gstrfuncs: Fix regression in types accepted by g_str_equal()

The new macro form of `g_str_equal()` had stricter type checking than
the original function form. That would be nice, except it causes new
compiler warnings in third party projects, which counts as an API break
for us, so unfortunately we can’t do it.

Add some tests to prevent regressions on this again.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #2809

glib/ghash.h

@@ -162,7 +162,7 @@ gboolean g_str_equal    (gconstpointer  v1,
                          gconstpointer  v2);
 
 /* Macro for optimization in the case it is not used as callback function */
-#define g_str_equal(v1, v2) (strcmp ((v1), (v2)) == 0)
+#define g_str_equal(v1, v2) (strcmp ((gconstpointer) (v1), (gconstpointer) (v2)) == 0)
 
 GLIB_AVAILABLE_IN_ALL
 guint    g_str_hash     (gconstpointer  v);

glib/tests/strfuncs.c

@@ -2178,6 +2178,31 @@ test_transliteration (void)
   g_free (out);
 }
 
+static void
+test_str_equal (void)
+{
+  const guchar *unsigned_a = (const guchar *) "a";
+
+  g_test_summary ("Test macro and function forms of g_str_equal()");
+
+  /* Test function form. */
+  g_assert_true ((g_str_equal) ("a", "a"));
+  g_assert_false ((g_str_equal) ("a", "b"));
+
+  /* Test macro form. */
+  g_assert_true (g_str_equal ("a", "a"));
+  g_assert_false (g_str_equal ("a", "b"));
+
+  /* As g_str_equal() is defined for use with GHashTable, it takes gconstpointer
+   * arguments, so can historically accept unsigned arguments. We need to
+   * continue to support that. */
+  g_assert_true ((g_str_equal) (unsigned_a, "a"));
+  g_assert_false ((g_str_equal) (unsigned_a, "b"));
+
+  g_assert_true (g_str_equal (unsigned_a, "a"));
+  g_assert_false (g_str_equal (unsigned_a, "b"));
+}
+
 /* Testing g_strv_contains() function with various cases */
 static void
 test_strv_contains (void)
@@ -2676,6 +2701,7 @@ main (int   argc,
   g_test_add_func ("/strfuncs/strv-length", test_strv_length);
   g_test_add_func ("/strfuncs/test-is-to-digit", test_is_to_digit);
   g_test_add_func ("/strfuncs/transliteration", test_transliteration);
+  g_test_add_func ("/strfuncs/str-equal", test_str_equal);
 
   return g_test_run();
 }