TST: Add regression test for bad promotion error/path

Author: seberg

numpy/_core/tests/test_stringdtype.py

@@ -845,6 +845,15 @@ def test_add_promoter(string_list):
         np.add("a", "b", signature=("U", "U", StringDType))
 
 
+def test_add_no_legacy_promote_with_signature():
+    # Possibly misplaced, but useful to test with string DType.  We check that
+    # if there is clearly no loop found, a stray `dtype=` doesn't break things
+    # Regression test for the bad error in gh-26735
+    # (If legacy promotion is gone, this can be deleted...)
+    with pytest.raises(TypeError, match=".*did not contain a loop"):
+        np.add("3", 6, dtype=StringDType)
+
+
 def test_add_promoter_reduce():
     # Exact TypeError could change, but ensure StringDtype doesn't match
     with pytest.raises(TypeError, match="the resolved dtypes are not"):