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+;;; test-eat-config--charset-never-nil.el --- Tests for the EAT charset nil-guard -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Unit tests for `cj/--eat-charset-never-nil', the `:filter-args' guard on
+;; `eat--t-set-charset'.  eat 0.9.4 accepts more charset-designation final
+;; bytes in its parser than its store step maps, so an unmapped designation
+;; (e.g. ESC ( A) stores nil as that slot's charset; the next character write
+;; then trips (cl-assert charset) in `eat--t-write', repeatedly, via the
+;; output-queue timer.  The guard coerces a nil charset to `us-ascii' before
+;; it is ever stored, so the assertion can't fire.
+
+;;; Code:
+
+(require 'ert)
+
+;; Stub keymap dep before loading the module (matches the other module tests).
+(defvar cj/custom-keymap (make-sparse-keymap)
+  "Stub keymap for testing.")
+
+(require 'eat-config)
+
+(ert-deftest test-eat-config-charset-never-nil-normal-real-charset-unchanged ()
+  "Normal: a real charset value passes through unchanged."
+  (should (equal (cj/--eat-charset-never-nil '(g0 us-ascii))
+                 '(g0 us-ascii)))
+  (should (equal (cj/--eat-charset-never-nil '(g1 dec-line-drawing))
+                 '(g1 dec-line-drawing))))
+
+(ert-deftest test-eat-config-charset-never-nil-error-nil-becomes-us-ascii ()
+  "Error: a nil charset (unmapped designation) is coerced to `us-ascii'."
+  (should (equal (cj/--eat-charset-never-nil '(g0 nil))
+                 '(g0 us-ascii)))
+  (should (equal (cj/--eat-charset-never-nil '(g3 nil))
+                 '(g3 us-ascii))))
+
+(ert-deftest test-eat-config-charset-never-nil-boundary-preserves-slot ()
+  "Boundary: the slot symbol is preserved for every slot, coerced or not."
+  (dolist (slot '(g0 g1 g2 g3))
+    (should (eq (car (cj/--eat-charset-never-nil (list slot nil))) slot))
+    (should (eq (cadr (cj/--eat-charset-never-nil (list slot nil))) 'us-ascii))))
+
+(provide 'test-eat-config--charset-never-nil)
+;;; test-eat-config--charset-never-nil.el ends here