Improve constructor for builtins.dict (#8517)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

test_cases/stdlib/builtins/test_dict.py

@@ -0,0 +1,49 @@
+# pyright: reportUnnecessaryTypeIgnoreComment=true
+
+from typing import Dict, Generic, Iterable, Tuple, TypeVar
+from typing_extensions import assert_type
+
+# These do follow `__init__` overloads order:
+# mypy and pyright have different opinions about this one:
+# mypy raises: 'Need type annotation for "bad"'
+# pyright is fine with it.
+# bad = dict()
+good: Dict[str, str] = dict()
+assert_type(good, Dict[str, str])
+
+assert_type(dict(arg=1), Dict[str, int])
+
+_KT = TypeVar("_KT")
+_VT = TypeVar("_VT")
+
+
+class KeysAndGetItem(Generic[_KT, _VT]):
+    def keys(self) -> Iterable[_KT]:
+        ...
+
+    def __getitem__(self, __k: _KT) -> _VT:
+        ...
+
+
+kt1: KeysAndGetItem[int, str] = KeysAndGetItem()
+assert_type(dict(kt1), Dict[int, str])
+dict(kt1, arg="a")  # type: ignore
+
+kt2: KeysAndGetItem[str, int] = KeysAndGetItem()
+assert_type(dict(kt2, arg=1), Dict[str, int])
+
+
+def test_iterable_tuple_overload(x: Iterable[Tuple[int, str]]) -> Dict[int, str]:
+    return dict(x)
+
+
+i1: Iterable[Tuple[int, str]] = [(1, "a"), (2, "b")]
+test_iterable_tuple_overload(i1)
+dict(i1, arg="a")  # type: ignore
+
+i2: Iterable[Tuple[str, int]] = [("a", 1), ("b", 2)]
+assert_type(dict(i2, arg=1), Dict[str, int])
+
+i3: Iterable[str] = ["a.b"]
+assert_type(dict(string.split(".") for string in i3), Dict[str, str])
+dict(["foo", "bar", "baz"])  # type: ignore