SymPy: operator stubs incorrectly return only NotImplementedType #407

Summary

Several SymPy operator stubs declare NotImplementedType as their only return type. However, these methods return concrete values for supported operands.

The declarations are present on the current main branch:

• DomainMatrix declarations
• PolyElement declarations

Runtime Reproducer

Tested with SymPy 1.14.0:

from sympy import QQ, ZZ
from sympy.polys.matrices.domainmatrix import DM
from sympy.polys.rings import ring

matrix = DM([[1]], QQ)
_, x, y = ring("x,y", ZZ)

operations = [
    ("matrix + matrix", matrix + matrix),
    ("matrix - matrix", matrix - matrix),
    ("matrix / 2", matrix / 2),
    ("x * y", x * y),
    ("2 * x", 2 * x),
    ("2 - x", 2 - x),
    ("divmod(2, x)", divmod(2, x)),
    ("2 % x", 2 % x),
    ("0 / x", 0 / x),
]

for expression, result in operations:
    print(f"{expression}: {type(result).__name__}")

Output:

matrix + matrix: DomainMatrix
matrix - matrix: DomainMatrix
matrix / 2: DomainMatrix
x * y: PolyElement
2 * x: PolyElement
2 - x: PolyElement
divmod(2, x): tuple
2 % x: PolyElement
0 / x: PolyElement

Affected Declarations

These methods currently return only NotImplementedType in the stubs:

• DomainMatrix: __add__ (L64), __sub__ (L65), __truediv__ (L79).
• PolyElement: __mul__ (L131), __rmul__ (L132), __rsub__ (L130), __rdivmod__ (L136), __rmod__ (L138), __rtruediv__ (L140).

Expected Behavior

Include the concrete return types. Retain NotImplementedType where the method can reject unsupported operands.

For example, DomainMatrix.__add__ needs a matrix return type, and PolyElement.__mul__ needs a polynomial return type.

This surfaced while testing a change to Pyrefly's handling of NotImplemented. A checker that interprets sentinel-only returns as definitive rejection can report unsupported-operation errors for these valid expressions.