SILGen: Fix inconsistency between vtable layout and vtable entry emission.

lib/SILGen/SILGenType.cpp

@@ -112,10 +112,18 @@ SILGenModule::emitVTableMethod(ClassDecl *theClass, SILDeclRef derived,
 
   // If the base method is less visible than the derived method, we need
   // a thunk.
+  //
+  // Note that we check the visibility of the derived method's immediate base
+  // here, rather than the ultimate base of the vtable entry, because it is
+  // possible through import visibility for an intermediate base class in one
+  // file to have public visibility to the ultimate base via a public import,
+  // but then in turn be overridden by a derived class in another file in
+  // the same module that either doesn't import the ultimate base class's
+  // module or else imports it non-publicly in its file.
   bool baseLessVisibleThanDerived =
     (!usesObjCDynamicDispatch &&
      !derivedDecl->isFinal() &&
-     derivedDecl->isMoreVisibleThan(baseDecl));
+     derivedDecl->isMoreVisibleThan(derivedDecl->getOverriddenDecl()));
 
   // Determine the derived thunk type by lowering the derived type against the
   // abstraction pattern of the base.

test/SILGen/Inputs/vtable_internal_imported_ancestor/Repro1.swift

@@ -0,0 +1,3 @@
+open class Base {
+    public required init() {}
+}

test/SILGen/Inputs/vtable_internal_imported_ancestor/Repro2.swift

@@ -0,0 +1,4 @@
+public import Repro1
+
+open class MidDerived: Base {
+}

test/SILGen/vtable_internal_imported_ancestor.swift

@@ -0,0 +1,14 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -enable-library-evolution -emit-module-path %t/Repro1.swiftmodule -module-name Repro1 %S/Inputs/vtable_internal_imported_ancestor/Repro1.swift
+// RUN: %target-swift-frontend -enable-upcoming-feature InternalImportsByDefault -I %t -emit-silgen %S/Inputs/vtable_internal_imported_ancestor/Repro2.swift -primary-file %s | %FileCheck %s
+
+// REQUIRES: swift_feature_InternalImportsByDefault
+
+import Repro1
+
+public class MostDerived: MidDerived {
+}
+
+// CHECK-NOT: vtable thunk
+// CHECK-LABEL: sil_vtable{{.*}} MostDerived {
+// CHECK:         #Base.init!allocator: {{.*}} @$s6Repro211MostDerivedCACycfC [override]

test/SILGen/vtables_multifile.swift

@@ -225,15 +225,15 @@ public final class FinalDerived : Base<Int> {
 // --
 
 // CHECK-LABEL: sil_vtable [serialized] MostDerived {
-// CHECK-NEXT:   #Base.privateMethod1: <T> (Base<T>) -> () -> () : @$s17vtables_multifile11MostDerivedC14privateMethod1yyFAA4BaseCAD33_63E5F2521A3C787F5F9EFD57FB9237EALLyyFTV [override]     // vtable thunk for Base.privateMethod1() dispatching to MostDerived.privateMethod1()
-// CHECK-NEXT:   #Base.privateMethod2: <T> (Base<T>) -> (AnyObject) -> () : @$s17vtables_multifile11MostDerivedC14privateMethod2yyyXlSgFAA4BaseCAD33_63E5F2521A3C787F5F9EFD57FB9237EALLyyyXlFTV [override]    // vtable thunk for Base.privateMethod2(_:) dispatching to MostDerived.privateMethod2(_:)
-// CHECK-NEXT:   #Base.privateMethod3: <T> (Base<T>) -> (Int) -> () : @$s17vtables_multifile11MostDerivedC14privateMethod3yySiSgFAA4BaseCAD33_63E5F2521A3C787F5F9EFD57FB9237EALLyySiFTV [override]    // vtable thunk for Base.privateMethod3(_:) dispatching to MostDerived.privateMethod3(_:)
-// CHECK-NEXT:   #Base.privateMethod4: <T> (Base<T>) -> (T) -> () : @$s17vtables_multifile11MostDerivedC14privateMethod4yySiFAA4BaseCAD33_63E5F2521A3C787F5F9EFD57FB9237EALLyyxFTV [override] // vtable thunk for Base.privateMethod4(_:) dispatching to MostDerived.privateMethod4(_:)
+// CHECK-NEXT:   #Base.privateMethod1: <T> (Base<T>) -> () -> () : @$s17vtables_multifile11MostDerivedC14privateMethod1yyF
+// CHECK-NEXT:   #Base.privateMethod2: <T> (Base<T>) -> (AnyObject) -> () : @$s17vtables_multifile11MostDerivedC14privateMethod2yyyXlSgF
+// CHECK-NEXT:   #Base.privateMethod3: <T> (Base<T>) -> (Int) -> () : @$s17vtables_multifile11MostDerivedC14privateMethod3yySiSgF
+// CHECK-NEXT:   #Base.privateMethod4: <T> (Base<T>) -> (T) -> () : @$s17vtables_multifile11MostDerivedC14privateMethod4yySiF
 // CHECK-NEXT:   #Base.init!allocator: <T> (Base<T>.Type) -> () -> Base<T> : @$s17vtables_multifile11MostDerivedCACycfC [override]   // MostDerived.__allocating_init()
-// CHECK-NEXT:   #Derived.privateMethod1: (Derived) -> () -> () : @$s17vtables_multifile11MostDerivedC14privateMethod1yyFAA0D0CADyyFTV [override]     // vtable thunk for Derived.privateMethod1() dispatching to MostDerived.privateMethod1()
-// CHECK-NEXT:   #Derived.privateMethod2: (Derived) -> (AnyObject?) -> () : @$s17vtables_multifile11MostDerivedC14privateMethod2yyyXlSgFAA0D0CADyyAEFTV [override]    // vtable thunk for Derived.privateMethod2(_:) dispatching to MostDerived.privateMethod2(_:)
-// CHECK-NEXT:   #Derived.privateMethod3: (Derived) -> (Int?) -> () : @$s17vtables_multifile11MostDerivedC14privateMethod3yySiSgFAA0D0CADyyAEFTV [override]   // vtable thunk for Derived.privateMethod3(_:) dispatching to MostDerived.privateMethod3(_:)
-// CHECK-NEXT:   #Derived.privateMethod4: (Derived) -> (Int) -> () : @$s17vtables_multifile11MostDerivedC14privateMethod4yySiFAA0D0CADyySiFTV [override]      // vtable thunk for Derived.privateMethod4(_:) dispatching to MostDerived.privateMethod4(_:)
+// CHECK-NEXT:   #Derived.privateMethod1: (Derived) -> () -> () : @$s17vtables_multifile11MostDerivedC14privateMethod1yyF
+// CHECK-NEXT:   #Derived.privateMethod2: (Derived) -> (AnyObject?) -> () : @$s17vtables_multifile11MostDerivedC14privateMethod2yyyXlSgF
+// CHECK-NEXT:   #Derived.privateMethod3: (Derived) -> (Int?) -> () : @$s17vtables_multifile11MostDerivedC14privateMethod3yySiSgF
+// CHECK-NEXT:   #Derived.privateMethod4: (Derived) -> (Int) -> () : @$s17vtables_multifile11MostDerivedC14privateMethod4yySiF
 // CHECK-NEXT:   #MoreDerived.privateMethod1: (MoreDerived) -> () -> () : @$s17vtables_multifile11MostDerivedC14privateMethod1yyF [override] // MostDerived.privateMethod1()
 // CHECK-NEXT:   #MoreDerived.privateMethod2: (MoreDerived) -> (AnyObject?) -> () : @$s17vtables_multifile11MostDerivedC14privateMethod2yyyXlSgF [override] // MostDerived.privateMethod2(_:)
 // CHECK-NEXT:   #MoreDerived.privateMethod3: (MoreDerived) -> (Int?) -> () : @$s17vtables_multifile11MostDerivedC14privateMethod3yySiSgF [override] // MostDerived.privateMethod3(_:)