class member '{member_name}' missing from class-body locals

Error message

class member '{member_name}' missing from class-body locals

What it means

After preparing a class body, every declared class member must exist in the class-body scope's local slot map. If a member's name has no slot, class preparation panics — the members pass and the locals pass disagree about what the class body defines. It is an internal compiler invariant, not a user-facing error.

Source

Thrown at crates/monty/src/prepare.rs:1807

        };
        let FunctionState {
            locals: inner_locals,
            free_var_map: inner_free_var_map,
            cell_var_map: inner_cell_var_map,
            ..
        } = *inner_state;
        let namespace_size = inner_locals.len();
        drop(inner_prepare);

        // Resolve each member to its class-body-local slot. Every member is
        // assigned in the class body (a method `def` or a class-var `Assign`),
        // so it is always present as a plain local.
        let members = members
            .into_iter()
            .map(|member| {
                let slot = inner_locals.get(member.name_id).unwrap_or_else(|| {
                    let member_name = self.interner.get_str(member.name_id);
                    panic!("class member '{member_name}' missing from class-body locals")
                });
                Identifier::new_with_scope(member.name_id, member.position, slot, NameScope::Local)
            })
            .collect::<Vec<_>>();

        // Same-name collision (a known divergence — see `limitations/classes.md`):
        // a class-body owned cell means a method captured a class-body local that
        // ALSO has the same name as a variable in an enclosing scope. CPython keeps
        // these distinct (class-dict entry vs. closure cell); Monty maps one name
        // to a single slot, so it cannot represent both. Reject cleanly rather than
        // miscompile (the alternative is a runtime "expected cell reference" crash).
        if let Some(&name_id) = inner_cell_var_map.keys().next() {
            let name_str = self.interner.get_str(name_id);
            return Err(ParseError::not_implemented(
                format!(
                    "class member '{name_str}' that shadows a captured variable of the same name from an enclosing scope"
                ),
                position,

View on GitHub (pinned to adc986b362)

Solutions

Fix member collection so it only reports names the class-body locals pass registers.
Ensure the class-body scope analysis runs before members are mapped to slots.
Reduce the failing class definition and compare slot registration against member enumeration.

Example fix

// before
let slot = inner_locals.get(member.name_id).unwrap_or_else(|| panic!("class member missing"));
// after
// ensure member enumeration uses the same pass that fills inner_locals
let members = collect_class_members(&body); // shared source of truth
let slot = inner_locals.get(member.name_id).unwrap_or_else(|| panic!("class member '{member_name}' missing from class-body locals"));

Defensive patterns

Strategy: validation

Validate before calling

// derive members and locals from the same AST pass
let members = collect_class_members(&body);
for m in &members { assert!(inner_locals.contains_key(m.name_id)); }

Prevention

Use one shared pass for member enumeration and local registration
Test class bodies with del, conditional defs and walrus expressions
Keep class preparation tests in the consolidated class test files

When it happens

Trigger: Compiling a class whose members list (from AST analysis) contains a name the class-body scope never registered as a local — e.g. names introduced by exotic statements inside the class body, or a mismatch introduced by changes to member collection.

Common situations: Contributors modifying class-body preparation or member enumeration; Python test code with unusual class bodies (del statements, conditional defs, walrus in class scope).

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

AI-assisted analysis of pydantic/monty@adc986b362 (2026-09-13). Data as JSON: /api/errors/b568b5e8ed667c88. Report an issue: GitHub.