Reject == and != between void expressions

src/fpy/semantics.py

@@ -1741,10 +1741,11 @@ def pick_intermediate_type(
         if op in BOOLEAN_OPERATORS:
             return BOOL
 
-        # for == and !=, non-numeric same-type comparisons are valid
+        # for == and !=, non-numeric same-type comparisons are valid, as long
+        # as the operands have a value at all
         if op in (BinaryStackOp.EQUAL, BinaryStackOp.NOT_EQUAL):
             if len(arg_types) == 2 and arg_types[0] == arg_types[1]:
-                if not arg_types[0].is_numerical:
+                if not arg_types[0].is_numerical and arg_types[0] != NOTHING:
                     # non-numeric equality (struct, array, enum, time)
                     return arg_types[0]
 

test/fpy/test_functions.py

@@ -313,6 +313,30 @@ def noop():
 """
         assert_compile_failure(fprime_test_api, seq)
 
+    def test_compare_void_function_results_equal(self, fprime_test_api):
+        """A void call has no value, so it cannot be an operand of ==."""
+        seq = """
+def f():
+    pass
+
+b: bool = f() == f()
+"""
+        assert_compile_failure(fprime_test_api, seq, match="undefined for Nothing")
+
+    def test_compare_void_function_results_not_equal(self, fprime_test_api):
+        """A void call has no value, so it cannot be an operand of !=."""
+        seq = """
+def f():
+    pass
+
+def g():
+    pass
+
+if f() != g():
+    exit(1)
+"""
+        assert_compile_failure(fprime_test_api, seq, match="undefined for Nothing")
+
     def test_call_embedded_in_bare_expression(self, fprime_test_api):
         """A bare expression statement whose top-level node isn't itself
         side-effecting (an == comparison) still embeds a call that is; the