🐛 should report duplicate identifier error

[bvanjoi]

Environment information

CLI:
  Version:                      1.6.4
  Color support:                true

Platform:
  CPU Architecture:             aarch64
  OS:                           macos

Environment:
  BIOME_LOG_DIR:                unset
  NO_COLOR:                     unset
  TERM:                         "xterm-256color"
  JS_RUNTIME_VERSION:           "v20.11.1"
  JS_RUNTIME_NAME:              "node"
  NODE_PACKAGE_MANAGER:         "npm/10.2.4"

Biome Configuration:
  Status:                       Loaded successfully
  Formatter disabled:           false
  Linter disabled:              false
  Organize imports disabled:    false
  VCS disabled:                 true

Workspace:
  Open Documents:               0

What happened?

• code:

// npx @biomejs/biome check --apply code.ts
function f(a: number) {
  const a = 1; // expected throw error here but not.
}

• playground:

https://biomejs.dev/playground/?code=ZgB1AG4AYwB0AGkAbwBuACAAZgAoAGEAOgAgAG4AdQBtAGIAZQByACkAIAB7AAoAIAAgAGMAbwBuAHMAdAAgAGEAIAA9ACAAMQA7AAoAfQA%3D

Expected result

Report errors like duplicated identifier

Code of Conduct

• I agree to follow Biome's Code of Conduct

[Conaclos]

The problem arises from the fact that noRedeclare checks for redeclarations in a given scope. Formal parameters and the function body are in different scopes.

[10Derozan]

Similarly, type parameters should not be in different scopes too.
For example:

function identity<T>(arg: T): T {
    const a: T = 1; // expect error
    return arg;
}

[Conaclos]

function identity(arg: T): T {
const a: T = 1; // expect error
return arg;
}

I don't see any issue in your example...

[10Derozan]

function identity(arg: T): T {
const a: T = 1; // expect error
return arg;
}

I don't see any issue in your example...

Sorry, maybe they are not the similar error, in ts check it will fail.

The example should be

function identity<T>(arg: T): T {
    type T = number;
}