SILGen: Fix SIL mandatory inlining crash that could occur with public override of an internal property

When we override a property to add a didSet, Sema also synthesizes
a getter that simply delegates to the superclass property getter.

The synthesized getter is marked @_transparent. This means that if
the superclass getter is also transparent, but less visible than
the override, mandatory inlining will crash with an assertion.

To fix this, make sure SILGen does not emit a direct reference to
a superclass method if the current function is marked [fragile]
but the method is not.

Fixes <rdar://problem/26408353>.

Author: slavapestov

lib/SILGen/SILGenApply.cpp

@@ -111,14 +111,32 @@ static CanSILFunctionType getDynamicMethodLoweredType(SILGenFunction &gen,
   return replaceSelfTypeForDynamicLookup(ctx, methodTy, selfTy, methodName);
 }
 
+/// Check if we can perform a dynamic dispatch on a super method call.
 static bool canUseStaticDispatch(SILGenFunction &gen,
                                  SILDeclRef constant) {
+  auto *funcDecl = cast<AbstractFunctionDecl>(constant.getDecl());
+
+  if (funcDecl->isFinal())
+    return true;
+
+  // We cannot form a direct reference to a method body defined in
+  // Objective-C.
   if (constant.isForeign)
     return false;
 
-  auto *funcDecl = cast<AbstractFunctionDecl>(constant.getDecl());
+  // If we cannot form a direct reference due to resilience constraints,
+  // we have to dynamic dispatch.
+  if (gen.F.isFragile() && !constant.isFragile())
+    return false;
+
+  // If the method is defined in the same module, we can reference it
+  // directly.
   auto thisModule = gen.SGM.M.getSwiftModule();
-  return funcDecl->isFinal() || (thisModule == funcDecl->getModuleContext());
+  if (thisModule == funcDecl->getModuleContext())
+    return true;
+
+  // Otherwise, we must dynamic dispatch.
+  return false;
 }
 
 namespace {

test/SILGen/properties.swift

@@ -1043,3 +1043,27 @@ protocol ProtocolWithReadWriteSubscript {
 struct CrashWithUnnamedSubscript : ProtocolWithReadWriteSubscript {
   subscript(_: Int) -> Int { get { } set { } }
 }
+
+
+/// <rdar://problem/26408353> crash when overriding internal property with
+/// public property
+
+public class BaseClassWithInternalProperty {
+  var x: () = ()
+}
+
+public class DerivedClassWithPublicProperty : BaseClassWithInternalProperty {
+  public override var x: () {
+    didSet {}
+  }
+}
+
+// CHECK-LABEL: sil hidden [transparent] @_TFC10properties29BaseClassWithInternalPropertyg1xT_
+
+// CHECK-LABEL: sil [transparent] [fragile] @_TFC10properties30DerivedClassWithPublicPropertyg1xT_
+// CHECK:       bb0([[SELF:%.*]] : $DerivedClassWithPublicProperty):
+// CHECK:         strong_retain [[SELF]] : $DerivedClassWithPublicProperty
+// CHECK-NEXT:    [[SUPER:%.*]] = upcast [[SELF]] : $DerivedClassWithPublicProperty to $BaseClassWithInternalProperty
+// CHECK-NEXT:    [[METHOD:%.*]] = super_method [[SELF]] : $DerivedClassWithPublicProperty, #BaseClassWithInternalProperty.x!getter.1 : (BaseClassWithInternalProperty) -> () -> () , $@convention(method) (@guaranteed BaseClassWithInternalProperty) -> ()
+// CHECK-NEXT:    [[RESULT:%.*]] = apply [[METHOD]]([[SUPER]]) : $@convention(method) (@guaranteed BaseClassWithInternalProperty) -> ()
+// CHECK-NEXT:    strong_release [[SUPER]] : $BaseClassWithInternalProperty