diff --git a/artiq/py2llvm/typing.py b/artiq/py2llvm/typing.py
index 310819530..aef05b9fd 100644
--- a/artiq/py2llvm/typing.py
+++ b/artiq/py2llvm/typing.py
@@ -358,7 +358,11 @@ class Printer(algorithm.Visitor):
 def main():
     import sys, fileinput, os
 
-    inference_mode = True
+    if sys.argv[1] == '+diag':
+        del sys.argv[1]
+        inference_mode = False
+    else:
+        inference_mode = True
 
     engine = diagnostic.Engine(all_errors_are_fatal=True)
     try:
@@ -375,6 +379,8 @@ def main():
         if inference_mode:
             print("\n".join(e.diagnostic.render()), file=sys.stderr)
             exit(1)
+        else:
+            print("\n".join(e.diagnostic.render(only_line=True)))
 
 if __name__ == "__main__":
     main()
diff --git a/lit-test/harness.py b/lit-test/harness.py
index 1779e3266..9821ae61f 100644
--- a/lit-test/harness.py
+++ b/lit-test/harness.py
@@ -12,8 +12,10 @@ emulate the same behavior when invoked under lit.
 import sys, os, argparse, importlib
 
 parser = argparse.ArgumentParser(description=__doc__)
-parser.add_argument('-m', metavar='mod', type=str, help='run library module as a script')
-parser.add_argument('args', type=str, nargs='+', help='arguments passed to program in sys.argv[1:]')
+parser.add_argument('-m', metavar='mod', type=str, required=True,
+                    help='run library module as a script')
+parser.add_argument('args', type=str, nargs='+',
+                    help='arguments passed to program in sys.argv[1:]')
 args = parser.parse_args(sys.argv[1:])
 
 artiq_path = os.path.abspath(os.path.join(os.path.dirname(__file__), '..'))
diff --git a/lit-test/py2llvm/typing/error_unify.py b/lit-test/py2llvm/typing/error_unify.py
new file mode 100644
index 000000000..859e49205
--- /dev/null
+++ b/lit-test/py2llvm/typing/error_unify.py
@@ -0,0 +1,8 @@
+# RUN: %python -m artiq.py2llvm.typing +diag %s >%t
+# RUN: OutputCheck %s --file-to-check=%t
+
+a = 1
+b = []
+
+# CHECK-L: ${LINE:+1}: fatal: cannot unify int(width='a) with list(elt='b)
+a = b