feat(semantic): report TS1228 for invalid type predicates

[camc314]

• Emit TS1228 for invalid type predicate positions from semantic syntax checks.
• Treat valid predicates as only those that are the immediate return type of a function, method, function type, call signature, or non-accessor method signature.

Why semantic

TypeScript parses type predicates as part of the normal type grammar, then validates their placement later in checker logic. This is not a parser recovery decision: the syntax can be represented in the AST, but whether it is legal depends on the predicate node's parent chain.

Keeping this check in semantic avoids parser mode plumbing and matches that model. The semantic checker can inspect ancestry to reject predicates in property annotations, constructor types, accessors, construct signatures, and nested or parenthesized type positions while reporting the span on the predicate node itself.

[codspeed-hq]

Merging this PR will not alter performance

✅ 62 untouched benchmarks
⏩ 9 skipped benchmarks¹

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_{Comparing codex/ts1228-diagnostics (a6064c1) with main (2409a10)}

Footnotes

1. 9 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[Copilot]

Pull request overview

This PR adds a semantic (post-parse) TypeScript syntax check to emit TS1228 when a TSTypePredicate/asserts predicate appears in an invalid position, matching TypeScript’s model where predicates parse as types but are validated later based on their ancestor chain.

Changes:

• Add a new TS diagnostic constructor for TS(1228) (“type predicate only allowed in return type position for functions and methods”).
• Introduce an ancestry-based checker that validates TSTypePredicate placement and reports TS1228 when invalid.
• Update the TypeScript parser coverage snapshot to reflect the new semantic errors (moving some cases out of “Expect Syntax Error” and into TS diagnostics output).

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
tasks/coverage/snapshots/parser_typescript.snap	Snapshot update reflecting new TS1228 semantic diagnostics for previously “syntax error” coverage cases.
crates/oxc_semantic/src/diagnostics.rs	Adds a TS(1228) diagnostic helper (type_predicate_only_in_return_type).
crates/oxc_semantic/src/checker/typescript.rs	Adds check_ts_type_predicate and ancestry-based allowed-position logic for type predicates.
crates/oxc_semantic/src/checker/mod.rs	Wires AstKind::TSTypePredicate into the semantic checker dispatch.

[camc314]

Merge activity

• Jun 11, 7:31 PM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• Jun 11, 7:49 PM UTC: camc314 added this pull request to the Graphite merge queue.
• Jun 11, 7:54 PM UTC: Merged by the Graphite merge queue.