This is a genre of invalid output expression that we've seen quite
commonly while testing with 0.11.0-rc1, so we'll call it out specifically
in the upgrade guide and suggest how to fix it.
We are recommending that as of 0.11 all provider configurations be placed
in the root module and, where necessary, be explicitly passed down via
a providers map to customize which configurations are seen by each
child module.
This new section attempts to guide users through such refactoring in the
common case where a child module defines its own provider configuration
based on a value passed in an input variable, and then uses that as
some context to link to the more detailed docs to help those who have
more complex configurations.
This changed close to the release of beta1 to use underscores as the
separator and to use a lower-case "v" to avoid any issues on
case-insensitive filesystems.
James Bardin