params: fix diff bt active branch and workspace

dvc/repo/params/diff.py

@@ -13,6 +13,12 @@ def diff(repo, *args, a_rev=None, b_rev=None, **kwargs):
 
     params = repo.params.show(*args, **kwargs, revs=[a_rev, b_rev])
 
+    # workspace may have been replaced by active branch
+    workspace_rev = (a_rev == "workspace") or (b_rev == "workspace")
+    if workspace_rev and "workspace" not in params:
+        active_branch = repo.scm.active_branch()
+        params["workspace"] = params[active_branch]
+
     old = params.get(a_rev, {}).get("data", {})
     new = params.get(b_rev, {}).get("data", {})
 

tests/func/params/test_diff.py

@@ -268,3 +268,11 @@ def test_diff_top_level_params(tmp_dir, dvc, scm, dvcfile, params_file):
             "foo": {"diff": 2, "new": 5, "old": 3}
         }
     }
+
+
+def test_diff_active_branch_no_changes(tmp_dir, scm, dvc):
+    tmp_dir.gen("params.yaml", "foo: bar")
+    dvc.run(cmd="echo params.yaml", params=["foo"], single_stage=True)
+    scm.add(["params.yaml", "Dvcfile"])
+    scm.commit("bar")
+    assert dvc.params.diff(a_rev=tmp_dir.scm.active_branch()) == {}