[Serialization] Serialize unresolved types as errors if allowing errors

Author: bnbarham

lib/Serialization/Serialization.cpp

@@ -4168,6 +4168,13 @@ class Serializer::TypeSerializer : public TypeVisitor<TypeSerializer> {
   }
 
   void visitUnresolvedType(const UnresolvedType *) {
+    // If for some reason we have an unresolved type while compiling with
+    // errors, just serialize an ErrorType and continue.
+    if (S.getASTContext().LangOpts.AllowModuleWithCompilerErrors) {
+      visitErrorType(
+          cast<ErrorType>(ErrorType::get(S.getASTContext()).getPointer()));
+      return;
+    }
     llvm_unreachable("should not serialize an UnresolvedType");
   }
 

validation-test/Serialization/AllowErrors/unresolved-type.swift

@@ -0,0 +1,12 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-swift-frontend -module-name errors -emit-module -o %t/errors.swiftmodule -experimental-allow-module-with-compiler-errors %s
+
+protocol SomeProto {}
+
+extension SomeProto {
+    func someFunc(arg:
+
+enum SomeEnum {
+    case a
+}