Fix decorated class methods with Symbol names to emit valid code

[Copilot]

tsgo emitted invalid JavaScript for decorated class methods with computed property names like [Symbol()]. The generated __decorate call referenced an undefined variable, causing ReferenceError: _a is not defined at runtime.

export class A {
  @(fakeDecorator as any)
  [Symbol()]() {}
}

Before (broken):

class A {
    [Symbol()]() { }  // Expression evaluated here
}
__decorate([...], A.prototype, _a, null);  // _a undefined

After (fixed):

var _a;
class A {
    [_a = Symbol()]() { }  // Expression captured
}
__decorate([...], A.prototype, _a, null);  // _a defined

Changes

• Added visitPropertyNameOfClassElement in legacydecorators.go to detect decorated class elements with non-simple computed property names
• For such elements: create a generated name, hoist its declaration via AddVariableDeclaration, and update the computed property to assign the expression to that name
• Updated visitMethodDeclaration, visitPropertyDeclaration, visitGetAccessorDeclaration, and visitSetAccessorDeclaration to use this new function
• Added test case legacyDecoratorSymbolMethodName.ts covering the reported scenario

Original prompt

---

This section details on the original issue you should resolve

<issue_title>tsgo emits invalid code for decorated class methods with Symbol names</issue_title>
<issue_description>## Steps to reproduce

tsgo compiles the following code incorrectly. Note the method name in the class:

export class A {
  @(fakeDecorator as any)
  [Symbol()]() {}
}

function fakeDecorator() {}

tsconfig:

{
  "compilerOptions": {
    "module": "nodenext",
    "declaration": true,
    "declarationMap": true,
    "sourceMap": true,
    "composite": true,
    "noImplicitOverride": true,
    "forceConsistentCasingInFileNames": true,
    "strict": true,
    "noImplicitReturns": true,
    "skipLibCheck": true,
    "target": "es2023",
    "strictPropertyInitialization": false,
    "noImplicitAny": true,
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "removeComments": false,
  }
}

Behavior with typescript@5.9

Valid code

Behavior with tsgo

Emits a non-existent method name - _a. If you run the compiled code, it throws an error:

], A.prototype, _a, null);
                ^

ReferenceError: _a is not defined

</issue_description>

<agent_instructions>Create a test case first which shows the issue; you will want to set the module setting, strict true, experimentalDecorators, emitDecoratorMetadata. Commit that test and its baselines, then work on the issue. Print debugging with go test -v might be helpful.</agent_instructions>

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

• Fixes tsgo emits invalid code for decorated class methods with Symbol names #2446

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[jakebailey]

I independently discovered this in #2956.