[5.5][Serialization] Skip MissingMembers when allowing errors

lib/Serialization/Serialization.cpp

@@ -1611,6 +1611,8 @@ static bool shouldSerializeMember(Decl *D) {
     llvm_unreachable("decl should never be a member");
 
   case DeclKind::MissingMember:
+    if (D->getASTContext().LangOpts.AllowModuleWithCompilerErrors)
+      return false;
     llvm_unreachable("should never need to reserialize a member placeholder");
 
   case DeclKind::IfConfig:

test/Serialization/AllowErrors/invalid-xref.swift

@@ -0,0 +1,41 @@
+// RUN: %empty-directory(%t)
+// RUN: %empty-directory(%t/mods)
+
+// RUN: touch %t/empty.swift
+// RUN: %{python} %utils/split_file.py -o %t %s
+
+// We're going to swap A and B around to cause an invalid xref
+// RUN: %target-swift-frontend -emit-module -o %t/mods/A.swiftmodule -module-name A %t/lib.swift
+// RUN: %target-swift-frontend -emit-module -o %t/mods/B.swiftmodule -module-name B %t/empty.swift
+
+// Compile using SomeType from A
+// RUN: %target-swift-frontend -module-name errors -emit-module -o %t/mods/errorsmain.partial.swiftmodule -I %t/mods %t/errors.swift
+// Empty module so we can do a merge modules step
+// RUN: %target-swift-frontend -module-name errors -emit-module -o %t/mods/errorsempty.partial.swiftmodule %t/empty.swift
+
+// Swap A and B
+// RUN: %target-swift-frontend -emit-module -o %t/mods/A.swiftmodule -module-name A %t/empty.swift
+// RUN: %target-swift-frontend -emit-module -o %t/mods/B.swiftmodule -module-name B %t/lib.swift
+
+// RUN: %target-swift-frontend -module-name errors -emit-module -o %t/mods/errors.swiftmodule -experimental-allow-module-with-compiler-errors %t/mods/errorsmain.partial.swiftmodule %t/mods/errorsempty.partial.swiftmodule
+
+// Expect this to crash without allowing errors (we should never get into a
+// situation where merge modules is run with MissingMemberDecls)
+// RUN: not --crash %target-swift-frontend -module-name errors -emit-module -o %t/mods/errors.swiftmodule %t/mods/errorsmain.partial.swiftmodule %t/mods/errorsempty.partial.swiftmodule
+
+// BEGIN lib.swift
+public struct SomeType {
+    public init() {}
+}
+
+
+// BEGIN errors.swift
+import A
+import B
+
+public class SomeClass {
+  public let member: SomeType
+  public init(member: SomeType) {
+    self.member = member
+  }
+}