Fix signature help

Author: ahejlsberg

src/services/signatureHelp.ts

@@ -372,8 +372,7 @@ namespace ts.SignatureHelp {
             if (isTypeParameterList) {
                 isVariadic = false; // type parameter lists are not variadic
                 prefixDisplayParts.push(punctuationPart(SyntaxKind.LessThanToken));
-                // Use `.mapper` to ensure we get the generic type arguments even if this is an instantiated version of the signature.
-                const typeParameters = candidateSignature.typeParameters; // !!! candidateSignature.mapper ? candidateSignature.mapper.mappedTypes : candidateSignature.typeParameters;
+                const typeParameters = (candidateSignature.target || candidateSignature).typeParameters;
                 signatureHelpParameters = typeParameters && typeParameters.length > 0 ? map(typeParameters, createSignatureHelpParameterForTypeParameter) : emptyArray;
                 suffixDisplayParts.push(punctuationPart(SyntaxKind.GreaterThanToken));
                 const parameterParts = mapToDisplayParts(writer =>