Allow downloading of sdists for specific platform with --no-deps

src/pip/_internal/commands/download.py

@@ -139,12 +139,17 @@ class DownloadCommand(RequirementCommand):
             options.implementation,
         ])
         binary_only = FormatControl(set(), {':all:'})
-        if dist_restriction_set and options.format_control != binary_only:
+        no_sdist_dependencies = (
+            options.format_control != binary_only and
+            not options.ignore_dependencies
+        )
+        if dist_restriction_set and no_sdist_dependencies:
             raise CommandError(
-                "--only-binary=:all: must be set and --no-binary must not "
-                "be set (or must be set to :none:) when restricting platform "
-                "and interpreter constraints using --python-version, "
-                "--platform, --abi, or --implementation."
+                "When restricting platform and interpreter constraints using "
+                "--python-version, --platform, --abi, or --implementation, "
+                "either --no-deps must be set, or --only-binary=:all: must be "
+                "set and --no-binary must not be set (or must be set to "
+                ":none:)."
             )
 
         options.src_dir = os.path.abspath(options.src_dir)

tests/functional/test_download.py

@@ -187,10 +187,30 @@ def test_only_binary_set_then_download_specific_platform(script, data):
     )
 
 
-def test_not_only_binary_then_download_specific_platform_fails(script, data):
+def test_no_deps_set_then_download_specific_platform(script, data):
     """
     Confirm that specifying an interpreter/platform constraint
-    enforces that ``--only-binary=:all:`` is set.
+    is allowed when ``--no-deps`` is set.
+    """
+    fake_wheel(data, 'fake-1.0-py2.py3-none-any.whl')
+
+    result = script.pip(
+        'download', '--no-index', '--find-links', data.find_links,
+        '--no-deps',
+        '--dest', '.',
+        '--platform', 'linux_x86_64',
+        'fake'
+    )
+    assert (
+        Path('scratch') / 'fake-1.0-py2.py3-none-any.whl'
+        in result.files_created
+    )
+
+
+def test_download_specific_platform_fails(script, data):
+    """
+    Confirm that specifying an interpreter/platform constraint
+    enforces that ``--no-deps`` or ``--only-binary=:all:`` is set.
     """
     fake_wheel(data, 'fake-1.0-py2.py3-none-any.whl')