Workaround methods for mypy type inference problems with converters

See python/mypy#5738 and  python-attrs/attrs#519

immutablecollections/converter_utils.py

@@ -0,0 +1,63 @@
+from typing import Any, Iterable, Mapping, Optional, Tuple, Union
+
+from immutablecollections import (
+    ImmutableDict,
+    ImmutableListMultiDict,
+    ImmutableSet,
+    ImmutableSetMultiDict,
+    immutabledict,
+    immutablelistmultidict,
+    immutableset,
+    immutablesetmultidict,
+)
+
+
+def _to_tuple(val: Iterable[Any]) -> Tuple[Any, ...]:
+    """Needed until https://github.com/python/mypy/issues/5738
+    and https://github.com/python-attrs/attrs/issues/519 are fixed.
+    """
+    return tuple(val)
+
+
+def _to_immutableset(val: Optional[Iterable[Any]]) -> ImmutableSet[Any]:
+    """Needed until https://github.com/python/mypy/issues/5738
+        and https://github.com/python-attrs/attrs/issues/519 are fixed.
+    """
+    return immutableset(val)
+
+
+def _to_immutabledict(
+    val: Optional[
+        Union[Iterable[Tuple[Any, Any]], Mapping[Any, Any], ImmutableDict[Any, Any]]
+    ]
+) -> ImmutableDict[Any, Any]:
+    """Needed until https://github.com/python/mypy/issues/5738
+        and https://github.com/python-attrs/attrs/issues/519 are fixed.
+    """
+    return immutabledict(val)
+
+
+def _to_immutablesetmultidict(
+    val: Optional[
+        Union[
+            Iterable[Tuple[Any, Any]], Mapping[Any, Any], ImmutableSetMultiDict[Any, Any]
+        ]
+    ]
+) -> ImmutableSetMultiDict[Any, Any]:
+    """Needed until https://github.com/python/mypy/issues/5738
+        and https://github.com/python-attrs/attrs/issues/519 are fixed.
+    """
+    return immutablesetmultidict(val)
+
+
+def _to_immutablelistmultidict(
+    val: Optional[
+        Union[
+            Iterable[Tuple[Any, Any]], Mapping[Any, Any], ImmutableListMultiDict[Any, Any]
+        ]
+    ]
+) -> ImmutableListMultiDict[Any, Any]:
+    """Needed until https://github.com/python/mypy/issues/5738
+        and https://github.com/python-attrs/attrs/issues/519 are fixed.
+    """
+    return immutablelistmultidict(val)