Use stack to build executable of a package dependency

Just one clarification first: Do you need that-exec only built as part of the dependency, or do you need the executable available to run via stack exec that-exec from the main project?

that-project is currently under extra-deps, not under packages. That's why Stack says: Unknown project package: that-Project Meaning, that-project is known as a dependency, but not as a project package.

stack build that-project:exe:that-exec

Stack is looking for that-project inside packages:, but it only finds . there. The Bitbucket dependenccy is in extra-deps, so Stack does not treat it as a project package component target.

Move the git dependency from extra-deps to packages:

resolver: lts-xx.y
packages:
  - .
  - git: git@bitbucket.org:workspace/that-project.git
    commit: 0123456789abcdef0123456789abcdef01234567

with above code, this should make more sense stack build that-project:exe:that-exec now that-project is treated as a project package, not just an extra dependency.

Using cabal instead worked. I created a cabal.project file:

source-repository-package
  type: git
  location: git@bitbucket.org:workspace/that-project.git
  tag: 0123456789abcdef0123456789abcdef01234567

and cabal build that-project:exe:that-exe worked.