cd module
cp foo.go foo.go.orig

# Initially, the Go language version is too low.
gofumpt foo.go
cmp stdout foo.go.orig

# We can give an explicitly newer version.
gofumpt -lang=1.13 foo.go
cmp stdout foo.go.golden

# If we bump the version in go.mod, it should be picked up.
exec go mod edit -go=1.13
gofumpt foo.go
cmp stdout foo.go.golden

gofumpt -d foo.go.golden
! stdout .

# We can give an explicitly older version.
gofumpt -lang=v1 foo.go
cmp stdout foo.go.orig

-- module/go.mod --
module test

go 1.12
-- module/foo.go --
package p

const (
	i = 0
	j = 022
	k = 0o_7_5_5
	l = 1022
)
-- module/foo.go.golden --
package p

const (
	i = 0
	j = 0o22
	k = 0o_7_5_5
	l = 1022
)