feat: support variance annotations on type parameters (TypeScript 4.7)

[marionebl]

Summary

TypeScript 4.7 introduced in/out variance annotations on generic type parameters to explicitly declare variance:

interface Covariant<out T> {}       // covariant
interface Contravariant<in T> {}    // contravariant
interface Invariant<in out T> {}    // explicitly invariant

These were previously parsed as ERROR nodes. In effect-ts for example, 316 out of 1 767 .ts files contain variance annotations, producing 3 160 parse errors.

Changes

common/define-grammar.js

• New named variance rule (modelled after override_modifier):

  variance: _ => choice(seq('in', 'out'), 'in', 'out'),

• type_parameter gains optional($.variance) between the existing optional('const') and field('name', …).

Using a named node (rather than inline anonymous tokens) makes the annotation queryable via tree-sitter node queries.

test/corpus/types.txt — two new corpus test cases:

• Variance annotations — all three forms + multiple params in a type alias
• Variance annotations with constraints and defaults — real-world patterns from effect-ts

Test results

113 tests: 113 passed, 0 failed

Reference

• TypeScript 4.7 release notes — Variance Annotations
• TypeScript PR #48240