[red-knot] Type narrowing for assertions (#17149)

## Summary

Fixes #17147 

## Test Plan

Add new narrow/assert.md test file

---------

Co-authored-by: Carl Meyer <carl@astral.sh>

crates/red_knot_python_semantic/resources/mdtest/narrow/assert.md

@@ -0,0 +1,53 @@
+# Narrowing with assert statements
+
+## `assert` a value `is None` or `is not None`
+
+```py
+def _(x: str | None, y: str | None):
+    assert x is not None
+    reveal_type(x)  # revealed: str
+    assert y is None
+    reveal_type(y)  # revealed: None
+```
+
+## `assert` a value is truthy or falsy
+
+```py
+def _(x: bool, y: bool):
+    assert x
+    reveal_type(x)  # revealed: Literal[True]
+    assert not y
+    reveal_type(y)  # revealed: Literal[False]
+```
+
+## `assert` with `is` and `==` for literals
+
+```py
+from typing import Literal
+
+def _(x: Literal[1, 2, 3], y: Literal[1, 2, 3]):
+    assert x is 2
+    reveal_type(x)  # revealed: Literal[2]
+    assert y == 2
+    reveal_type(y)  # revealed: Literal[1, 2, 3]
+```
+
+## `assert` with `isinstance`
+
+```py
+def _(x: int | str):
+    assert isinstance(x, int)
+    reveal_type(x)  # revealed: int
+```
+
+## `assert` a value `in` a tuple
+
+```py
+from typing import Literal
+
+def _(x: Literal[1, 2, 3], y: Literal[1, 2, 3]):
+    assert x in (1, 2)
+    reveal_type(x)  # revealed: Literal[1, 2]
+    assert y not in (1, 2)
+    reveal_type(y)  # revealed: Literal[3]
+```

crates/red_knot_python_semantic/src/semantic_index/builder.rs

@@ -534,7 +534,6 @@ impl<'db> SemanticIndexBuilder<'db> {
     }
 
     /// Records a visibility constraint by applying it to all live bindings and declarations.
-    #[must_use = "A visibility constraint must always be negated after it is added"]
     fn record_visibility_constraint(
         &mut self,
         predicate: Predicate<'db>,
@@ -1292,6 +1291,17 @@ where
                     );
                 }
             }
+
+            ast::Stmt::Assert(node) => {
+                self.visit_expr(&node.test);
+                let predicate = self.record_expression_narrowing_constraint(&node.test);
+                self.record_visibility_constraint(predicate);
+
+                if let Some(msg) = &node.msg {
+                    self.visit_expr(msg);
+                }
+            }
+
             ast::Stmt::Assign(node) => {
                 debug_assert_eq!(&self.current_assignments, &[]);
 

crates/red_knot_python_semantic/src/types/infer.rs

@@ -3188,7 +3188,7 @@ impl<'db> TypeInferenceBuilder<'db> {
             msg,
         } = assert;
 
-        let test_ty = self.infer_expression(test);
+        let test_ty = self.infer_standalone_expression(test);
 
         if let Err(err) = test_ty.try_bool(self.db()) {
             err.report_diagnostic(&self.context, &**test);