go/types: tweaks to ArgumentError to be more idiomatic

This CL makes a few changes to the new ArgumentError type to be more
idiomatic:
 - Use a pointer receiver for methods.
 - Export fields, similarly to Error. ArgumentError has a clear meaning
   (an error associated with an index), so there is no need to hide its
   representation.
 - Add an Unwrap method to access the underlying error.
 - Say explicitly that the error returned from Instantiate may wrap
   *ArgumentError. There is no need to commit to an API that always
   returns an error with dynamic type *ArgumentError.

Updates #47916

Change-Id: Ib1a43e921f247794e7155280ccbf5a6775ed3978
Reviewed-on: https://go-review.googlesource.com/c/go/+/351335
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
3 files changed
tree: 44ca55d33267d9171d37d339e083a4169346d456
  1. .gitattributes
  2. .github/
  3. .gitignore
  4. AUTHORS
  5. CONTRIBUTING.md
  6. CONTRIBUTORS
  7. LICENSE
  8. PATENTS
  9. README.md
  10. SECURITY.md
  11. api/
  12. codereview.cfg
  13. doc/
  14. lib/
  15. misc/
  16. src/
  17. test/
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