Remove restriction that OUTPUT cannot be in any input directory

We need this for nancy-invoice, for example.

This restriction was moved to Nancy from Linton. It makes sense in
Linton (and in many or even most Nany uses), but not always for Nancy.

README.md

@@ -51,8 +51,6 @@ options:
 
 The INPUT-PATH is a ':'-separated list; the inputs are merged
 in left-to-right order.
-
-OUTPUT cannot be in any input directory.
 ```
 
 ## Operation <a name="operation"></a>

nancy/__init__.py

@@ -269,8 +269,7 @@ def main(argv: List[str] = sys.argv[1:]) -> None:
         description="A simple templating system.",
         formatter_class=argparse.RawDescriptionHelpFormatter,
         epilog=f"The INPUT-PATH is a '{os.path.pathsep}'-separated list; the inputs are merged\n"
-        + "in left-to-right order.\n\n"
-        + "OUTPUT cannot be in any input directory.",
+        + "in left-to-right order."
     )
     parser.add_argument(
         "input",
@@ -303,25 +302,10 @@ def main(argv: List[str] = sys.argv[1:]) -> None:
 
         # Deal with special case where INPUT is a single file and --path is not
         # given.
-        single_file_no_path = False
-        if args.path is None and len(inputs) == 1:
-            if os.path.isfile(inputs[0]):
-                single_file_no_path = True
-                args.path = inputs[0]
-                inputs[0] = os.getcwd()
-
-        # Check output is not under an input path provided we're not dealing with
-        # the special case above.
-        if not single_file_no_path:
-            for root in inputs:
-                relative = os.path.relpath(args.output, start=root)
-                if (
-                    args.output != "-"
-                    and relative != ""
-                    and not relative.startswith("..")
-                    and not os.path.isabs(relative)
-                ):
-                    raise ValueError("output cannot be in any input directory")
+        if args.path is None and len(inputs) == 1 and os.path.isfile(inputs[0]):
+            args.path = inputs[0]
+            inputs[0] = os.getcwd()
+
         expand(inputs, args.output, args.path)
     except Exception as err:  # pylint: disable=broad-exception-caught
         if "DEBUG" in os.environ:

tests/test_nancy.py

@@ -292,20 +292,6 @@ def test_absolute_build_path_causes_an_error(
         )
 
 
-def test_output_to_subdirectory_of_input_causes_an_error(
-    capsys: CaptureFixture[str],
-    caplog: LogCaptureFixture,
-) -> None:
-    with chdir(tests_dir):
-        failing_cli_test(
-            capsys,
-            caplog,
-            ["webpage-src"],
-            "output cannot be in any input directory",
-            "webpage-src/foo",
-        )
-
-
 # pylint: disable-next=invalid-name
 def test_empty_INPUT_PATH_causes_an_error(
     capsys: CaptureFixture[str],