yak(mutation-testing-with-muex-blogex-pure-app-code-1341): mutation testing with muex (blogex + pure app code)

blogex/test/blogex/link_validator_test.exs

@@ -245,6 +245,24 @@ defmodule Blogex.LinkValidatorTest do
   end
 
   describe "validate_body/2" do
+    # The guard is a conjunction, not a disjunction: both arguments must have
+    # the documented types. Mutating `and` to `or` would let a string blog id
+    # (or a non-binary body) through the guard to crash somewhere else, so pin
+    # the contract at the door.
+    test "rejects a non-atom blog id at the function clause guard" do
+      assert_raise FunctionClauseError, fn ->
+        # apply/3 keeps the compiler's type checker quiet: the point is the
+        # runtime guard, and the test deliberately passes an invalid type.
+        apply(LinkValidator, :validate_body, ["Just text, no links.", "engineering"])
+      end
+    end
+
+    test "rejects a non-binary body at the function clause guard" do
+      assert_raise FunctionClauseError, fn ->
+        apply(LinkValidator, :validate_body, [[], :engineering])
+      end
+    end
+
     test "returns :ok when body has no internal blog links" do
       body = "Just text, no links."