Fix coercion of scalar fields in modules of differential forms (trac #27658)

Author: egourgoulhon

src/sage/manifolds/differentiable/diff_form_module.py

@@ -24,26 +24,28 @@
 - [Lee2013]_
 
 """
-#******************************************************************************
+# *****************************************************************************
 #       Copyright (C) 2015 Eric Gourgoulhon <[email protected]>
 #       Copyright (C) 2016 Travis Scrimshaw <[email protected]>
 #
 #  Distributed under the terms of the GNU General Public License (GPL)
 #  as published by the Free Software Foundation; either version 2 of
 #  the License, or (at your option) any later version.
-#                  http://www.gnu.org/licenses/
-#******************************************************************************
+#                  https://www.gnu.org/licenses/
+# *****************************************************************************
 
 from sage.misc.cachefunc import cached_method
 from sage.structure.unique_representation import UniqueRepresentation
 from sage.structure.parent import Parent
 from sage.categories.modules import Modules
 from sage.rings.integer import Integer
 from sage.tensor.modules.ext_pow_free_module import ExtPowerDualFreeModule
+from sage.manifolds.scalarfield import ScalarField
 from sage.manifolds.differentiable.diff_form import DiffForm, DiffFormParal
 from sage.manifolds.differentiable.tensorfield import TensorField
 from sage.manifolds.differentiable.tensorfield_paral import TensorFieldParal
 
+
 class DiffFormModule(UniqueRepresentation, Parent):
     r"""
     Module of differential forms of a given degree `p` (`p`-forms) along a
@@ -354,6 +356,10 @@ def _element_constructor_(self, comp=[], frame=None, name=None,
             else:
                 raise TypeError("cannot convert the {} ".format(tensor) +
                                 "to an element of {}".format(self))
+        if isinstance(comp, ScalarField):
+            # since the degree of self is >= 1, we cannot coerce scalar fields:
+            raise TypeError("cannot convert the {} ".format(comp) +
+                            "to an element of {}".format(self))
         # standard construction
         resu = self.element_class(self._vmodule, self._degree, name=name,
                                   latex_name=latex_name)
@@ -545,7 +551,7 @@ def degree(self):
         """
         return self._degree
 
-#******************************************************************************
+# *****************************************************************************
 
 class DiffFormFreeModule(ExtPowerDualFreeModule):
     r"""
@@ -790,6 +796,12 @@ def _element_constructor_(self, comp=[], frame=None, name=None,
             sage: A(0) is A.zero()
             True
 
+        Check that #27658 is fixed::
+
+            sage: f = M.scalar_field(x)
+            sage: f in A
+            False
+
         """
         if isinstance(comp, (int, Integer)) and comp == 0:
             return self.zero()
@@ -815,6 +827,10 @@ def _element_constructor_(self, comp=[], frame=None, name=None,
             else:
                 raise TypeError("cannot convert the {} ".format(tensor) +
                                 "to an element of {}".format(self))
+        if isinstance(comp, ScalarField):
+            # since the degree of self is >= 1, we cannot coerce scalar fields:
+            raise TypeError("cannot convert the {} ".format(comp) +
+                            "to an element of {}".format(self))
         resu = self.element_class(self._fmodule, self._degree, name=name,
                                   latex_name=latex_name)
         if comp != []: