Diagnostics

Cyclic module import detected: {0}

Module A imports B and B imports A (directly or transitively). Break the cycle by extracting shared types into a third module that both can import.

Duplicate definition of '{0}' in module '{1}'

Two types, messages, or services share the same name. Rename one of them.

Type '{0}' not found or is not a standard builtin type

The base type for an enum or attribute argument is not a valid built-in type. Use a primitive like u1, u4, string, etc.

Type '{0}' is not allowed in attribute arguments

Attribute arguments must be scalar types. You cannot use a message or union type as an attribute parameter.

Attribute '{0}' not found

The referenced attribute is not declared. Either declare it with attribute @name(...); or import the module that defines it.

Unresolved reference to type '{0}'

The type doesn't exist in any imported module. If it's a typo, the compiler will suggest the closest match: "Did you mean '{1}'?" (via Levenshtein distance).

Duplicate enum item name '{0}' in enum '{1}'

Two members in the same enum/flags share the same name.

Invalid value '{0}' for enum '{1}'

Enum values must be constant integer expressions (literals or bit-shift expressions like 1 << 3).

Duplicate enum value in enum '{1}'

Two members resolve to the same numeric value.

Enum item '{0}' has overlapping bits with '{2}'

Flags-specific: two members share overlapping bits, which would make them indistinguishable when combined.

Union '{0}' declares shared fields but contains type reference case

Unions with type reference cases (e.g., union R { UserData, ErrorInfo }) cannot also have shared fields. Use inline cases instead.

Method '{0}' declares multiple stream parameters

Only one parameter per service method can be marked as stream.

Circular type reference detected: {0}

Types form a cycle (A → B → A). This would cause infinite recursion during serialization. Break the cycle by making one of the references optional (T?).

Breaking change: field '{0}' (index {1}) was removed from '{2}'

A field that was locked has been removed. Use --update-lock to acknowledge.

Breaking change: field '{0}' changed index from {2} to {3}

Field order determines wire identity. Reordering breaks all existing clients.

Breaking change: field '{0}' changed type from '{2}' to '{3}'

Changing a field's type changes the wire encoding. Existing clients cannot deserialize.

Breaking change: definition '{0}' was removed

A locked type/service was completely removed from the schema.

Breaking change: definition '{0}' changed kind from '{1}' to '{2}'

E.g., changing a msg to an enum.

Service '{0}' removed method '{1}'

Removing a service method will break existing clients that call it.

Breaking change: method '{0}.{1}' signature changed

Method arguments or return type changed.

Breaking change: enum/flags '{1}' member '{2}' changed value

Changing an enum or flags member's numeric value breaks wire compatibility.

Breaking change: union '{0}' case '{1}' changed index

Union case index is the wire discriminator. Reordering changes the meaning of each case.

Field '{0}' added to '{1}' is not nullable

Adding a required field is safe for new clients but older clients reading the previous schema won't have this field. Consider making it optional (T?).

Circular module dependency detected: {0}

Module A depends on B and B depends on A (directly or transitively). Break the cycle by extracting shared types into a third module.

Module '{0}' config not found at path '{1}'

The module path in ion.config.json doesn't point to a directory with a valid ion.config.json. Check that the relative path is correct.

Unknown module '{0}'

A #import references a module that is not declared in the modules section of ion.config.json.

Type '{0}' not found in module '{1}'

The imported type does not exist in the target module. Verify the type name and ensure it is defined in one of the module's .ion files.

Type '{0}' not found in module '{1}'. Did you mean '{2}'?

Similar to ION0043 but the compiler found a close match — check for typos.

Imported type '{0}' from module '{1}' is never used

Remove the unused import to keep your code clean.

Module '{0}' content hash has changed since last lock

The module's source files changed since the lock file was generated. Run ionc lock update to acknowledge.

#use is deprecated

Migrate to the #import { Type } from "module" syntax. See Directives.

Type '{0}' has the same name as a type from module '{1}'

Cross-module name collision. Rename one of the types to avoid ambiguity.

Type '{0}' is defined but never referenced

The type is not used by any service method, field, or union case. It may be safe to remove.

Import '{0}' is unused

No types from this imported file are referenced. Remove the import.

Field '{0}' in '{1}' is never used

The field is not referenced by any service method.