+      msg = 0;
+      return TC_Failed;
+    }
   } else
     Kind = CK_NoOp;
 

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

@@ -1,5 +1,4 @@
 // RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify %s
-// expected-no-diagnostics
 
 // 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).
@@ -23,3 +22,67 @@ void test(A &a, B &b) {
   const A &&ar10 = static_cast<const A&&>(xvalue<A>());
   const A &&ar11 = static_cast<const A&&>(xvalue<B>());
 }
+
+struct C : private A { // expected-note 4 {{declared private here}}
+    C&& that();
+
+    void f() {
+        (void)static_cast<A&&>(*this);
+        (void)static_cast<const A&&>(*this);
+
+        (void)static_cast<A&&>(that());
+        (void)static_cast<const A&&>(that());
+    }
+};
+C c;
+const C cc;
+
+void f() {
+    static_cast<A&&>(c);        // expected-error {{cannot cast 'C' to its private base class 'A'}}
+    static_cast<A&&>(c.that()); // expected-error {{cannot cast 'C' to its private base class 'A'}}
+
+    static_cast<const A&&>(c);        // expected-error {{cannot cast 'C' to its private base class 'const A'}}
+    static_cast<const A&&>(c.that()); // expected-error {{cannot cast 'C' to its private base class 'const A'}}
+}
+
+constexpr auto v = (
+    (A&&)c,
+    (A&&)(C&&)c,
+    (A&&)cc,
+    (A&&)(const C&&)c,
+    (const A&&)c,
+    (const A&&)(C&&)c,
+    (const A&&)cc,
+    (const A&&)(const C&&)c
+);
+
+struct D : A, B { // expected-warning {{direct base 'A' is inaccessible due to ambiguity}}
+    D&& rv();
+};
+D d;
+
+void g(const D cd) {
+    static_cast<A&&>(d);      // expected-error {{ambiguous conversion from derived class 'D' to base class 'A'}}
+    static_cast<A&&>(d.rv()); // expected-error {{ambiguous conversion from derived class 'D' to base class 'A'}}
+
+    static_cast<const A&&>(d);      // expected-error {{ambiguous conversion from derived class 'D' to base class 'const A'}}
+    static_cast<const A&&>(d.rv()); // expected-error {{ambiguous conversion from derived class 'D' to base class 'const A'}}
+
+    (A&&)d;                  // expected-error {{ambiguous conversion from derived class 'D' to base class 'A'}}
+    (A&&)(D&&)d;             // expected-error {{ambiguous conversion from derived class 'D' to base class 'A'}}
+    (A&&)cd;                 // expected-error {{ambiguous conversion from derived class 'D' to base class 'A'}}
+    (A&&)(const D&&)d;       // expected-error {{ambiguous conversion from derived class 'D' to base class 'A'}}
+    (const A&&)d;            // expected-error {{ambiguous conversion from derived class 'D' to base class 'A'}}
+    (const A&&)(D&&)d;       // expected-error {{ambiguous conversion from derived class 'D' to base class 'A'}}
+    (const A&&)cd;           // expected-error {{ambiguous conversion from derived class 'D' to base class 'A'}}
+    (const A&&)(const D&&)d; // expected-error {{ambiguous conversion from derived class 'D' to base class 'A'}}
+}
+
+template<class T, class U>
+auto h(U u = {}) -> decltype(static_cast<T&&>(u));
+
+template<class, class>
+int h();
+
+int i = h<A, C>();
+int j = h<A, D>();