Reproduces a label_flag bug in bazel

repro.sh

#!/bin/bash

function executable() {
  local name="$1"
  cat > "${name}.cc" <<EOF
#include <cstdlib>
#include <iostream>

int main(int argc, char *argv[]) {
  std::cout << "${name}\n";
  return EXIT_SUCCESS;
}
EOF
}

function workspace() {
  local name="$1"
  local location="$2"
}


readonly tmp=$(mktemp -d)
pushd "${tmp}"
mkdir label
cat > "label/BUILD.tpl" <<EOF
cc_library(name = "placeholder")
label_flag(
    name = "label",
    build_setting_default = "@test_label//:placeholder",
    visibility = ["//visibility:public"],
)
EOF

cat > "label/WORKSPACE.tpl" <<EOF
workspace(name = "test_label")
EOF

cat > "label/BUILD" <<EOF
exports_files(["BUILD.tpl", "WORKSPACE.tpl"])
EOF

cat > repo.bzl <<EOF
def _repo_impl(repository_ctx):
    repository_ctx.template("BUILD", Label("@test//label:BUILD.tpl"))
    repository_ctx.template("WORKSPACE", Label("@test//label:WORKSPACE.tpl"))

make_repo = repository_rule(implementation = _repo_impl)
EOF

cat > WORKSPACE <<EOF
workspace(name = "test")

load(":repo.bzl", "make_repo")
make_repo(name = "test_label")
EOF

executable "foo"
executable "bar"

cat > BUILD <<EOF
cc_library(name = "foo")

cc_binary(
    name = "qux",
    srcs = select({
        ":is_foo": ["foo.cc"],
        "//conditions:default": ["bar.cc"],
    }),
)

config_setting(
    name = "is_foo",
    flag_values = {"@test_label//:label": "//:foo"},
)
EOF

# Should print "foo", but is a command line parse error.
bazel build --@test_label:label=//:foo :qux

# Should print "foo", but is an error about a missing BUILD file?
bazel build --//external/test_label:label=//:foo :qux