Actually honor MissingMemberDecls in ClassMetadataScanner.

...which is sufficient to correctly invoke methods in a vtable
even when members have been deleted. 🎉

Author: jrose-apple

include/swift/SIL/SILVTableVisitor.h

@@ -94,6 +94,8 @@ template <class T> class SILVTableVisitor {
         maybeAddMethod(fd);
       else if (auto *cd = dyn_cast<ConstructorDecl>(member))
         maybeAddConstructor(cd);
+      else if (auto *placeholder = dyn_cast<MissingMemberDecl>(member))
+        asDerived().addPlaceholder(placeholder);
     }
   }
 };

lib/IRGen/ClassMetadataLayout.h

@@ -181,6 +181,12 @@ class ClassMetadataScanner : public ClassMetadataLayout<Impl> {
                               ClassDecl *forClass) {
     addPointer();
   }
+  void addPlaceholder(MissingMemberDecl *MMD) {
+    for (auto i : range(MMD->getNumberOfVTableEntries())) {
+      (void)i;
+      addPointer();
+    }
+  }
 
 private:
   // Our layout here assumes that there will never be unclaimed space

lib/IRGen/GenMeta.cpp

@@ -3502,6 +3502,10 @@ namespace {
       }
     }
 
+    void addPlaceholder(MissingMemberDecl *) {
+      llvm_unreachable("cannot generate metadata with placeholders in it");
+    }
+
     void addMethodOverride(SILDeclRef baseRef, SILDeclRef declRef) {}
 
     void addGenericArgument(CanType argTy, ClassDecl *forClass) {

test/Serialization/Recovery/typedefs.swift

@@ -1,5 +1,5 @@
 // RUN: rm -rf %t && mkdir -p %t
-// RUN: %target-swift-frontend -emit-module -o %t -module-name Lib -I %S/Inputs/custom-modules %s
+// RUN: %target-swift-frontend -emit-sil -o - -emit-module-path %t/Lib.swiftmodule -module-name Lib -I %S/Inputs/custom-modules %s | %FileCheck -check-prefix CHECK-VTABLE %s
 
 // RUN: %target-swift-ide-test -source-filename=x -print-module -module-to-print Lib -I %t -I %S/Inputs/custom-modules | %FileCheck %s
 
@@ -9,6 +9,7 @@
 
 // RUN: %target-swift-frontend -typecheck -I %t -I %S/Inputs/custom-modules -Xcc -DBAD -DTEST -enable-experimental-deserialization-recovery -DVERIFY %s -verify
 // RUN: %target-swift-frontend -emit-silgen -I %t -I %S/Inputs/custom-modules -Xcc -DBAD -DTEST -enable-experimental-deserialization-recovery %s | %FileCheck -check-prefix CHECK-SIL %s
+// RUN: %target-swift-frontend -emit-ir -I %t -I %S/Inputs/custom-modules -Xcc -DBAD -DTEST -enable-experimental-deserialization-recovery %s | %FileCheck -check-prefix CHECK-IR %s
 
 #if TEST
 
@@ -24,6 +25,16 @@ func testSymbols() {
   _ = Lib.usesAssoc
 } // CHECK-SIL: end sil function '_T08typedefs11testSymbolsyyF'
 
+// CHECK-IR-LABEL: define{{.*}} void @_T08typedefs18testVTableBuildingy3Lib4UserC4user_tF
+public func testVTableBuilding(user: User) {
+  // The important thing in this CHECK line is the "i64 23", which is the offset
+  // for the vtable slot for 'lastMethod()'. If the layout here
+  // changes, please check that offset 23 is still correct.
+  // CHECK-IR-NOT: ret
+  // CHECK-IR: getelementptr inbounds void (%T3Lib4UserC*)*, void (%T3Lib4UserC*)** %{{[0-9]+}}, i64 23
+  _ = user.lastMethod()
+} // CHECK-IR: ret void
+
 #if VERIFY
 let _: String = useAssoc(ImportedType.self) // expected-error {{cannot convert call result type '_.Assoc?' to expected type 'String'}}
 let _: Bool? = useAssoc(ImportedType.self) // expected-error {{cannot convert value of type 'Int32?' to specified type 'Bool?'}}
@@ -93,12 +104,34 @@ open class User {
 
   // CHECK: required init(wrappedRequired: WrappedInt)
   // CHECK-RECOVERY: /* placeholder for init(wrappedRequired:) */
-  // CHECK-RECOVERY: /* placeholder for init(wrappedRequired:) */
   public required init(wrappedRequired: WrappedInt) {}
+
+  public func lastMethod() {}
 }
 // CHECK: {{^}$}}
 // CHECK-RECOVERY: {{^}$}}
 
+// This is mostly to check when changes are necessary for the CHECK-IR lines
+// above.
+// CHECK-VTABLE-LABEL: sil_vtable User {
+// (8 words of normal class metadata)
+// 9 CHECK-VTABLE-NEXT: #User.unwrappedProp!getter.1:
+// 10 CHECK-VTABLE-NEXT: #User.unwrappedProp!setter.1:
+// 11 CHECK-VTABLE-NEXT: #User.unwrappedProp!materializeForSet.1:
+// 12 CHECK-VTABLE-NEXT: #User.wrappedProp!getter.1:
+// 13 CHECK-VTABLE-NEXT: #User.wrappedProp!setter.1:
+// 14 CHECK-VTABLE-NEXT: #User.wrappedProp!materializeForSet.1:
+// 15 CHECK-VTABLE-NEXT: #User.returnsUnwrappedMethod!1:
+// 16 CHECK-VTABLE-NEXT: #User.returnsWrappedMethod!1:
+// 17 CHECK-VTABLE-NEXT: #User.subscript!getter.1:
+// 18 CHECK-VTABLE-NEXT: #User.init!initializer.1:
+// 19 CHECK-VTABLE-NEXT: #User.init!initializer.1:
+// 20 CHECK-VTABLE-NEXT: #User.init!initializer.1:
+// 21 CHECK-VTABLE-NEXT: #User.init!allocator.1:
+// 22 CHECK-VTABLE-NEXT: #User.init!initializer.1:
+// 23 CHECK-VTABLE-NEXT: #User.lastMethod!1:
+// CHECK-VTABLE: }
+
 
 // CHECK-LABEL: class UserConvenience
 // CHECK-RECOVERY-LABEL: class UserConvenience