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cmd/compile: confusing error if composite literal field that is a method is specified #29855

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pwaller opened this issue Jan 21, 2019 · 4 comments

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commented Jan 21, 2019

What version of Go are you using (go version)?

$ go version
tip, go playground.

Does this issue reproduce with the latest release?

Yes.

What did you do?

Playground link: https://play.golang.org/p/llL2iikbJmd

package main

type T struct {
	GlobalName string
}
var t = T{Name: "foo"}

func (t T) Name() string {
	return t.GlobalName
}

(Note: the field is called GlobalName but the composite literal specifies the incorrect field Name, which is also a method).

What did you expect to see?

An error message telling me that I'm trying to declare a composite literal field which doesn't exist (or is a method, not a struct field).

What did you see instead?

prog.go:6:11: cannot use promoted field Name in struct literal of type T

This lead to some confusion - in my case because the name of the method (Name) was similar to the name of the field I was trying to declare (GlobalName). The issue occurred because a dependency I was using changed the Name field to GlobalName and introduced an accessor method called (T).Name(). So I was in a larger process of code repair.

The struct in question also had a number of embedded fields, so it took a moment before I found the root cause (that there was Name method defined far away).

See also #23609 which improved error messages for embedding but perhaps made things more confusing in this case.

@mvdan

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commented Jan 21, 2019

/cc @mdempsky @griesemer @ChrisALiles, who worked on the previous issue.

@beoran

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commented Jan 22, 2019

I agree that this would probably be more clear in this case:

prog.go:6:11: cannot use method Name in struct literal of type T

@mdempsky

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commented Jan 22, 2019

@pwaller Thanks for the report, and sorry about the confusing error message.

Looks like

 p, _ := dotpath(l.Sym, t, nil, true)
 if p == nil {
     yyerror("unknown field '%v' in struct literal of type %v", l.Sym, t)

in typecheck.go:typecheckcomplit needs to be changed to used dotpath's save parameter, and then we need to validate that the returned *types.Field is a field and not a method (i.e., f.Type.Etype != TFUNC || f.Type.Recv() == nil).

This should probably be abstracted into an IsMethod method on *types.Field since it's non-intuitive and appears in a few places already:

reflect.go:		if f.Type.Etype != TFUNC || f.Type.Recv() == nil {
subr.go:			if f.Sym == s || (ignorecase && f.Type.Etype == TFUNC && f.Type.Recv() != nil && strings.EqualFold(f.Sym.Name, s.Name)) {
subr.go:		if f.Type.Etype != TFUNC || f.Type.Recv() == nil {
subr.go:	if m.Type.Etype != TFUNC || m.Type.Recv() == nil {

Should be a relatively straight forward CL if anyone wants to volunteer to take it.

@mdempsky mdempsky added the NeedsFix label Jan 22, 2019

@mdempsky mdempsky added this to the Go1.13 milestone Jan 22, 2019

@mdempsky mdempsky added the Suggested label Jan 22, 2019

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commented Jan 23, 2019

Change https://golang.org/cl/158938 mentions this issue: cmd/compile: confusing error if composite literal field is a method

@gopherbot gopherbot closed this in 6d781de Feb 26, 2019

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