Allow build deps as long as at least one setup file is in sources list.

Passing ``--build-deps-for`` or ``--all-build-deps`` should not emit an error as long as at least one file in the sources list is a setup file.

piptools/scripts/compile.py

@@ -332,13 +332,6 @@ def cli(
     setup_file_found = False
     for src_file in src_files:
         is_setup_file = os.path.basename(src_file) in METADATA_FILENAMES
-        if not is_setup_file and build_deps_targets:
-            msg = (
-                "--build-deps-for and --all-build-deps can be used only with the "
-                "setup.py, setup.cfg and pyproject.toml specs."
-            )
-            raise click.BadParameter(msg)
-
         if src_file == "-":
             # pip requires filenames and not files. Since we want to support
             # piping from stdin, we need to briefly save the input from stdin
@@ -430,6 +423,13 @@ def cli(
         msg = "--extra has effect only with setup.py and PEP-517 input formats"
         raise click.BadParameter(msg)
 
+    if build_deps_targets and not setup_file_found:
+        msg = (
+            "--build-deps-for and --all-build-deps can be used only with the "
+            "setup.py, setup.cfg and pyproject.toml specs."
+        )
+        raise click.BadParameter(msg)
+
     primary_packages = {
         key_from_ireq(ireq) for ireq in constraints if not ireq.constraint
     }

tests/test_cli_compile.py

@@ -2964,6 +2964,55 @@ def test_build_deps_fail_without_setup_file(runner, tmpdir, option):
     assert exp in out.stderr
 
 
+@backtracking_resolver_only
+@pytest.mark.parametrize("option", ("--all-build-deps", "--build-deps-for=wheel"))
+def test_build_deps_does_not_error_when_setup_included_with_multiple_source_files(
+    fake_dists_with_build_deps,
+    runner,
+    tmp_path,
+    monkeypatch,
+    current_resolver,
+    option,
+):
+    """
+    Test that passing ``--build-deps-for`` or ``--all-build-deps`` does not emit an
+    error as long as at least one file in the sources list is a setup file.
+    """
+    src_pkg_path = pathlib.Path(PACKAGES_PATH) / "small_fake_with_build_deps"
+    # When used as argument to the runner it is not passed to pip
+    monkeypatch.setenv("PIP_FIND_LINKS", fake_dists_with_build_deps)
+
+    requirements_path = pathlib.Path(tmp_path) / "requirements.in"
+    requirements_path.write_text("\n")
+
+    pyproject_toml_path = pathlib.Path(tmp_path) / "pyproject.toml"
+    pyproject_toml_path.write_text("\n")
+
+    with runner.isolated_filesystem(tmp_path) as tmp_pkg_path:
+        shutil.copytree(src_pkg_path, tmp_pkg_path, dirs_exist_ok=True)
+        out = runner.invoke(
+            cli,
+            [
+                "--allow-unsafe",
+                "--output-file",
+                "-",
+                "--quiet",
+                "--no-emit-options",
+                "--no-header",
+                option,
+                os.fspath(requirements_path),
+                os.fspath(pyproject_toml_path)
+            ],
+        )
+
+    exp = (
+        "--build-deps-for and --all-build-deps can be used only with the "
+        "setup.py, setup.cfg and pyproject.toml specs."
+    )
+    assert exp not in out.stderr
+    assert out.exit_code == 0
+
+
 def test_extras_fail_with_requirements_in(runner, tmpdir):
     """
     Test that passing ``--extra`` with ``requirements.in`` input file fails.