Fix isinstance with unions of tuples (#20677)

Author: hauntsaninja

mypy/checker.py

@@ -8646,12 +8646,13 @@ def flatten(t: Expression) -> list[Expression]:
 def flatten_types(t: Type) -> list[Type]:
     """Flatten a nested sequence of tuples into one list of nodes."""
     t = get_proper_type(t)
+    if isinstance(t, UnionType):
+        return [b for a in t.items for b in flatten_types(a)]
     if isinstance(t, TupleType):
         return [b for a in t.items for b in flatten_types(a)]
     elif is_named_instance(t, "builtins.tuple"):
         return [t.args[0]]
-    else:
-        return [t]
+    return [t]
 
 
 def expand_func(defn: FuncItem, map: dict[TypeVarId, Type]) -> FuncItem:

test-data/unit/check-isinstance.test

@@ -2938,3 +2938,26 @@ def foo(x: object, t: type[Any]):
     if isinstance(x, t):
         reveal_type(x)  # N: Revealed type is "Any"
 [builtins fixtures/isinstance.pyi]
+
+[case testIsInstanceUnionOfTuples]
+# flags: --strict-equality --warn-unreachable
+from __future__ import annotations
+from typing import TypeVar, Iterator
+
+T1 = TypeVar("T1")
+T2 = TypeVar("T2")
+T3 = TypeVar("T3")
+
+def extract(
+    values: object,
+    ts: (
+        tuple[type[T1]]
+        | tuple[type[T1], type[T2]]
+        | tuple[type[T1], type[T2], type[T3]]
+    )
+) -> Iterator[T1 | T2 | T3]:
+    if isinstance(values, ts):
+        reveal_type(values)  # N: Revealed type is "T1`-1 | T2`-2 | T3`-3"
+        yield values
+    raise
+[builtins fixtures/primitives.pyi]