Log a warning if the user specifies -P and the output file is present but empty

piptools/scripts/compile.py

@@ -444,6 +444,16 @@ def cli(
     # Proxy with a LocalRequirementsRepository if --upgrade is not specified
     # (= default invocation)
     if not upgrade and os.path.exists(output_file.name):
+        output_file_is_empty = os.path.getsize(output_file.name) == 0
+        if upgrade_install_reqs and output_file_is_empty:
+            log.warning(
+                f"WARNING: the output file {output_file.name} exists but is empty. "
+                "Pip-tools cannot upgrade only specific packages (using -P/--upgrade-package) "
+                "without an existing pin file to provide constraints. "
+                "This often occurs if you redirect standard output to your output file, "
+                "as any existing content is truncated."
+            )
+
         # Use a temporary repository to ensure outdated(removed) options from
         # existing requirements.txt wouldn't get into the current repository.
         tmp_repository = PyPIRepository(pip_args, cache_dir=cache_dir)

tests/test_cli_compile.py

@@ -858,6 +858,27 @@ def test_upgrade_packages_option_no_existing_file(pip_conf, runner):
     assert out.exit_code == 0
     assert "small-fake-a==0.2" in out.stderr.splitlines()
     assert "small-fake-b==0.3" in out.stderr.splitlines()
+    assert (
+        "WARNING: the output file requirements.txt exists but is empty"
+        not in out.stderr
+    )
+
+
+def test_upgrade_packages_empty_target_file_warning(pip_conf, runner):
+    """
+    piptools warns the user if --upgrade-package/-P is specified and the
+    output file exists, but is empty.
+    """
+    with open("requirements.in", "w") as req_in:
+        req_in.write("small-fake-a==0.2")
+    with open("requirements.txt", "w") as req_txt:
+        req_txt.write("")
+
+    out = runner.invoke(cli, ["--no-annotate", "-P", "small-fake-a"])
+
+    assert out.exit_code == 0
+    assert "small-fake-a==0.2" in out.stderr.splitlines()
+    assert "WARNING: the output file requirements.txt exists but is empty" in out.stderr
 
 
 @pytest.mark.parametrize(