Match exhaustiveness with class inheriting from tuple

[tobiasBora]

Bug Report

If I run:

# Run the type checking with "mypy --strict foo.py"

class Add(tuple[int,int]):
 pass

class Const(int):
 pass

Expr = Add | Const

def my_eval(e : Expr) -> int:
 match e:
 case Add((a,b)):
 return a + b
 case Const(x):
 return x

print(my_eval(Const(42)))
print(my_eval(Add((42,45))))

with python 11, it runs just fine. But if I run mypy on this file (with or without strict), I get an error:

$ mypy --strict foo.py
foo.py:11: error: Missing return statement [return]
Found 1 error in 1 file (checked 1 source file)

Usually I get this error when a branch in a match is forgotten… but here I have covered all cases.

Note that the following code does work, but I would prefer the above code that is less verbose and error prone:

# Run the type checking with "mypy --strict foo.py"

class Add:
 left: int
 right: int
 def __init__(self, left:int, right:int):
 self.left = left
 self.right = right

class Const:
 val: int
 def __init__(self, val:int):
 self.val = val

Expr = Add | Const

def my_eval(e : Expr) -> int:
 match e:
 case Add():
 return e.left + e.right
 case Const():
 return e.val

print(my_eval(Const(42)))
print(my_eval(Add(42,45)))

To Reproduce

Gist URL: https://gist.github.com/mypy-play/5e61f50c5c61bf4c90b5e1403f2b6be1
Playground URL: https://mypy-play.net/?mypy=latest&python=3.12&gist=5e61f50c5c61bf4c90b5e1403f2b6be1

Expected Behavior

I would expect this to type fine. I need this for instance in this context https://stackoverflow.com/questions/78345055/python-static-type-checking-for-union-type-and-pattern-maching/78345797#78345797

I would also expect this to work when using forward reference:

# Run the type checking with "mypy --strict foo.py"

class Add(tuple['Expr','Expr']):
 pass

class Const(int):
 pass

Expr = Add | Const

def my_eval(e : Expr) -> int:
 match e:
 case Add((a,b)):
 return my_eval(a) + my_eval(b)
 case Const(x):
 return x

print(my_eval(Const(42)))
print(my_eval(Add((Const(42),Const(45)))))

Actual Behavior

I get an error.

Your Environment

• Mypy version used: 1.5.1
• Mypy command-line flags: None or strict
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.11.8

[JelleZijlstra]

The problem isn't the Foo(int), but the Add(tuple[int, int]). This works fine (https://mypy-play.net/?mypy=latest&python=3.12&gist=5e61f50c5c61bf4c90b5e1403f2b6be1):

# Run the type checking with "mypy --strict foo.py"

class Add(int):
 pass

class Const(int):
 pass

Expr = Add | Const

def my_eval(e : Expr) -> int:
 match e:
 case Add(a):
 return a
 case Const(x):
 return x

print(my_eval(Const(42)))
print(my_eval(Add(4)))

As for the solution, tuple types are often special; something will have to change in mypy's handling of them but I'm not sure where.