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feat(compiler): improve error message when type alias is used as valu… #62774

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feat(compiler): improve error message when type alias is used as valu… #62774

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5 changes: 5 additions & 0 deletions src/compiler/checker.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -27656,6 +27656,11 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
return Diagnostics.No_value_exists_in_scope_for_the_shorthand_property_0_Either_declare_one_or_provide_an_initializer;
}
else {
// Check if there's a type symbol with the same name
const typeSymbol = resolveName(node, node.escapedText, SymbolFlags.Type, /*nameNotFoundMessage*/ undefined, /*isUse*/ false);
if (typeSymbol && !(typeSymbol.flags & SymbolFlags.Value)) {
return Diagnostics.Cannot_find_name_0_1_only_refers_to_a_type_but_is_being_used_as_a_value_here;
}
Comment on lines +27660 to +27663
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Copilot AI Nov 18, 2025

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The diagnostic message requires two parameters ('{0}' for the identifier name and '{1}' for the type symbol name), but the current implementation doesn't provide a way to pass the second parameter. When this diagnostic is used in onFailedToResolveSymbol (line 3282 in checker.ts), it will call error(errorLocation, nameNotFoundMessage, diagnosticName(nameArg)) which only provides one argument. You need to either: (1) change the diagnostic message to only use one parameter like the existing "'{0}' only refers to a type, but is being used as a value here." (code 2693), or (2) modify the error reporting mechanism to pass both the identifier name and the type symbol name as separate arguments. The existing diagnostic 2693 might already serve this purpose.

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return Diagnostics.Cannot_find_name_0;
}
}
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4 changes: 4 additions & 0 deletions src/compiler/diagnosticMessages.json
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Expand Up @@ -2828,6 +2828,10 @@
"category": "Error",
"code": 2552
},
"Cannot find name '{0}'. '{1}' only refers to a type, but is being used as a value here.": {
"category": "Error",
"code": 2888
},
Comment on lines +2831 to +2834
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Copilot AI Nov 18, 2025

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The error code 2888 is significantly out of sequence. It's placed between codes 2552 and 2553 in the file, but 2888 is over 300 codes higher. This breaks the sequential ordering convention used throughout the file. The code should likely be 2553 or the next available sequential code in the 2500s range to maintain consistency with the surrounding diagnostic codes.

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"Computed values are not permitted in an enum with string valued members.": {
"category": "Error",
"code": 2553
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26 changes: 26 additions & 0 deletions tests/cases/compiler/typeAliasUsedAsValue.ts
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@@ -0,0 +1,26 @@
// @filename: mytypes.ts
export class MyClass {
public a: number;
}

export type MyClassAlias = MyClass;

// @filename: main.ts
import {MyClassAlias} from './mytypes';

let a: MyClassAlias = new MyClassAlias(); // Error: should show better message
let b = MyClassAlias; // Error: should show better message

// Test with local type alias
type LocalAlias = string;
let c = LocalAlias; // Error: should show better message

// Test with interface
interface MyInterface {
prop: string;
}
let d = MyInterface; // Error: should show better message

// Test with generic type alias
type GenericAlias<T> = T[];
let e = GenericAlias; // Error: should show better message
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