cmd/vet: printfunc false negative on struct without String method

[josharian]

package main

import "fmt"

type T int

func (T) String() string { return "T" }

type S struct {
	t T
}

func main() {
	var t T
	fmt.Printf("%s\n", t)
	var s S
	fmt.Printf("%s\n", s)
}

$ go tool vet x.go
$ go run x.go 
T
{%!s(main.T=0)}

vet should know that S isn't a fmt.Stringer. The printfunc's "satisfies interface" check needs improvements. Related is #17057.

cc @robpike @valyala

[valyala]

I'll look into this and the #17057 on the weekend.

[valyala]

FYI, it looks like the cl 28959 breaks printf check on errror interfaces:

err := errors.New("foobar")
fmt.Printf("%d", err) // vet silently skips this

[valyala]

I think it would be better to fix these issues in go 1.8.

[josharian]

CL 28959 doesn't change cmd/vet at all. It only changes cmd/vet/all, which is a script that runs cmd/vet over the standard library.

I think it would be better to fix these issues in go 1.8.

If the changes are fairly safe and we can get them in soon, I think that's fine.

[josharian]

FYI, it looks like the cl 28959 breaks printf check on errror interfaces:

Btw, this is intentional. See #16314.

[valyala]

This looks like a bug in package fmt - it must properly print all the struct fields if they implement fmt.Stringer according to the rule:

For compound operands such as slices and structs, the format applies to the elements of each operand, recursively, not to the operand as a whole.

In this case struct S contains only a single field of type T, which implements Stringer. So fmt.Printf must print contents of the struct - {T} instead of printing {%!s(main.T=0)}.

fmt.Printf works as expected for slices, arrays and maps with fmt.Stringer values, but doesn't work for structs:

type S [5]T
type S []T
type S map[string]T

[valyala]

cc'ing @ALTree , @martisch and @robpike

[martisch]

type S struct {
	t T
}

the field t is not exported. If t is changed to be exported fmt will print {T}. fmt is in another package and does not have access to call the String method of t if it is not exported.

see also #18281 #17409 #16698

[gopherbot]

CL https://golang.org/cl/44831 mentions this issue.

[mvdan]

CL was marked as R=go1.10, so moving the issue milestone too.