TST: optimize.minimize: test improved error message

scipy/optimize/_trustregion_constr/equality_constrained_sqp.py

@@ -86,7 +86,7 @@ def equality_constrained_sqp(fun_and_constr, grad_and_jac, lagr_hess,
             raise ValueError(
                 "The 'expected square matrix' error can occur if there are"
                 " more equality constraints than independent variables."
-                " Consider how your constraints are setup, or use"
+                " Consider how your constraints are set up, or use"
                 " factorization_method='SVDFactorization'."
             ) from e
         else:

scipy/optimize/tests/test_minimize_constrained.py

@@ -10,7 +10,8 @@
                             Bounds,
                             minimize,
                             BFGS,
-                            SR1)
+                            SR1,
+                            rosen)
 
 
 class Maratos:
@@ -747,6 +748,22 @@ def nci(x):
     assert_allclose(res.fun, ref.fun)
 
 
+def test_gh20665_too_many_constraints():
+    # gh-20665 reports a confusing error message when there are more equality
+    # constraints than variables. Check that the error message is improved.
+    message = "...more equality constraints than independent variables..."
+    with pytest.raises(ValueError, match=message):
+        x0 = np.ones((2,))
+        A_eq, b_eq = np.arange(6).reshape((3, 2)), np.ones((3,))
+        g = NonlinearConstraint(lambda x:  A_eq @ x, lb=b_eq, ub=b_eq)
+        minimize(rosen, x0, method='trust-constr', constraints=[g])
+    # no error with `SVDFactorization`
+    with np.testing.suppress_warnings() as sup:
+        sup.filter(UserWarning)
+        minimize(rosen, x0, method='trust-constr', constraints=[g],
+                 options={'factorization_method': 'SVDFactorization'})
+
+
 class TestBoundedNelderMead:
 
     @pytest.mark.parametrize('bounds, x_opt',