go/types: better error when converting untyped values in assignments

The error returned by convertUntyped is 'cannot convert _ to _', which
can be misleading in contexts where an explicit conversion would be
allowed.

Arguably the error message from convertUntyped should just be 'cannot
use _ as _', as 'convert' has an explicit meaning within the spec.
Making that change caused a large number of test failures, so for now we
just fix this for assignments by interpreting the error.

For #22070

Change-Id: I4eed6f39d1a991e8df7e035ec301d28a05150eb5
Reviewed-on: https://go-review.googlesource.com/c/go/+/242083
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
1 file changed
tree: 6db01004ad88145751dba3f7c6ce76aec340f9dc
  1. .github/
  2. api/
  3. doc/
  4. lib/
  5. misc/
  6. src/
  7. test/
  8. .gitattributes
  9. .gitignore
  10. AUTHORS
  11. CONTRIBUTING.md
  12. CONTRIBUTORS
  13. favicon.ico
  14. LICENSE
  15. PATENTS
  16. README.md
  17. robots.txt
  18. SECURITY.md
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