Can't figure out how variant dependency works with virtual packages

Steps to reproduce

I am trying to write a PR to allow Scalapack build with 8-byte integers as described at https://github.com/Reference-ScaLAPACK/scalapack/blob/master/README#L141-L149 and as discussed yesterday at the concall with @scheibelp and others.

For now I wrote just some boilerplate as follows

$ git diff var/
diff --git a/var/spack/repos/builtin/packages/netlib-scalapack/package.py b/var/spack/repos/builtin/packages/netlib-scalapack/package.py
index 245dcfaf0c..13c2e9fd95 100644
--- a/var/spack/repos/builtin/packages/netlib-scalapack/package.py
+++ b/var/spack/repos/builtin/packages/netlib-scalapack/package.py
@@ -15,12 +15,14 @@ class ScalapackBase(CMakePackage):

     variant("shared", default=True, description="Build the shared library version")
     variant("pic", default=False, description="Build position independent code")
+    variant("int8", default=False, description="Build with 8-byte (long) integers rather than the regular 4-byte ones")

     provides("scalapack")

     depends_on("mpi")
     depends_on("lapack")
-    depends_on("blas")
+    depends_on("blas", when="~int8")
+    depends_on("blas+int8", when="+int8")
     depends_on("cmake", when="@2.0.0:", type="build")

     # See: https://github.com/Reference-ScaLAPACK/scalapack/issues/9
diff --git a/var/spack/repos/builtin/packages/openblas/package.py b/var/spack/repos/builtin/packages/openblas/package.py
index 2785f3a06d..71c1affee7 100644
--- a/var/spack/repos/builtin/packages/openblas/package.py
+++ b/var/spack/repos/builtin/packages/openblas/package.py
@@ -69,6 +69,7 @@ class Openblas(CMakePackage, MakefilePackage):
         description="w/o a Fortran compiler, OpenBLAS will build an f2c-converted LAPACK",
     )

+    variant("int8", default=False, description="Build with 8-byte (long) integers rather than the regular 4-byte ones")
     variant("ilp64", default=False, description="Force 64-bit Fortran native integers")
     variant("pic", default=True, description="Build position independent code")
     variant("shared", default=True, description="Build shared libraries")

Error message

and unfortunately fails as follows

$ spack spec scalapack+int8
==> Error: Spack concretizer internal error. Please submit a bug report and include the command, environment if applicable and the following error message.
    scalapack+int8 is unsatisfiable

Am I doing something wrong?

Information on your system

• Spack: 0.23.1 (2bfcc69fa870d3c6919be87593f22647981b648a)
• Python: 3.6.8
• Platform: linux-rhel8-icelake

General information

• I have run spack debug report and reported the version of Spack/Python/Platform
• I have searched the issues of this repo and believe this is not a duplicate
• I have run the failing commands in debug mode and reported the output

Reproduce the failure with spack spec scalapack+int8 using the versions and platform reported. Read var/spack/repos/builtin/packages/netlib-scalapack/package.py and var/spack/repos/builtin/packages/openblas/package.py, then trace how variant-qualified virtual dependencies are handled by the concretizer. Done means the intended Scalapack specification no longer reports an unsatisfiable concretizer error, with coverage for this dependency combination.