Refine `ResolvableType` type variable resolution

[sdeleuze]

While exploring using ResolvableType in Spring AI with @tzolov and Spring Cloud Function with @olegz, we figured out that the type variable algorithm is not working as expected in all use cases.

This gist illustrates various use cases where either:

• Type resolution does not happen despite the information provided beeing enough to resolve them
• GenericTypeResolver#resolveType(Type, Class<?>) is required for the getGeneric(0) use case, and most users miss GenericTypeResolver that is not easily discoverable.

The purpose of this issue is to refine the type resolver to cover the broken use cases, and to explore if that would make sense or not to provide GenericTypeResolver#resolveType(Type, Class<?>) capabilities as part of ResolvableType.

[olegz]

Here is what does the trick for me. . basically some recursive logic that effectively does the same thing as ResolvableType.toString()

In other words what goes into the method, comes out the same but as ParameterizedType (not just String)

public static Type resolveType(ResolvableType typeWithGenerics) {
	if (typeWithGenerics.hasResolvableGenerics()) {
		ResolvableType[] generics = typeWithGenerics.getGenerics();
		List<ResolvableType> resolvedGenerics = new ArrayList<>();
		for (int i = 0; i < generics.length; i++) {
			ResolvableType genericType = typeWithGenerics.getGenerics()[i];
			resolvedGenerics.add(ResolvableType.forType(resolveType(genericType)));
		}
		return ResolvableType.forClassWithGenerics(typeWithGenerics.resolve(),
			resolvedGenerics.toArray(new ResolvableType[0])).getType();
	}
	else {
		return typeWithGenerics.resolve();
	}
}

https://github.com/spring-cloud/spring-cloud-function/blob/main/spring-cloud-function-context/src/main/java/org/springframework/cloud/function/context/catalog/FunctionTypeUtils.java#L378