Expose method for retrieving inferred generic type arguments for a signature on the TypeChecker (microsoft#59637)

Author: tristanmenzel

src/compiler/checker.ts

@@ -1927,8 +1927,14 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         isTypeParameterPossiblyReferenced,
         typeHasCallOrConstructSignatures,
         getSymbolFlags,
+        getTypeArgumentsForResolvedSignature,
     };
 
+    function getTypeArgumentsForResolvedSignature(signature: Signature) {
+        if (signature.mapper === undefined) return undefined
+        return instantiateTypes((signature.target || signature).typeParameters, signature.mapper);
+    }
+
     function getCandidateSignaturesForStringLiteralCompletions(call: CallLikeExpression, editingArgument: Node) {
         const candidatesSet = new Set<Signature>();
         const candidates: Signature[] = [];

src/compiler/types.ts

@@ -5426,6 +5426,8 @@ export interface TypeChecker {
     /** @internal */ typeHasCallOrConstructSignatures(type: Type): boolean;
     /** @internal */ getSymbolFlags(symbol: Symbol): SymbolFlags;
     /** @internal */ fillMissingTypeArguments(typeArguments: readonly Type[], typeParameters: readonly TypeParameter[] | undefined, minTypeArgumentCount: number, isJavaScriptImplicitAny: boolean): Type[];
+
+    getTypeArgumentsForResolvedSignature(signature: Signature): readonly Type[] | undefined
 }
 
 /** @internal */

tests/baselines/reference/api/typescript.d.ts

@@ -6349,6 +6349,7 @@ declare namespace ts {
          * and the operation is cancelled, then it should be discarded, otherwise it is safe to keep.
          */
         runWithCancellationToken<T>(token: CancellationToken, cb: (checker: TypeChecker) => T): T;
+        getTypeArgumentsForResolvedSignature(signature: Signature): readonly Type[] | undefined;
     }
     enum NodeBuilderFlags {
         None = 0,