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TypeScript: variance annotations (<out A>, <in E>, <in out T>) are parse errors #3276
Description
TypeScript 4.7 variance annotations on type parameters (<out A>, <in E>, <in out T>, <const in T>) are parse errors in the bundled TypeScript grammar, tree-sitter-typescript v0.23.2 (grammars.ts:386). That is upstream tree-sitter/tree-sitter-typescript#370, which is still open. The grammar repo has had no commit since 2025-01-30.
Repro (main f72c9d2e, bundled tree-sitter-typescript.wasm, web-tree-sitter 0.25.10)
| source | hasError |
tree |
|---|---|---|
interface S<out A, in E> {} |
true | (type_parameters (type_parameter name: …) (ERROR (identifier)) …) |
function f<in out T>(x: T) {} |
true | (type_parameters (type_parameter name: …) (ERROR (identifier) (identifier))) |
class C<const in T> {} |
true | (type_parameters (type_parameter name: …) (ERROR (identifier))) |
interface S<A, E> {} (control) |
false |
Expected: each parses clean, with the modifier attached to its type_parameter.
What it costs today
- Error recovery is local in small files, so declarations in them are still extracted. Indexing
packages/effect/srcfrom Effect-TS/effect7a87ff9gives the same 31,478 nodes with and without a grammar that accepts the annotations. - The graph does lose edges. With the fixed grammar, the same index has 8 more
referencesedges. For example,makeEquivalenceinRedacted.ts:321gains its references toEquivalenceandRedacted, which sit afterexport interface Redacted<out A = string>(line 58). - Across the whole Effect repo, 239 of 2,054 files parse with an error; 185 still do once the annotations parse. The rest come from other grammar bugs, such as bug: anonymous generic call signatures fail to parse when separated only by line breaks tree-sitter/tree-sitter-typescript#335.
Libraries that use variance annotations heavily (Effect, and code that copies its patterns) are the ones affected.
A fix is up as #3226. It carries a patch for the grammar, following the existing docs/grammars/*.patch pattern.
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