Port TypeScript PR #62170: Enhance type argument completions

[Copilot]

• Add HasTypeArguments helper function in internal/ast/utilities.go
• Add getUninstantiatedSignatures function in internal/checker/services.go
• Add getTypeParameterConstraintForPositionAcrossSignatures function in internal/checker/services.go
• Rewrite getTypeArgumentConstraint in internal/checker/services.go to handle call-like expressions, decorators, instantiation expressions, and type references
• Add GetElementTypeOfArrayType public API to checker
• Update getConstraintOfTypeArgumentProperty in internal/ls/completions.go to handle more node kinds (ColonToken, OpenBracketToken) and remove isTypeReferenceType restriction
• Update getCompletionData in internal/ls/completions.go to fall back to getConstraintOfTypeArgumentProperty for contextual type
• Update fromUnionableLiteralType in internal/ls/string_completions.go to handle more expression kinds (CallExpression, NewExpression, JSX, TaggedTemplate) and PropertySignature
• Remove all completionListInTypeLiteralInTypeParameter tests from internal/fourslash/_scripts/failingTests.txt
• Revert incorrect commit character change for TupleType/IndexSignature; make test 21 manual with correct DefaultCommitCharacters
• Build passes
• All completionListInTypeLiteralInTypeParameter tests 1-21 pass
• Full test suite passes
• Lint passes with 0 issues

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[Copilot]

Pull request overview

Ports TypeScript PR #62170 to improve type-argument-related completions in the tsgo checker and language service, expanding where constraints/contextual types are derived so completions are more accurate in additional syntactic contexts.

Changes:

• Added ast.HasTypeArguments and expanded Checker.GetTypeArgumentConstraint to support more parent node kinds (call-like, decorators, instantiation expressions, type refs).
• Updated LS completion logic to better derive contextual types/constraints for literals and string completions (including more node kinds and property signatures).
• Tweaked commit character behavior for [ contexts and updated fourslash failing test expectations.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
internal/ast/utilities.go	Adds HasTypeArguments helper to safely detect nodes supporting type arguments.
internal/checker/services.go	Adds array element-type public API and rewrites type-argument constraint computation across more expression kinds.
internal/ls/completions.go	Improves contextual type fallback via type-argument-property constraints; adjusts commit characters and constraint-walking.
internal/ls/string_completions.go	Expands unionable literal handling for more expression kinds and adds property-signature support.
internal/fourslash/_scripts/failingTests.txt	Removes entries for tests that now pass after the port.

[jakebailey]

@copilot Look at the suggestions, but check against the original code and do not make a change if it differs from the original implementation. Doing nothing is acceptable, but explain your decision for each.

[Copilot]

@copilot Look at the suggestions, but check against the original code and do not make a change if it differs from the original implementation. Doing nothing is acceptable, but explain your decision fo...

Checked each suggestion against the original TypeScript implementation:

1. getTypeParameterConstraintForPositionAcrossSignatures returning nil for empty constraints — The original TS code calls getUnionType(relevantTypeParameterConstraints) directly without guarding for empty arrays. getUnionType([]) returns never in both TS and Go. My implementation matches the original. No change.

2. Unwrapping decorator expression in getUninstantiatedSignatures — The original TS code for SyntaxKind.Decorator simply does getTypeOfExpression(node.expression) with no special unwrapping of ExpressionWithTypeArguments. My implementation matches the original. No change.

3. KindPropertySignature returning nil for empty string literal types — The original TS code directly returns { kind: StringLiteralCompletionKind.Types, types: getStringLiteralTypes(...), isNewIdentifier: false } without checking if types are empty. My implementation matches the original. No change.

[gabritto]

This looks wrong and isn't in the original PR.

[jakebailey]

Indeed, this was in 460b995

[jakebailey]

@copilot Address this; this fix seems weird. There must be something else wrong.

[gabritto]

It may be then that this test needs to be manually fixed with regards to commit characters. IIRC the test converter tries to guess what commit characters to expect, but it cannot always get it right (and Strada used to just ignore it if not set...).

[Copilot]

Reverted in 171cecd. The original TypeScript code returns allCommitCharacters for all four kinds (ArrayLiteralExpression, IndexSignature, TupleType, ComputedPropertyName) in the OpenBracketToken case. The test failure was caused by the Go test generator mapping isNewIdentifierLocation: true to empty commit characters, which is incorrect for this context. Made test 21 manual with the correct DefaultCommitCharacters instead.

[gabritto]

I think this looks correct, including the commit character test update (what new identifier could you possibly type in that type position??), but for some reason Strada seemed to return empty commit characters in the scenario of that test. 🤷‍♀️