Default type alias type parameters

Summary

Introduce syntax to provide default type values inside the type alias type parameter list.

Motivation

Luau has support for type parameters for type aliases and functions. In languages with similar features like C++ and TypeScript, it is possible to specify default values and users with experience in those languages would like to use that in Luau.

Here is an example that is coming up frequently during development of GraphQL Luau library:

export type GraphQLFieldResolver<
    TSource,
    TContext,
    TArgs = { [string]: any }
> = (TSource, TArgs, TContext, GraphQLResolveInfo) -> any

If we could specify defaults like that, we won't have to write long type names when type alias is used unless specific customization is required. Some engineers already skip these extra arguments and use any to save time, which gives worse typechecking quality.

Without default arguments it's also harder to refactor the code as each type alias reference that uses 'common' type arguments has to be updated.

While previous example uses a concrete type for default type value, it should also be possible to reference generic types from the same list:

type Eq<T, U = T> = (l: T, r: U) -> boolean

local a: Eq<number> = ...
local b: Eq<number, string> = ...

Generic functions in Luau also have a type parameter list, but it's not possible to specify type arguments at the call site and because of that, default type argument values for generic functions are not proposed.

Design

If a default type argument value is assigned, following type parameters (on the right) must also have default type argument values.

type A<T, U = string, V> = ... -- not allowed

Default type argument values can reference type parameters which were defined earlier (to the left):

type A<T, U = T> = ...-- ok

type A<T, U = V, V = T> = ... -- not allowed

Default type arguments are not allowed for type pack parameters.

Syntax for type alias type argument is extended as follows:

typearg ::= Name [`=' typeannotation]

Instead of storing a simple array of names in AstStatTypeAlias, we will store an array of structs containing the name and an optional default type value.

When type alias is referenced, missing type parameters are replaced with default type values, if they are available.

If all type parameters have a default type value, it is now possible to reference that without providing a type parameter list:

type All<T = string, U = number> = ...

local a: All -- ok
local b: All<> -- not allowed

If type is exported from a module, default type parameter values will still be available when module is imported.

Drawbacks

Previously, it was possible to reference type alias without type arguments inside that alias (recursive types). We disallow that now, but if we decide to return that, there will be an ambiguity if all type parameters have a default value.

Alternatives

We could use : instead of = to define default type parameter values.

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