[Bug Report] Generic Type Parameter Incorrectly Inferred as NoneType with Lambda Callable

[MetalOxideSemi]

Environment

• Codon version: v0.19.3
• OS: Debian Bullseye

Description

Codon fails to correctly infer generic type parameters when a class uses CallableTrait with a lambda function. The type parameter is incorrectly inferred as NoneType instead of the actual return type of the callable, causing a compilation error.

Reproduction Code

from collections import Counter

class Item:
    def __init__(self, x, y, z):
        self.x = x
        self.y = y
        self.z = z

class Count:
    Key: type

    def __init__(self, project: CallableTrait[[Item], Key]):
        self.project = project
        self.counter = Counter[Key]()

    def visit(self, item: Item):
        self.counter[self.project(item)] += 1

items = [Item(1, 2, 3), Item(1, 2, 4), Item(1, 2, 5)]
count = Count(lambda item: item.z)  # Lambda returns int
for item in items:
    count.visit(item)
print(count)

Compilation fails with the following error:

crash_on_filter.py:15 (9-13): error: 'Count[NoneType]' object has no attribute 'project'
╰─ crash_on_filter.py:23 (9-35): error: during the realization of __init__(self: Count[NoneType], project: %_lambda_95(...))