[red-knot] Support typing.TYPE_CHECKING

Author: sharkdp

crates/red_knot_python_semantic/resources/mdtest/known_constants.md

@@ -0,0 +1,52 @@
+# 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]
+```

crates/red_knot_python_semantic/src/types.rs

@@ -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)