diff --git a/lib/spack/spack/solver/concretize.lp b/lib/spack/spack/solver/concretize.lp
index d706663aaae..22ba77142a3 100644
--- a/lib/spack/spack/solver/concretize.lp
+++ b/lib/spack/spack/solver/concretize.lp
@@ -227,6 +227,7 @@ provider_weight(Package, 100)
     provider(Package, Virtual),
     not default_provider_preference(Virtual, Package, _).
 
+#defined possible_provider/2.
 #defined provider_condition/3.
 #defined required_provider_condition/3.
 #defined required_provider_condition/4.