extensions/typescript-language-features/src/languageFeatures/signatureHelp.ts/TypeScriptSignatureHelpProvider/getActiveParameter
class TypeScriptSignatureHelpProvider:	private getActiveParameter(info: Proto.SignatureHelpItems): number {
		const activeSignature = info.items[info.selectedItemIndex];
		if (activeSignature?.isVariadic) {
			return Math.min(info.argumentIndex, activeSignature.parameters.length - 1);
		}
		return info.argumentIndex;
	}