[Bug]: There's seemingly no way to see what arguments were actually specified vs defaulted for a template instantiation

[TomerAberbach]

Describe the bug

For example, suppose you have the following:

model TemplateModel<A, B = string> {
  a: A,
  b: B
}

model InstantiatedModel1 is TemplateModel<int32>;
model InstantiatedModel2 is TemplateModel<int32, string>;

As far as I can tell, there's nothing in the TypeSpec AST that lets me differentiate between TemplateModel<int32> and TemplateModel<int32, string>. In both cases the second argument is considered to be string and there's no simple indication that in one case it was from a default and in the other case it was explicitly specified.

This matters for my use-case because I'm generating functions for templated models (see TomerAberbach/typespec-fast-check#76 if you're curious) and it would be nice to make the emitted function calls (i.e. template instantiations) not unnecessarily emit default arguments that were not specified in the TypeSpec file.

Related to #5448.

Reproduction

See above.

Checklist

• Follow our Code of Conduct
• Check that there isn't already an issue that request the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion.
• The provided reproduction is a minimal reproducible example of the bug.