fixes for minos everywhere when fcn is n-dim (#591)

src/iminuit/_minuit.py

@@ -793,6 +793,8 @@ def minos(self, *parameters, cl=None, ncall=None):
 
             self
         """
+        ncall = 0 if ncall is None else int(ncall)
+
         if cl is None:
             factor = 1.0
         else:
@@ -803,19 +805,20 @@ def minos(self, *parameters, cl=None, ncall=None):
             factor = chi2(1).ppf(cl)
 
         if not self._fmin:
-            # create a FunctionMinimum for MnMinos
+            # create a seed minimum for MnMinos
             fm = FunctionMinimum(
                 self._fcn, self._last_state, self._strategy, self._tolerance
             )
+            # running MnHesse on seed is necessary for MnMinos to work
+            hesse = MnHesse(self.strategy)
+            hesse(self._fcn, fm, ncall)
+            self._last_state = fm.state
+            self._make_covariance()
         else:
             fm = self._fmin._src
 
-        ncall = 0 if ncall is None else int(ncall)
-
         if not fm.is_valid:
-            raise RuntimeError(
-                "Function minimum is not valid. Make sure MIGRAD converged."
-            )
+            raise RuntimeError("Function minimum is not valid.")
 
         if len(parameters) == 0:
             pars = [par for par in self.parameters if not self.fixed[par]]

tests/test_minuit.py

@@ -1461,10 +1461,14 @@ def test_minos_new_min():
     assert m.merrors["x"].upper == approx(1.1, abs=1e-2)
 
 
-@pytest.mark.parametrize("x", (0.9, 1.0, 1.1))
-def test_minos_without_migrad(x):
-    m = Minuit(lsq(lambda x: (x - 1) ** 2), x)
+def test_minos_without_migrad():
+    m = Minuit(lsq(lambda x, y: (x - 1) ** 2 + (y / 2) ** 2), 1.001, 0.001)
     m.minos()
     me = m.merrors["x"]
-    assert x + me.lower == approx(0, abs=5e-3)
-    assert x + me.upper == approx(2, abs=5e-3)
+    assert me.is_valid
+    assert me.lower == approx(-1, abs=5e-3)
+    assert me.upper == approx(1, abs=5e-3)
+    me = m.merrors["y"]
+    assert me.is_valid
+    assert me.lower == approx(-2, abs=5e-3)
+    assert me.upper == approx(2, abs=5e-3)