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Two crashes on recursive named tuples #3340

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ilevkivskyi opened this issue May 8, 2017 · 1 comment · Fixed by #3952
Closed

Two crashes on recursive named tuples #3340

ilevkivskyi opened this issue May 8, 2017 · 1 comment · Fixed by #3952

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@ilevkivskyi
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While working on #3322 I have found two more crashes:

N = NamedTuple('N', [('x', N)])
n: N

crashes with RecursionError and

class N(NamedTuple):
    x: N
class M(NamedTuple):
    x: M

n: N
m: M
lst = [n, m]

crashes with AssertionError in join_instances_via_supertype. I didn't try TypedDict but most probably it will have similar problems.

There is actually a fundamental question here: what should be the join(N, M)? @JukkaL do you have any ideas? One possible option would be to detect recursive joins and just return object for them (maybe also UninhabitedType for recursive meets).

@JukkaL
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JukkaL commented May 8, 2017

object would be consistent as the result of join(N, M). A recursive type like T where T = Tuple[T] would perhaps also be a reasonable result, assuming we were able to represent them. (The only reasonable common use case where recursive types could be useful that I've seen is JSON, so this is not a valid motivating example for recursive types.)

JukkaL pushed a commit that referenced this issue Sep 27, 2017
Forward references didn't work with anything apart from classes, for example 
this didn't work:

```
x: A
A = NamedTuple('A', [('x', int)])
```

The same situation was with `TypedDict`, `NewType`, and type aliases. The 
root problem is that these synthetic types are neither detected in first pass, 
nor fixed in third pass. In certain cases this can lead to crashes (first six issues 
below are various crash scenarios). This fixes these crashes by applying some 
additional patches after third pass.

Here is the summary of the PR:

* New simple wrapper type `ForwardRef` with only one field `link` is introduced 
  (with updates to type visitors)
* When an unknown type is found in second pass, the corresponding 
  `UnboundType` is wrapped in `ForwardRef`, it is given a "second chance" in 
  third pass.
* After third pass I record the "suspicious" nodes, where forward references and 
  synthetic types have been encountered and append patches (callbacks) to fix 
  them after third pass. Patches use the new visitor `TypeReplacer` (which is the 
  core of this PR).

Fixes #3340
Fixes #3419
Fixes #3674
Fixes #3685
Fixes #3799
Fixes #3836
Fixes #3881
Fixes #867
Fixes #2241
Fixes #2399
Fixes #1701
Fixes #3016
Fixes #3054
Fixes #2762
Fixes #3575
Fixes #3990
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