Fix import elision with type-only usage

[Copilot]

• Understand the issue: tsgo emits a require statement when a value is imported without the type keyword but is only used in type-level positions
• Investigate root cause using print debugging
• Identify issue: isElisionBlocked returns true when TypeEraserTransformer modifies import clause
• Fix importelision.go to elide imports even when isElisionBlocked is true if import clause becomes nil
• Run full test suite - all tests pass
• Multiple diff files deleted indicating tsgo now matches TypeScript behavior
• Code review and security checks passed
• Remove redundant test case since existing submodule tests cover the behavior
• Simplify import elision logic by removing unnecessary conditional checks

Original prompt

---

This section details on the original issue you should resolve

<issue_title>tsgo emits a require statement when a value is imported without the type keyword but only used in type-level positions</issue_title>
<issue_description>## Steps to reproduce

Minimal reproduction: https://github.com/psm14/tsgo-type-only-import-reproduction

This extra require statement can cause different behavior at runtime if the imported file performs any top-level side effects.

// Import Value as a value (no `type` keyword), but only use it in type positions
import { Value, type ValueData } from "./provider";

// Value is ONLY used in type positions:
export interface Record {
  getValue(): Value; // return type
  setValue(value: Value): void; // parameter type
  readonly currentValue: Value; // type annotation
}

export function processRecord(
  value: Value, // parameter type
  callback: (result: Value) => void, // parameter type in callback
): Value {
  // return type
  callback(value);
  return value;
}

Behavior with typescript@5.9

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.BaseProcessor = void 0;
exports.processRecord = processRecord;
// NO require("./provider")

function processRecord(value, callback) {
  callback(value);
  return value;
}

class BaseProcessor {
  constructor() {
    this.current = null;
  }
}
exports.BaseProcessor = BaseProcessor;

Behavior with tsgo

"use strict";
Object.defineProperty(exports, "__esModule", { value: true });
exports.BaseProcessor = void 0;
exports.processRecord = processRecord;
require("./provider"); // <-- EMITTED!

function processRecord(value, callback) {
  callback(value);
  return value;
}

class BaseProcessor {
  current = null;
}
exports.BaseProcessor = BaseProcessor;

</issue_description>

<agent_instructions>Create a test case out of the linked repo that shows the bug first, commit that, then work on the issue.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes tsgo emits a require statement when a value is imported without the type keyword but only used in type-level positions #2636

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[jakebailey]

@copilot as you said, this is a partial fix. Think harder and figure out how to fully fix it. Cross compare against checker.ts. Use print debugging with fmt.Println and go test -v if you have to.

[Copilot]

@copilot as you said, this is a partial fix. Think harder and figure out how to fully fix it. Cross compare against checker.ts. Use print debugging with fmt.Println and go test -v if you have to...

Fixed in e624128. The root cause was that isElisionBlocked returned true when the TypeEraserTransformer had already modified the import clause (by removing type-only specifiers), which prevented proper elision. The fix now visits the import clause first and elides the entire import declaration if the import clause becomes nil, regardless of isElisionBlocked. This also fixed several other baseline diffs where tsgo now matches TypeScript's behavior.

[Copilot]

Pull request overview

Fixes incorrect import elision where tsgo could emit runtime require(...) calls for imports that are only used in type positions, aligning emitted output more closely with the TypeScript reference baselines.

Changes:

• Update ImportElisionTransformer to drop ImportDeclarations when the visited ImportClause is fully elided, preventing conversion into side-effect-only imports.
• Elide type-only ImportSpecifiers directly in ImportElisionTransformer.
• Update reference baselines and remove now-unnecessary .diff baselines where tsgo output converged with the reference behavior.

Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
internal/transformers/tstransforms/importelision.go	Changes import elision behavior for ImportDeclaration and ImportSpecifier.
testdata/baselines/reference/submodule/conformance/nodeModulesImportModeDeclarationEmitErrors1(module=nodenext).js.diff	Removed baseline-diff file now that output matches reference.
testdata/baselines/reference/submodule/conformance/nodeModulesImportModeDeclarationEmitErrors1(module=nodenext).js	Updates expected emitted JS to no longer include a require("pkg").
testdata/baselines/reference/submodule/conformance/nodeModulesImportModeDeclarationEmitErrors1(module=node20).js.diff	Removed baseline-diff file now that output matches reference.
testdata/baselines/reference/submodule/conformance/nodeModulesImportModeDeclarationEmitErrors1(module=node20).js	Updates expected emitted JS to no longer include a require("pkg").
testdata/baselines/reference/submodule/conformance/nodeModulesImportModeDeclarationEmitErrors1(module=node18).js.diff	Removed baseline-diff file now that output matches reference.
testdata/baselines/reference/submodule/conformance/nodeModulesImportModeDeclarationEmitErrors1(module=node18).js	Updates expected emitted JS to no longer include a require("pkg").
testdata/baselines/reference/submodule/conformance/nodeModulesImportModeDeclarationEmitErrors1(module=node16).js.diff	Removed baseline-diff file now that output matches reference.
testdata/baselines/reference/submodule/conformance/nodeModulesImportModeDeclarationEmitErrors1(module=node16).js	Updates expected emitted JS to no longer include a require("pkg").
testdata/baselines/reference/submodule/conformance/nodeModulesImportAttributesModeDeclarationEmitErrors(module=nodenext).js.diff	Removed baseline-diff file now that output matches reference.
testdata/baselines/reference/submodule/conformance/nodeModulesImportAttributesModeDeclarationEmitErrors(module=nodenext).js	Updates expected emitted JS to no longer include a require("pkg").
testdata/baselines/reference/submodule/conformance/nodeModulesImportAttributesModeDeclarationEmitErrors(module=node20).js.diff	Removed baseline-diff file now that output matches reference.
testdata/baselines/reference/submodule/conformance/nodeModulesImportAttributesModeDeclarationEmitErrors(module=node20).js	Updates expected emitted JS to no longer include a require("pkg").
testdata/baselines/reference/submodule/conformance/nodeModulesImportAttributesModeDeclarationEmitErrors(module=node18).js.diff	Removed baseline-diff file now that output matches reference.
testdata/baselines/reference/submodule/conformance/nodeModulesImportAttributesModeDeclarationEmitErrors(module=node18).js	Updates expected emitted JS to no longer include a require("pkg").
testdata/baselines/reference/submodule/conformance/nodeModulesImportAttributesModeDeclarationEmitErrors(module=node16).js.diff	Removed baseline-diff file now that output matches reference.
testdata/baselines/reference/submodule/conformance/nodeModulesImportAttributesModeDeclarationEmitErrors(module=node16).js	Updates expected emitted JS to no longer include a require("pkg").
testdata/baselines/reference/submodule/conformance/importSpecifiers1.js.diff	Removed baseline-diff file now that output matches reference.
testdata/baselines/reference/submodule/conformance/importSpecifiers1.js	Updates expected emitted JS for d.js to export {};.

[Copilot]

The PR description says the fix is to elide imports even when isElisionBlocked is true only when the visited ImportClause becomes nil, but the implementation removes the isElisionBlocked gate for all ImportDeclarations. This changes behavior for any import declaration that was modified earlier in the pipeline (including by custom before transforms) and may re-enable elision in cases the isElisionBlocked contract is meant to prevent. Consider restoring the isElisionBlocked check for the general case and only bypassing it for the specific scenario where the visited ImportClause becomes nil (or adjust isElisionBlocked to treat TypeEraser updates as non-blocking).

[jakebailey]

The PR description was not updated to reflect the later changes, unfortunately

[jakebailey]

isElisionBlocked seems to only be needed for custom transformers, which we don't have, too.

[jakebailey]

I investigated this more and figured out that the problem is that the old TS transform has been split into a type eraser and an import elider. And so, the "fix" is actually the removal of the isElisionBlocked call itself, which was originally designed to bail on import nodes that had been modified by custom transformers that happened before ours. But that's exactly what this looks like now, where there are two steps, and the second thinks that a previous step did the work because parts of the AST had been modified through erasing TS code.

The extra bit is that both contain basically the same code for erasing imports, which makes me think that we shouldn't have a separate import elider at all.

[jakebailey]

Sent #2673 instead