Fix when_spec inheritance for extensions (#7746)

Popping the when spec from kwargs in the extends directive breaks
class inheritance. Inheriting classes do not find their when spec.

We now get the when spec from kwargs instead, leaving it to be found
by any downstream package classes.

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/spack/spack/directives.py b/lib/spack/spack/directives.py
index 1e2435eac2..eca984093d 100644
--- a/lib/spack/spack/directives.py
+++ b/lib/spack/spack/directives.py
@@ -381,7 +381,7 @@ def extends(spec, **kwargs):
         #     msg = 'Packages can extend at most one other package.'
         #     raise DirectiveError(directive, msg)
 
-        when = kwargs.pop('when', pkg.name)
+        when = kwargs.get('when', pkg.name)
         _depends_on(pkg, spec, when=when)
         pkg.extendees[spec] = (Spec(spec), kwargs)
     return _execute_extends