Retain uninitialized binding patterns for variable declarations in declaration emit by weswigham · Pull Request #4775 · microsoft/typescript-go

weswigham · 2026-07-28T17:40:33Z

As a side-effect, this makes exported binding patterns (potentially) isolatedDeclarations-emittable, since we can lookup their local type node/assertion/inference result the same as any other variable declaration.

Honestly, we've supported binding patterns in declaration files for... as long as I can remember, at least (and we've emitted them in parameter positions for many many major releases at this point), so I don't think there's really any harm in this - unless you'd get mad that when you write export const [,...rest] = [first(), ...others()] we now expose the type of first() in the declaration file even though it's not actually an accessible part of the exported shape. Still - that's probably worthwhile in exchange for making binding patterns isolatedDeclarations-emittable.

weswigham · 2026-07-28T17:43:09Z

-				t = b.ch.instantiateType(b.ch.getWriteTypeOfSymbol(symbol), b.ctx.mapper)
-			} else if symbol != nil && (symbol.Flags&(ast.SymbolFlagsTypeLiteral|ast.SymbolFlagsSignature) == 0) {
-				t = b.ch.instantiateType(b.ch.getWidenedLiteralType(b.ch.getTypeOfSymbol(symbol)), b.ctx.mapper)
+		if symbol == nil {


The "interesting" part implementation-wise is that variable declarations hosting binding patterns do have an overall type (ofc they do), but don't have a symbol bound to them (only the individual names in the binding pattern do). Somehow, this is the first time we've needed to serialize such symbol-less declarations, so I had to adjust the pipeline to actually handle nil symbols.

(Compare with parameters where even the nameless binding pattern parameters get a bound symbol that refers to them, since that's how the argument type is looked up externally.)

Copilot

Pull request overview

Preserves variable binding patterns in declaration emit and updates type lookup to support symbol-less declarations.

Changes:

Emits destructuring patterns rather than flattened declarations.
Adds regression coverage and updates affected baselines.
Handles symbol-less declaration type resolution safely.

Reviewed changes

Copilot reviewed 50 out of 50 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
`testdata/tests/cases/compiler/declarationEmitVariableBindingPatternsRetained.ts`	Adds regression coverage.
`testdata/submoduleAccepted.txt`	Registers accepted baseline differences.
`testdata/baselines/reference/submoduleAccepted/conformance/stringLiteralTypesAndTuples01.js.diff`	Accepts tuple-pattern emit.
`testdata/baselines/reference/submoduleAccepted/conformance/dependentDestructuredVariables.js.diff`	Accepts object-pattern emit.
`testdata/baselines/reference/submoduleAccepted/compiler/isolatedDeclarationErrorsExpressions.errors.txt.diff`	Accepts isolated-declaration diagnostics.
`testdata/baselines/reference/submoduleAccepted/compiler/destructuredDeclarationEmit.js.diff`	Accepts nested-pattern emit.
`testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitRetainsJsdocyComments.js.diff`	Updates comment-preserving emit.
`testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitOptionalMappedTypePropertyNoStrictNullChecks3.js.diff`	Accepts mapped-type pattern emit.
`testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitOptionalMappedTypePropertyNoStrictNullChecks2.js.diff`	Accepts mapped-type pattern emit.
`testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitOptionalMappedTypePropertyNoStrictNullChecks1.js.diff`	Accepts mapped-type pattern emit.
`testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitNonExportedBindingPattern.js.diff`	Accepts non-exported patterns.
`testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitExpressionInExtends6.js.diff`	Updates destructured class dependency emit.
`testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitDestructuringPrivacyError.js.diff`	Updates privacy-case emit.
`testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitDestructuringObjectLiteralPattern2.js.diff`	Accepts nested object patterns.
`testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitDestructuringObjectLiteralPattern1.js.diff`	Accepts object patterns.
`testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitDestructuringObjectLiteralPattern.js.diff`	Accepts comprehensive object patterns.
`testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitDestructuringArrayPattern5.js.diff`	Accepts omitted array elements.
`testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitDestructuringArrayPattern4.js.diff`	Accepts array rest patterns.
`testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitDestructuringArrayPattern3.js.diff`	Accepts namespace array patterns.
`testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitDestructuringArrayPattern2.js.diff`	Accepts nested array patterns.
`testdata/baselines/reference/submoduleAccepted/compiler/declarationEmitDestructuringArrayPattern1.js.diff`	Accepts basic array patterns.
`testdata/baselines/reference/submodule/conformance/stringLiteralTypesAndTuples01.js`	Updates tuple declaration output.
`testdata/baselines/reference/submodule/conformance/dependentDestructuredVariables.js`	Updates object declaration output.
`testdata/baselines/reference/submodule/compiler/isolatedDeclarationErrorsExpressions.errors.txt`	Removes binding-pattern diagnostics.
`testdata/baselines/reference/submodule/compiler/destructuredDeclarationEmit.js`	Records nested destructuring output.
`testdata/baselines/reference/submodule/compiler/declarationEmitRetainsJsdocyComments.js`	Records comments inside patterns.
`testdata/baselines/reference/submodule/compiler/declarationEmitOptionalMappedTypePropertyNoStrictNullChecks3.js`	Records mapped-type pattern output.
`testdata/baselines/reference/submodule/compiler/declarationEmitOptionalMappedTypePropertyNoStrictNullChecks2.js`	Records mapped-type pattern output.
`testdata/baselines/reference/submodule/compiler/declarationEmitOptionalMappedTypePropertyNoStrictNullChecks1.js`	Records mapped-type pattern output.
`testdata/baselines/reference/submodule/compiler/declarationEmitNonExportedBindingPattern.js`	Records non-exported patterns.
`testdata/baselines/reference/submodule/compiler/declarationEmitExpressionInExtends6.js`	Records namespace destructuring.
`testdata/baselines/reference/submodule/compiler/declarationEmitDestructuringPrivacyError.js`	Records privacy-case pattern.
`testdata/baselines/reference/submodule/compiler/declarationEmitDestructuringObjectLiteralPattern2.js`	Records nested object patterns.
`testdata/baselines/reference/submodule/compiler/declarationEmitDestructuringObjectLiteralPattern1.js`	Records object patterns.
`testdata/baselines/reference/submodule/compiler/declarationEmitDestructuringObjectLiteralPattern.js`	Records comprehensive object patterns.
`testdata/baselines/reference/submodule/compiler/declarationEmitDestructuringArrayPattern5.js`	Records omitted array elements.
`testdata/baselines/reference/submodule/compiler/declarationEmitDestructuringArrayPattern4.js`	Records array rest patterns.
`testdata/baselines/reference/submodule/compiler/declarationEmitDestructuringArrayPattern3.js`	Records namespace array patterns.
`testdata/baselines/reference/submodule/compiler/declarationEmitDestructuringArrayPattern2.js`	Records nested array patterns.
`testdata/baselines/reference/submodule/compiler/declarationEmitDestructuringArrayPattern1.js`	Records basic array patterns.
`testdata/baselines/reference/conformance/dependentDestructuredVariablesNoCrash3.js`	Updates malformed-pattern emit baseline.
`testdata/baselines/reference/compiler/subpathImportDeclarationEmit.js`	Preserves patterns across subpath imports.
`testdata/baselines/reference/compiler/declarationEmitVariableBindingPatternsRetained.types`	Records inferred types.
`testdata/baselines/reference/compiler/declarationEmitVariableBindingPatternsRetained.symbols`	Records symbol bindings.
`testdata/baselines/reference/compiler/declarationEmitVariableBindingPatternsRetained.js`	Records JavaScript and declaration emit.
`testdata/baselines/reference/compiler/declarationEmitSubpathImportsReexport.js`	Updates re-export declaration output.
`internal/transformers/declarations/transform.go`	Retains variable binding patterns.
`internal/pseudochecker/lookup.go`	Handles symbol-less variables.
`internal/checker/nodebuilderimpl.go`	Resolves symbol-less declaration types.
`internal/checker/emitresolver.go`	Avoids nil-symbol literal checks.

+const func = () => "ok"
+const b = { a: func }
+export const { a } = b


jakebailey · 2026-07-28T22:45:06Z

+-!!! error TS9019: Binding elements can't be exported directly with --isolatedDeclarations.
+     export const [, , b = 1]: [number, number, number | undefined] = [0, 1, 2];
+                       ~
+ !!! error TS9019: Binding elements can't be exported directly with --isolatedDeclarations.


Does this need a refreshed message that mentions the initializer? Sort of curious how this differs from export const foo = 1, though

Retain binding patterns for variable declarations in declaration emit

d6bf5e0

weswigham added this to the TypeScript 7.1 milestone Jul 28, 2026

weswigham requested review from Copilot, gabritto and jakebailey July 28, 2026 17:40

Copilot started reviewing on behalf of weswigham July 28, 2026 17:41 View session

weswigham commented Jul 28, 2026

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Copilot AI reviewed Jul 28, 2026

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Comment thread internal/transformers/declarations/transform.go

Comment thread testdata/tests/cases/compiler/declarationEmitVariableBindingPatternsRetained.ts

Comment on lines +2 to +4

const func = () => "ok"

const b = { a: func }

export const { a } = b

Only preserve binding patterns without initializers

197b017

weswigham changed the title ~~Retain binding patterns for variable declarations in declaration emit~~ Retain uninitialized binding patterns for variable declarations in declaration emit Jul 28, 2026

jakebailey reviewed Jul 28, 2026

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Retain uninitialized binding patterns for variable declarations in declaration emit - #4775

Retain uninitialized binding patterns for variable declarations in declaration emit#4775
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microsoft:mainfrom
weswigham:type-nodes-for-binding-patterns

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weswigham commented Jul 28, 2026 •

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