red-knot: support narrowing for bool(E) (#14668)

Resolves https://github.com/astral-sh/ruff/issues/14547 by delegating
narrowing to `E` for `bool(E)` where `E` is some expression.

This change does not include other builtin class constructors which
should also work in this position, like `int(..)` or `float(..)`, as the
original issue does not mention these. It should be easy enough to add
checks for these as well if we want to.

I don't see a lot of markdown tests for malformed input, maybe there's a
better place for the no args and too many args cases to go?

I did see after the fact that it looks like this task was intended for a
new hire.. my apologies. I got here from
https://github.com/astral-sh/ruff/issues/13694, which is marked
help-wanted.

---------

Co-authored-by: David Peter <mail@david-peter.de>

crates/red_knot_python_semantic/resources/mdtest/narrow/bool-call.md

@@ -0,0 +1,32 @@
+## Narrowing for `bool(..)` checks
+
+```py
+def flag() -> bool: ...
+
+x = 1 if flag() else None
+
+# valid invocation, positive
+reveal_type(x)  # revealed: Literal[1] | None
+if bool(x is not None):
+    reveal_type(x)  # revealed: Literal[1]
+
+# valid invocation, negative
+reveal_type(x)  # revealed: Literal[1] | None
+if not bool(x is not None):
+    reveal_type(x)  # revealed: None
+
+# no args/narrowing
+reveal_type(x)  # revealed: Literal[1] | None
+if not bool():
+    reveal_type(x)  # revealed: Literal[1] | None
+
+# invalid invocation, too many positional args
+reveal_type(x)  # revealed: Literal[1] | None
+if bool(x is not None, 5):  # TODO diagnostic
+    reveal_type(x)  # revealed: Literal[1] | None
+
+# invalid invocation, too many kwargs
+reveal_type(x)  # revealed: Literal[1] | None
+if bool(x is not None, y=5):  # TODO diagnostic
+    reveal_type(x)  # revealed: Literal[1] | None
+```

crates/red_knot_python_semantic/src/types/narrow.rs

@@ -388,18 +388,23 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
         let scope = self.scope();
         let inference = infer_expression_types(self.db, expression);
 
+        let callable_ty =
+            inference.expression_ty(expr_call.func.scoped_expression_id(self.db, scope));
+
         // TODO: add support for PEP 604 union types on the right hand side of `isinstance`
         // and `issubclass`, for example `isinstance(x, str | (int | float))`.
-        match inference
-            .expression_ty(expr_call.func.scoped_expression_id(self.db, scope))
-            .into_function_literal()
-            .and_then(|f| f.known(self.db))
-            .and_then(KnownFunction::constraint_function)
-        {
-            Some(function) if expr_call.arguments.keywords.is_empty() => {
-                if let [ast::Expr::Name(ast::ExprName { id, .. }), class_info] =
+        match callable_ty {
+            Type::FunctionLiteral(function_type) if expr_call.arguments.keywords.is_empty() => {
+                let function = function_type
+                    .known(self.db)
+                    .and_then(KnownFunction::constraint_function)?;
+
+                let [ast::Expr::Name(ast::ExprName { id, .. }), class_info] =
                     &*expr_call.arguments.args
-                {
+                else {
+                    return None;
+                };
+
                 let symbol = self.symbols().symbol_id_by_name(id).unwrap();
 
                 let class_info_ty =
@@ -407,9 +412,7 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
 
                 let to_constraint = match function {
                     KnownConstraintFunction::IsInstance => {
-                            |class_literal: ClassLiteralType<'db>| {
-                                Type::instance(class_literal.class)
-                            }
+                        |class_literal: ClassLiteralType<'db>| Type::instance(class_literal.class)
                     }
                     KnownConstraintFunction::IsSubclass => {
                         |class_literal: ClassLiteralType<'db>| {
@@ -425,9 +428,18 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
                         constraints
                     },
                 )
-                } else {
-                    None
             }
+            // for the expression `bool(E)`, we further narrow the type based on `E`
+            Type::ClassLiteral(class_type)
+                if expr_call.arguments.args.len() == 1
+                    && expr_call.arguments.keywords.is_empty()
+                    && class_type.class.is_known(self.db, KnownClass::Bool) =>
+            {
+                self.evaluate_expression_node_constraint(
+                    &expr_call.arguments.args[0],
+                    expression,
+                    is_positive,
+                )
             }
             _ => None,
         }