Fix crash on unhandled meet variadic tuple vs indtance

mypy/meet.py

@@ -1090,7 +1090,28 @@ def visit_tuple_type(self, t: TupleType) -> ProperType:
         elif isinstance(self.s, Instance):
             # meet(Tuple[t1, t2, <...>], Tuple[s, ...]) == Tuple[meet(t1, s), meet(t2, s), <...>].
             if self.s.type.fullname in TUPLE_LIKE_INSTANCE_NAMES and self.s.args:
-                return t.copy_modified(items=[meet_types(it, self.s.args[0]) for it in t.items])
+                arg = self.s.args[0]
+                new_items: list[Type] = []
+                for it in t.items:
+                    # Unpack items need to be handled by the caller.
+                    if isinstance(it, UnpackType):
+                        unpacked = get_proper_type(it.type)
+                        if isinstance(unpacked, TypeVarTupleType):
+                            # We can't infer anything in this case.
+                            new_arg = UninhabitedType()
+                            instance = unpacked.tuple_fallback
+                        else:
+                            assert (
+                                isinstance(unpacked, Instance)
+                                and unpacked.type.fullname == "builtins.tuple"
+                            )
+                            new_arg = meet_types(unpacked.args[0], arg)
+                            instance = unpacked
+                        new_items.append(UnpackType(instance.copy_modified(args=[new_arg])))
+                    else:
+                        # All other items can be processed in a regular way.
+                        new_items.append(meet_types(it, arg))
+                return t.copy_modified(items=new_items)
             elif is_proper_subtype(t, self.s):
                 # A named tuple that inherits from a normal class
                 return t

test-data/unit/check-typeis.test

@@ -213,8 +213,7 @@ def test5(t: tuple[A | B, ...]) -> None:
 
 def test6(t: tuple[B, Unpack[tuple[A | B, ...]], B]) -> None:
     if is_tuple_of_B(t):
-        # Should this be tuple[B, *tuple[B, ...], B]
-        reveal_type(t)  # N: Revealed type is "tuple[__main__.B, Never, __main__.B]"
+        reveal_type(t)  # N: Revealed type is "tuple[__main__.B, Unpack[builtins.tuple[__main__.B, ...]], __main__.B]"
     else:
         reveal_type(t)  # N: Revealed type is "tuple[__main__.B, Unpack[builtins.tuple[__main__.A | __main__.B, ...]], __main__.B]"
 [builtins fixtures/tuple.pyi]

test-data/unit/check-typevar-tuple.test

@@ -2873,3 +2873,15 @@ reveal_type(x)  # N: Revealed type is "builtins.list[tuple[()]]"
 reveal_type(y)  # N: Revealed type is "builtins.list[tuple[()]]"
 reveal_type(z)  # N: Revealed type is "builtins.list[tuple[()]]"
 [builtins fixtures/tuple.pyi]
+
+[case testMeetTupleWithUnpackVsInstanceWorks_no_verbose_reveal]
+from typing import TypeVar, Unpack, Union
+
+K = TypeVar("K")
+Prefix = tuple[Union[K, list[int]], ...]
+
+def g(x: K) -> None:
+    d: Union[Prefix[K], tuple[Unpack[Prefix[int]], int], None] = None
+    assert d is not None
+    reveal_type(d)  # N: Revealed type is "tuple[K | list[int], ...] | tuple[*tuple[int | list[int], ...], int]"
+[builtins fixtures/tuple.pyi]