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Fix unused type parameter checking to skip overload signatures and only report on implementations #1618

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Fix unused type parameter checking to skip overload signatures and only report on implementations #1618

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c5eeeec
Initial plan
Copilot Aug 21, 2025
e72f01e
Initial investigation: add test case and understand issue
Copilot Aug 22, 2025
64269ce
Fix unused type parameter diagnostic to use TS6133 instead of TS6196
Copilot Aug 22, 2025
469d286
Add comprehensive test case for unused type parameters
Copilot Aug 22, 2025
c72979a
Revert diagnostic changes - type variables are not values that can be…
Copilot Aug 22, 2025
beefab7
Fix unused type parameter checking in overloads to only report on imp…
Copilot Aug 22, 2025
0065dbe
Combine comments for unused parameter and type parameter checking
Copilot Aug 22, 2025
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3 changes: 2 additions & 1 deletion internal/checker/checker.go
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Expand Up @@ -6694,8 +6694,9 @@ func (c *Checker) checkUnusedIdentifiers(potentiallyUnusedIdentifiers []*ast.Nod
// Only report unused parameters on the implementation, not overloads.
if node.Body() != nil {
c.checkUnusedLocalsAndParameters(node)
// Only report unused type parameters on the implementation, not overloads.
c.checkUnusedTypeParameters(node)
}
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@jakebailey jakebailey Aug 22, 2025 •
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The original code for checkUnusedIdentifiers in checker.ts is:

case SyntaxKind.Constructor:
case SyntaxKind.FunctionExpression:
case SyntaxKind.FunctionDeclaration:
case SyntaxKind.ArrowFunction:
case SyntaxKind.MethodDeclaration:
case SyntaxKind.GetAccessor:
case SyntaxKind.SetAccessor:
    // Only report unused parameters on the implementation, not overloads.
    if (node.body) {
        checkUnusedLocalsAndParameters(node, addDiagnostic);
    }
    checkUnusedTypeParameters(node, addDiagnostic);
    break;

Which very much implies that we were checking unused type parameters on these.

I think the difference is actually in checkUnusedTypeParameters, which did this:

        // Only report errors on the last declaration for the type parameter container;
        // this ensures that all uses have been accounted for.
        const declarations = getSymbolOfDeclaration(node).declarations;
        if (!declarations || last(declarations) !== node) return;

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So, what I'm saying is, I think that this PR is actually correct as it skips the unused check exactly like the other code.

c.checkUnusedTypeParameters(node)
case ast.KindMethodSignature, ast.KindCallSignature, ast.KindConstructSignature, ast.KindFunctionType, ast.KindConstructorType,
ast.KindTypeAliasDeclaration, ast.KindJSTypeAliasDeclaration, ast.KindInterfaceDeclaration:
c.checkUnusedTypeParameters(node)
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24 changes: 24 additions & 0 deletions testdata/baselines/reference/compiler/overloadUnusedTypeParameters.errors.txt
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@@ -0,0 +1,24 @@
overloadUnusedTypeParameters.ts(3,30): error TS6196: 'T' is declared but never used.
overloadUnusedTypeParameters.ts(13,31): error TS6196: 'T' is declared but never used.


==== overloadUnusedTypeParameters.ts (2 errors) ====
export function tryParseJson<T>(text: string): unknown;
export function tryParseJson<T>(text: string, predicate: (parsed: unknown) => parsed is T): T | undefined;
export function tryParseJson<T>() {
~
!!! error TS6196: 'T' is declared but never used.
throw new Error("noop")
}

export function tryParseJson2<T>(text: string): unknown;
export function tryParseJson2<T>(text: string, predicate: (parsed: unknown) => parsed is T): T | undefined;
export function tryParseJson2() {
throw new Error("noop")
}

export function tryParseJson3<T>(_text: string): unknown {
~
!!! error TS6196: 'T' is declared but never used.
throw new Error("noop")
}
29 changes: 29 additions & 0 deletions testdata/baselines/reference/compiler/overloadUnusedTypeParameters.js
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@@ -0,0 +1,29 @@
//// [tests/cases/compiler/overloadUnusedTypeParameters.ts] ////

//// [overloadUnusedTypeParameters.ts]
export function tryParseJson<T>(text: string): unknown;
export function tryParseJson<T>(text: string, predicate: (parsed: unknown) => parsed is T): T | undefined;
export function tryParseJson<T>() {
throw new Error("noop")
}

export function tryParseJson2<T>(text: string): unknown;
export function tryParseJson2<T>(text: string, predicate: (parsed: unknown) => parsed is T): T | undefined;
export function tryParseJson2() {
throw new Error("noop")
}

export function tryParseJson3<T>(_text: string): unknown {
throw new Error("noop")
}

//// [overloadUnusedTypeParameters.js]
export function tryParseJson() {
throw new Error("noop");
}
export function tryParseJson2() {
throw new Error("noop");
}
export function tryParseJson3(_text) {
throw new Error("noop");
}
56 changes: 56 additions & 0 deletions testdata/baselines/reference/compiler/overloadUnusedTypeParameters.symbols
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@@ -0,0 +1,56 @@
//// [tests/cases/compiler/overloadUnusedTypeParameters.ts] ////

=== overloadUnusedTypeParameters.ts ===
export function tryParseJson<T>(text: string): unknown;
>tryParseJson : Symbol(tryParseJson, Decl(overloadUnusedTypeParameters.ts, 0, 0), Decl(overloadUnusedTypeParameters.ts, 0, 55), Decl(overloadUnusedTypeParameters.ts, 1, 106))
>T : Symbol(T, Decl(overloadUnusedTypeParameters.ts, 0, 29))
>text : Symbol(text, Decl(overloadUnusedTypeParameters.ts, 0, 32))

export function tryParseJson<T>(text: string, predicate: (parsed: unknown) => parsed is T): T | undefined;
>tryParseJson : Symbol(tryParseJson, Decl(overloadUnusedTypeParameters.ts, 0, 0), Decl(overloadUnusedTypeParameters.ts, 0, 55), Decl(overloadUnusedTypeParameters.ts, 1, 106))
>T : Symbol(T, Decl(overloadUnusedTypeParameters.ts, 1, 29))
>text : Symbol(text, Decl(overloadUnusedTypeParameters.ts, 1, 32))
>predicate : Symbol(predicate, Decl(overloadUnusedTypeParameters.ts, 1, 45))
>parsed : Symbol(parsed, Decl(overloadUnusedTypeParameters.ts, 1, 58))
>parsed : Symbol(parsed, Decl(overloadUnusedTypeParameters.ts, 1, 58))
>T : Symbol(T, Decl(overloadUnusedTypeParameters.ts, 1, 29))
>T : Symbol(T, Decl(overloadUnusedTypeParameters.ts, 1, 29))

export function tryParseJson<T>() {
>tryParseJson : Symbol(tryParseJson, Decl(overloadUnusedTypeParameters.ts, 0, 0), Decl(overloadUnusedTypeParameters.ts, 0, 55), Decl(overloadUnusedTypeParameters.ts, 1, 106))
>T : Symbol(T, Decl(overloadUnusedTypeParameters.ts, 2, 29))

throw new Error("noop")
>Error : Symbol(Error, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es2022.error.d.ts, --, --))
}

export function tryParseJson2<T>(text: string): unknown;
>tryParseJson2 : Symbol(tryParseJson2, Decl(overloadUnusedTypeParameters.ts, 4, 1), Decl(overloadUnusedTypeParameters.ts, 6, 56), Decl(overloadUnusedTypeParameters.ts, 7, 107))
>T : Symbol(T, Decl(overloadUnusedTypeParameters.ts, 6, 30))
>text : Symbol(text, Decl(overloadUnusedTypeParameters.ts, 6, 33))

export function tryParseJson2<T>(text: string, predicate: (parsed: unknown) => parsed is T): T | undefined;
>tryParseJson2 : Symbol(tryParseJson2, Decl(overloadUnusedTypeParameters.ts, 4, 1), Decl(overloadUnusedTypeParameters.ts, 6, 56), Decl(overloadUnusedTypeParameters.ts, 7, 107))
>T : Symbol(T, Decl(overloadUnusedTypeParameters.ts, 7, 30))
>text : Symbol(text, Decl(overloadUnusedTypeParameters.ts, 7, 33))
>predicate : Symbol(predicate, Decl(overloadUnusedTypeParameters.ts, 7, 46))
>parsed : Symbol(parsed, Decl(overloadUnusedTypeParameters.ts, 7, 59))
>parsed : Symbol(parsed, Decl(overloadUnusedTypeParameters.ts, 7, 59))
>T : Symbol(T, Decl(overloadUnusedTypeParameters.ts, 7, 30))
>T : Symbol(T, Decl(overloadUnusedTypeParameters.ts, 7, 30))

export function tryParseJson2() {
>tryParseJson2 : Symbol(tryParseJson2, Decl(overloadUnusedTypeParameters.ts, 4, 1), Decl(overloadUnusedTypeParameters.ts, 6, 56), Decl(overloadUnusedTypeParameters.ts, 7, 107))

throw new Error("noop")
>Error : Symbol(Error, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es2022.error.d.ts, --, --))
}

export function tryParseJson3<T>(_text: string): unknown {
>tryParseJson3 : Symbol(tryParseJson3, Decl(overloadUnusedTypeParameters.ts, 10, 1))
>T : Symbol(T, Decl(overloadUnusedTypeParameters.ts, 12, 30))
>_text : Symbol(_text, Decl(overloadUnusedTypeParameters.ts, 12, 33))

throw new Error("noop")
>Error : Symbol(Error, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --), Decl(lib.es2022.error.d.ts, --, --))
}
50 changes: 50 additions & 0 deletions testdata/baselines/reference/compiler/overloadUnusedTypeParameters.types
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@@ -0,0 +1,50 @@
//// [tests/cases/compiler/overloadUnusedTypeParameters.ts] ////

=== overloadUnusedTypeParameters.ts ===
export function tryParseJson<T>(text: string): unknown;
>tryParseJson : { <T>(text: string): unknown; <T>(text: string, predicate: (parsed: unknown) => parsed is T): T | undefined; }
>text : string

export function tryParseJson<T>(text: string, predicate: (parsed: unknown) => parsed is T): T | undefined;
>tryParseJson : { <T>(text: string): unknown; <T>(text: string, predicate: (parsed: unknown) => parsed is T): T | undefined; }
>text : string
>predicate : (parsed: unknown) => parsed is T
>parsed : unknown

export function tryParseJson<T>() {
>tryParseJson : { <T>(text: string): unknown; <T>(text: string, predicate: (parsed: unknown) => parsed is T): T | undefined; }

throw new Error("noop")
>new Error("noop") : Error
>Error : ErrorConstructor
>"noop" : "noop"
}

export function tryParseJson2<T>(text: string): unknown;
>tryParseJson2 : { <T>(text: string): unknown; <T>(text: string, predicate: (parsed: unknown) => parsed is T): T | undefined; }
>text : string

export function tryParseJson2<T>(text: string, predicate: (parsed: unknown) => parsed is T): T | undefined;
>tryParseJson2 : { <T>(text: string): unknown; <T>(text: string, predicate: (parsed: unknown) => parsed is T): T | undefined; }
>text : string
>predicate : (parsed: unknown) => parsed is T
>parsed : unknown

export function tryParseJson2() {
>tryParseJson2 : { <T>(text: string): unknown; <T>(text: string, predicate: (parsed: unknown) => parsed is T): T | undefined; }

throw new Error("noop")
>new Error("noop") : Error
>Error : ErrorConstructor
>"noop" : "noop"
}

export function tryParseJson3<T>(_text: string): unknown {
>tryParseJson3 : <T>(_text: string) => unknown
>_text : string

throw new Error("noop")
>new Error("noop") : Error
>Error : ErrorConstructor
>"noop" : "noop"
}
20 changes: 20 additions & 0 deletions testdata/tests/cases/compiler/overloadUnusedTypeParameters.ts
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// @strict: true
// @noUnusedLocals: true
// @noUnusedParameters: true
// @target: esnext

export function tryParseJson<T>(text: string): unknown;
export function tryParseJson<T>(text: string, predicate: (parsed: unknown) => parsed is T): T | undefined;
export function tryParseJson<T>() {
throw new Error("noop")
}

export function tryParseJson2<T>(text: string): unknown;
export function tryParseJson2<T>(text: string, predicate: (parsed: unknown) => parsed is T): T | undefined;
export function tryParseJson2() {
throw new Error("noop")
}

export function tryParseJson3<T>(_text: string): unknown {
throw new Error("noop")
}
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