go/types: better error when assigning to struct field in map
Identify this assignment case and instead of the more general error
prog.go:6: cannot assign to students["sally"].age (value of type int)
produce
prog.go:6: cannot directly assign to struct field students["sally"].age in map
that explains why the assignment is not possible. Used ExprString
instead of String of operand since the type of the field is not relevant
to the error.
Updates #13779.
Change-Id: I581251145ae6336ddd181b9ddd77f657c51b5aff
Reviewed-on: https://go-review.googlesource.com/21463 Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>