[MS] Allow access to ambiguous, inaccessible direct bases

Summary:
Clang typically warns that in the following class hierarchy, 'A' is
inaccessible because there is no series of casts that the user can
write to access it unambiguously:
  struct A { };
  struct B : A { };
  struct C : A, B { };

MSVC allows the user to convert from C* to A*, though, and we've
encountered this issue in the latest Windows SDK headers.

This patch allows this conversion when -fms-compatibility is set and
adds a warning for it under -Wmicrosoft-inaccessible-base.

Reviewers: rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D39389

llvm-svn: 316807

1 files changed, 20 insertions, 0 deletions

diff --git a/clang/test/CodeGenCXX/microsoft-inaccessible-base.cpp b/clang/test/CodeGenCXX/microsoft-inaccessible-base.cpp
new file mode 100644
index 00000000000..2c0d124eb01
--- /dev/null
+++ b/clang/test/CodeGenCXX/microsoft-inaccessible-base.cpp
@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -fms-compatibility -triple x86_64-windows-msvc %s -emit-llvm -o - | FileCheck %s
+
+// Make sure we choose the *direct* base path when doing these conversions.
+
+// CHECK: %struct.C = type { %struct.A, %struct.B }
+// CHECK: %struct.D = type { %struct.B, %struct.A }
+
+struct A { int a; };
+struct B : A { int b; };
+
+struct C : A, B { };
+extern "C" A *a_from_c(C *p) { return p; }
+// CHECK-LABEL: define %struct.A* @a_from_c(%struct.C* %{{.*}})
+// CHECK: bitcast %struct.C* %{{.*}} to %struct.A*
+
+struct D : B, A { };
+extern "C" A *a_from_d(D *p) { return p; }
+// CHECK-LABEL: define %struct.A* @a_from_d(%struct.D* %{{.*}})
+// CHECK: %[[p_i8:[^ ]*]] = bitcast %struct.D* %{{.*}} to i8*
+// CHECK: getelementptr inbounds i8, i8* %[[p_i8]], i64 8