fix of minos with var keyword and fixed parameters (#459)

Author: HDembinski

src/iminuit/_libiminuit.pyx

@@ -1023,9 +1023,11 @@ cdef class Minuit:
                 deref(self.cfmin), self.strategy
             )
 
-        for vname in self.parameters:
+
+        vnames = self.parameters if var is None else [var]
+        for vname in vnames:
             if self.fixed[vname]:
-                if var is not None:  #specifying vname but it's fixed
+                if var is not None and var == vname:
                     warn('Specified variable name for minos is fixed',
                          mutil.IMinuitWarning)
                     return None

src/iminuit/tests/test_iminuit.py

@@ -493,6 +493,17 @@ def test_minos_all(grad, sigma):
         m.merrors[-3]
 
 
+def test_minos_all_some_fix():
+    m = Minuit(func0, fix_x=True, pedantic=False)
+    m.migrad()
+    m.minos()
+    assert "x" not in m.merrors
+    me = m.merrors["y"]
+    assert me.name == "y"
+    assert me.lower == pytest.approx(-1)
+    assert me.upper == pytest.approx(1)
+
+
 @pytest.mark.parametrize("grad", (None, func0_grad))
 def test_minos_single(grad):
     m = Minuit(func0, grad=func0_grad, pedantic=False)
@@ -502,6 +513,16 @@ def test_minos_single(grad):
     assert m.ncalls < 15
 
 
+def test_minos_single_fixed():
+    m = Minuit(func0, fix_x=True, pedantic=False)
+    m.migrad()
+    m.minos("y")
+    me = m.merrors["y"]
+    assert me.name == "y"
+    assert me.lower == pytest.approx(-1)
+    assert me.upper == pytest.approx(1)
+
+
 def test_minos_single_fixed_raising():
     m = Minuit(func0, pedantic=False, fix_x=True)
     m.migrad()