New semantic analyzer: Support idem forward aliases

This is a follow-up for #6390

This tricky corner case is used in some real code and is supported by old semantic analyzer

C = C
class C:  # type: ignore
    pass

x: C

But with new analyzer this results in Invalid type since the first definition always wins. Probably we just need to special-case this somehow.

EDIT: The new behavior is actually correct, see below.

~~Another pattern that we need to support with new semantic analyzer is this:~~

class In:
    ...
class Out:
    In = In  # recognized as variable

~~this currently (both before and after assignment refactor) causes a bunch of troubles in internal code bases. Raising priority to high.~~

Oh, wait, this is actually correct, by the rules non-indexed assignments at class scope are always variables, so this was previously a false negative, OK back to normal priority.

@JukkaL

Another repro for this issue with a weird error:

x: C
class C:
    pass
C = C

fails with

main:2: error: Name 'C' already defined on line 4

cc @JukkaL

The examples look meaningless, since the code won't work at runtime. I wonder if the actual code where this comes up is different (involving try/except perhaps). This could also affect the way we'd fix this.

Yes, the actual code is like

try:
    C = C
except NameError:
    class C:
        ...

Also the second example works without try/except.

I think we should fix this before the release since this is actually used in some projects.

Yes, it would be good to make this better. I'm increasing priority.

But even that example seems incomplete, as that will always generate a NameError. I assume it's more like this:

try:
    ModuleNotFoundError = ModuleNotFoundError
except NameError:
    class ModuleNotFoundError(Exception):
        ...

The assignment will fail on some Python versions and pass on some.

Most likely, I don't remember TBH, also aiohttp (who first reported this) used to have tons of * imports. This may be some other name they define in another module depending on sys.platform and then star import it.

I did some digging and it looks like this happens sometimes after star imports from C acceleration modules.

New semantic analyzer: fix issues with 'X = X' assignments

New semantic analyzer: fix issues with 'X = X' assignments (#7144)

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New semantic analyzer: Support idem forward aliases #6404

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New semantic analyzer: Support idem forward aliases #6404

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ilevkivskyi commented on Mar 14, 2019

ilevkivskyi commented on Mar 14, 2019

ilevkivskyi commented on Jun 7, 2019

JukkaL commented on Jul 3, 2019

ilevkivskyi commented on Jul 3, 2019

JukkaL commented on Jul 3, 2019

ilevkivskyi commented on Jul 3, 2019

ilevkivskyi commented on Jul 3, 2019

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