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cmd/compile: provide better error message when directly assigning to struct field in map #13779

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dsnet opened this issue Dec 30, 2015 · 2 comments
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cmd/compile: provide better error message when directly assigning to struct field in map #13779

dsnet opened this issue Dec 30, 2015 · 2 comments
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Go1.7

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@dsnet
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dsnet commented Dec 30, 2015

Using go1.5

Compile the following code:

package main

func main() {
    type person struct{ age, weight, height int }
    students := map[string]person{"sally": person{12, 50, 32}}
    students["sally"].age = 3
}

Currently, I see the error:
prog.go:6: cannot assign to students["sally"].age
While the message tells you what went wrong, it doesn't tell you why it is wrong. It causes a programmer to scratch his/her head wondering why this is happening. "Did I misspell the field name?" "Is the map using the wrong type?" etc...

I expect to see something more informative:
prog.go:6: cannot directly assign to struct field students["sally"].age in map

This is related to #3117
@ianlancetaylor ianlancetaylor added this to the Go1.7 milestone Dec 31, 2015
@dr2chase dr2chase self-assigned this Jan 2, 2016
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CL https://golang.org/cl/21462 mentions this issue.

@gopherbot
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CL https://golang.org/cl/21463 mentions this issue.

gopherbot pushed a commit that referenced this issue Apr 4, 2016
@cmarcelo @dr2chase
go/types: better error when assigning to struct field in map
74c0fea 
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>
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