Go version

go 1.22

Output of go env in your module/workspace:

GOROOT='/opt/homebrew/Cellar/go/1.22.0/libexec'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/opt/homebrew/Cellar/go/1.22.0/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.22.0'
GCCGO='gccgo'
AR='ar'
CC='cc'
CXX='c++'
CGO_ENABLED='1'
GOMOD='/dev/null'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/n7/l86gpr1x5vb55kqd_k4g1mq00000gq/T/go-build852953591=/tmp/go-build -gno-record-gcc-switches -fno-common'

What did you do?

https://go.dev/play/p/gFzyGuJGwGq

For the type with Stringer interface custom formatting works only if printing directly this class, and doesn't work if it is a struct member:

type Frac struct {
	a int
	b int
}

func (f Frac) String() string { return fmt.Sprintf("\"%d/%d\"", f.a, f.b) }

func main() {
        f := Frac{1, 3}
	fmt.Printf("Fraction is %v\n", f)

	st := struct {
		one Frac
		two Frac
	}{Frac{1, 3}, Frac{3, 5}}
	fmt.Printf("Struct is %v\n", st)
}

What did you see happen?

printing type as fmt.Printf("Fraction is %v\n", f) calls String() method:

Fraction is "1/3"

printing struct fmt.Printf("Struct is %v\n", st) does not:

Struct is {{1 3} {3 5}}

What did you expect to see?

libexec/src/fmt/print.go:pp.printArg checks for the Printf argument type and if it doesn't have custom formatting methods it goes to print it as value, while struct fields can have their own Stringer/GoStringer implementations