Generalized = delete Declarations

Order

Recommended Issue order: 2.

Goal

Extend the current = delete support from type-specialization-only usage into a general negative declaration mechanism.

Current support can reject cases such as:

type<T> ref<ref<T>> = delete;

This design should define and implement where deleted declarations are legal, how they participate in overload and specialization resolution, and how diagnostics should point to the deleted declaration.

Dependencies

Prerequisites:

• Existing type alias specialization and overload/specialization resolution.

Unblocks:

• Enhanced Tuple Types, for deleted fallback projections such as invalid tuple_element.
• Auto Traits And Derive Traits, for negative or blocked auto-trait behavior.
• Future constraint modeling that needs "matched but forbidden" diagnostics.

Scope

In scope:

• Deleted type aliases and type specializations.
• Deleted function overloads or generic function specializations.
• Deleted const specializations if useful for constraint modeling.
• Resolution rule: deleted candidates may match, but selecting one is a compile-time error.
• Clear diagnostics showing the deleted declaration that matched.

Out of scope:

• Runtime behavior.
• Trait auto-derive interaction.
• const fun execution.

Acceptance Criteria

• type<T> X<Bad<T>> = delete; rejects matching type use.
• fun<T> foo(value: Bad<T>) = delete; rejects matching calls.
• A more specific deleted declaration beats a more general valid declaration.
• Non-matching deleted declarations do not affect valid overloads.
• Parser/typechecker tests cover exact match, generic pattern match, and fallback behavior.

Implementation Status

Implemented:

• Deleted primary and specialized type aliases.
• Deleted generic function overloads selected by argument type patterns.
• Deleted non-generic functions as forbidden callable signatures.
• Deleted const predicate specializations.
• Candidate selection treats deleted declarations as normal candidates first; selecting one is a compile-time error.
• More-specific deleted generic function patterns beat less-specific valid fallbacks.
• Parser and typechecker tests cover exact matches, generic pattern matches, fallback behavior, and const predicates.

Current limitations:

• Function overload selection is intentionally narrow and only models same-name generic candidates needed by deleted declarations.
• Runtime backends skip deleted functions; deleted declarations are expected to be rejected by type checking before execution.