go/types, cmd/compile/internal/types2: type parameter inference fails for constraints that succeed individually

[firelizzard18]

Go version

go version go1.22.1 linux/amd64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/firelizzard/.cache/go-build'
GOENV='/home/firelizzard/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/firelizzard/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/firelizzard/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/lib/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='local'
GOTOOLDIR='/usr/lib/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.22.1'
GCCGO='/usr/bin/gccgo'
GOAMD64='v1'
AR='ar'
CC='x86_64-pc-linux-gnu-gcc'
CXX='x86_64-pc-linux-gnu-g++'
CGO_ENABLED='1'
GOMOD='/home/firelizzard/src/yaegi/go-script/go.mod'
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 -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build3476193043=/tmp/go-build -gno-record-gcc-switches'

What did you do?

https://go.dev/play/p/fVSkvTEgYx_1

package main

type ptrTo[A any] interface{ ~*A }
type hasFoo[A any] interface{ foo(A) }
type both[A, B any] interface {
	ptrTo[A]
	hasFoo[B]
}

type fooer[A any] struct{}

func (f *fooer[A]) foo(A) {}

func withPtr[A ptrTo[B], B any]()       {}
func withFoo[A hasFoo[B], B any]()      {}
func withBoth[A both[B, C], B, C any]() {}

func main() {
	withPtr[*fooer[int]]()  // Ok
	withFoo[*fooer[int]]()  // Ok
	withBoth[*fooer[int]]() // Error: cannot infer C
}

What did you see happen?

Inference succeeds on all three calls.

What did you expect to see?

Inference succeeds on the first two but fails on the third. If inference of B for withPtr or withFoo failed, it would make sense. But inference succeeds on both of those so it's surprising and confusing that it fails for withBoth.

[findleyr]

CC @griesemer

[Garciat]

I ran into an inference issue that is very similar to OP's issue:

type X struct {}
func (x X) M() int { return 42 }

func CallM1[T interface{M() R}, R any](t T) R {
	return t.M()
}

func CallM2[T interface{X; M() R}, R any](t T) R {
	return t.M()
}

func main() {
	CallM1(X{}) // OK
	CallM2(X{}) // cannot infer R
}

I wasn't expecting CallM2 to fail inference on R because of the added T=X constraint.

[griesemer]

Simplified reproducer for original example:

package p

type S struct{}

func (*S) m(int) {}

func f[A interface {
	~*B
	m(C)
}, B, C any]() {
}

var _ = f[*S] // error: cannot infer C

[gopherbot]

Change https://go.dev/cl/617896 mentions this issue: go/types, types2: always try inference over methods when possible