Ambiguous call under spread scenarios

[pshao25]

Typespec:

alias ThingAlias = {
  @doc("age of the Thing")
  age?: int32;
  @doc("name of the Thing")
  name?: string;
};
op spreadAlias( ...ThingAlias ): void;

Actual:

public virtual async Task<Response> SpreadAliasAsync(int? age = null, string name = null, CancellationToken cancellationToken = default);
public virtual async Task<Response> SpreadAliasAsync(RequestContent content, RequestContext context = null)
{
    Argument.AssertNotNull(content, nameof(content));
}

This call is an ambiguity: SpreadAliasAsync(null). This will not happen to string, see details in #3587.

Expected:
In the normal case like

public virtual async Task<Response> SpreadAliasAsync(Model model, CancellationToken cancellationToken = default)
{
    Argument.AssertNotNull(model, nameof(model));
}
public virtual async Task<Response> SpreadAliasAsync(RequestContent content, RequestContext context = null)
{
    Argument.AssertNotNull(content, nameof(content));
}

Even there is an ambiguous call SpreadAliasAsync(null), we still leave RequestContext optional, because SpreadAliasAsync(null) makes no sense.

What makes this scenario special from others is SpreadAliasAsync(null) makes sense to convenience method. So I think the protocol method should change when the first parameter in convenience method is not string.

Solution:

public virtual async Task<Response> SpreadAliasAsync(RequestContent content, RequestContext context)
{
    Argument.AssertNotNull(content, nameof(content));
}

Update:

According to #3587 (comment), we don't take SpreadAliasAsync(null) into consideration, so leave RequestContext context = null.

[AlexanderSher]

This will not happen to string, see details in #3587.

RequestContent has an implicit cast from string, but no implicit cast to string, so during cast resolution string parameter wins.

[pshao25]

In the update section of description, this is by design.