IRGen: Deal with broken AST from batch mode

lib/IRGen/GenType.cpp

@@ -1836,9 +1836,12 @@ const TypeInfo *TypeConverter::convertType(CanType ty) {
   PrettyStackTraceType stackTrace(IGM.Context, "converting", ty);
 
   switch (ty->getKind()) {
-  case TypeKind::Error:
-    llvm_unreachable("found an ErrorType in IR-gen");
-
+  case TypeKind::Error: {
+    // We might see error types if type checking has failed.
+    // Try to do something graceful and return an zero sized type.
+    auto &ctx = ty->getASTContext();
+    return convertTupleType(cast<TupleType>(ctx.TheEmptyTupleType));
+  }
 #define UNCHECKED_TYPE(id, parent) \
   case TypeKind::id: \
     llvm_unreachable("found a " #id "Type in IR-gen");

test/IRGen/Inputs/batchmode_errors.swift

@@ -0,0 +1,14 @@
+import Foundation
+
+@objc
+protocol SomeProto {
+  func conform()
+}
+
+class AnotherClass {}
+
+class SomeClass {
+  // Intentionally referencing the wrong class name.
+  var x : AnotherClass2? = nil
+}
+

test/IRGen/batchmode_ast_errors.swift

@@ -0,0 +1,11 @@
+// RUN: not %target-swift-frontend -primary-file %s %S/Inputs/batchmode_errors.swift -emit-ir 2>&1 | %FileCheck %s
+
+// REQUIRES: objc_interop
+
+// Verify that we don't crash (in IRGen).
+
+// CHECK-NOT: Stack dump
+
+extension SomeClass : SomeProto {
+  func conform() {}
+}