Invalid collections returned when element type is not a class

[RomainMuller]

🐛 Bug Report

Affected Languages

• TypeScript or Javascript
• Python
• Java
• .NET (C#, F#, ...)

General Information

• JSII Version: 0.21.1 (build 9ff44cb), typescript 3.7.4
• Platform: any

What is the problem?

When a return value is typed as a List<SomeStruct>, the Java runtime does not correctly perform the conversion to the correct Java type, and instead returns a List<JsiiObject> where the JsiiObject instances however contain all the information necessary to permit the conversion.

The runtime needs to be corrected so that it actually returns a List<SomeStruct>, or else attempting to use the contents of the list is likely to cause a ClassCastException.

It is probable the same issue would affect Map<String, SomeStruct> instances, so this has to be checked as well.

Minimal Reproduction

A minimal reproduction using the AWS CDK can be achieved:

package com.myorg;

import software.amazon.awscdk.core.*;
import software.amazon.awscdk.services.cloudwatch.Dimension;
import software.amazon.awscdk.services.cloudwatch.IMetric;
import software.amazon.awscdk.services.cloudwatch.MetricConfig;
import software.amazon.awscdk.services.lambda.Code;
import software.amazon.awscdk.services.lambda.Function;
import software.amazon.awscdk.services.lambda.Runtime;

import java.util.List;

public class CwMetricsApp {
    public static void main(final String[] args) {
        final App app = new App();
        final Stack stack = new Stack(app, "ReproStack");

        final Function funct = Function.Builder.create(stack, "Function")
                .code(Code.fromInline("export function handler() {}"))
                .handler("index.handler")
                .runtime(Runtime.NODEJS_10_X)
                .build();

        final IMetric metric = funct.metricInvocations();

        final MetricConfig config = metric.toMetricConfig();

        // This list is, in fact (and incorrectly), a List<JsiiObject>
        final List<Dimension> dimensions = config.getMetricStat().getDimensions();
        for (final Dimension dimension : dimensions) {
            System.out.println(String.format("Dimension<%s, %s>", dimension.getName(), dimension.getValue()));
        }

        app.synth();
    }
}