cat buildtest.hcl
group "a" {
  targets = ["b", "c"]
}
group "b" {
  targets = ["d"]
}
group "c" {
  targets = ["b"]
}
target "d" {
  context = "."
  dockerfile = "./testdockerfile"
}

docker buildx bake a -f buildtest.hcl
[+] Building 0.0s (0/0)
error: failed to find target b