go/types, types2: better error messages for for-range clauses
Provide the exact error cause instead of reporting a missing
core type.
For #70128.
Change-Id: I835698fa1f22382711bd54b974d2c87ee17e9065
Reviewed-on: https://go-review.googlesource.com/c/go/+/651215 Reviewed-by: Robert Griesemer <gri@google.com> Reviewed-by: Robert Findley <rfindley@google.com>
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