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[red-knot] Support typing.TYPE_CHECKING (#14952)
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## Summary

Add support for `typing.TYPE_CHECKING` and
`typing_extensions.TYPE_CHECKING`.

relates to: #14170

## Test Plan

New Markdown-based tests
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sharkdp authored Dec 13, 2024
1 parent f52b1f4 commit e96b13c
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# Known constants

## `typing.TYPE_CHECKING`

This constant is `True` when in type-checking mode, `False` otherwise. The symbol is defined to be
`False` at runtime. In typeshed, it is annotated as `bool`. This test makes sure that we infer
`Literal[True]` for it anyways.

### Basic

```py
from typing import TYPE_CHECKING
import typing

reveal_type(TYPE_CHECKING) # revealed: Literal[True]
reveal_type(typing.TYPE_CHECKING) # revealed: Literal[True]
```

### Aliased

Make sure that we still infer the correct type if the constant has been given a different name:

```py
from typing import TYPE_CHECKING as TC

reveal_type(TC) # revealed: Literal[True]
```

### Must originate from `typing`

Make sure we only use our special handling for `typing.TYPE_CHECKING` and not for other constants
with the same name:

```py path=constants.py
TYPE_CHECKING: bool = False
```

```py
from constants import TYPE_CHECKING

reveal_type(TYPE_CHECKING) # revealed: bool
```

### `typing_extensions` re-export

This should behave in the same way as `typing.TYPE_CHECKING`:

```py
from typing_extensions import TYPE_CHECKING

reveal_type(TYPE_CHECKING) # revealed: Literal[True]
```
8 changes: 8 additions & 0 deletions crates/red_knot_python_semantic/src/types.rs
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Expand Up @@ -120,6 +120,14 @@ fn symbol_by_id<'db>(db: &'db dyn Db, scope: ScopeId<'db>, symbol: ScopedSymbolI

/// Shorthand for `symbol_by_id` that takes a symbol name instead of an ID.
fn symbol<'db>(db: &'db dyn Db, scope: ScopeId<'db>, name: &str) -> Symbol<'db> {
// We don't need to check for `typing_extensions` here, because `typing_extensions.TYPE_CHECKING`
// is just a re-export of `typing.TYPE_CHECKING`.
if name == "TYPE_CHECKING"
&& file_to_module(db, scope.file(db)).is_some_and(|module| module.name() == "typing")
{
return Symbol::Type(Type::BooleanLiteral(true), Boundness::Bound);
}

let table = symbol_table(db, scope);
table
.symbol_id_by_name(name)
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