Generics Type Inference

[carmichaeljr]

Go version

go version go1.23.3 linux/amd64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/jack/.cache/go-build'
GOENV='/home/jack/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/jack/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/jack/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.23.3'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/jack/.config/go/telemetry'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
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 -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build2450416570=/tmp/go-build -gno-record-gcc-switches'

What did you do?

I have made the below example code:

// You can edit this code!
// Click here and start typing.
package main

type (
	IFace1[T any]            interface{ Method1(v T) bool }
	Impl[I IFace1[T], T any] struct{}

	Thingy[T any] struct{}
	Thingy2       struct{}
)

func (t Thingy[T]) Method1(v T) bool    { return false }
func (t Thingy2) Method1(v string) bool { return false }

func Func1[I IFace1[T], T any](v I)         {}
func Func2[I IFace1[T], T any](v IFace1[T]) {}

func main() {
	var tmp Impl[Thingy[int]]
	var tmp2 Impl[Thingy2]
	_ = tmp
	_ = tmp2

	Func1[Thingy[int]](Thingy[int]{})
	// Func1[Thingy[int]](Thingy2{}) // Error as expected
	Func1[Thingy2](Thingy2{})

	Func2[Thingy[int]](Thingy[int]{})
	// Func2[Thingy[int]](Thingy2{}) // Error as expected
	Func2[Thingy2](Thingy2{})
}

Go playground link: https://go.dev/play/p/0v8JtrzGqQn

What did you see happen?

A compiler error:

./prog.go:17:10: not enough type arguments for type Impl: have 1, want 2
./prog.go:18:11: not enough type arguments for type Impl: have 1, want 2

What did you expect to see?

I expected the second type parameter (T) to be able to be inferred for two reasons:

• First reason: Method1 uses the T generic parameter in its signature. If an incorrect second type parameter is supplied the compiler complains about it, so it knows about this relation between the method signature and type parameter. For example, var tmp4 Impl[Thingy[int], string] results in:

./prog.go:21:16: Thingy[int] does not satisfy IFace1[string] (wrong type for method Method1)
		have Method1(int) bool
		want Method1(string) bool

Looking at the above example it appears like the user supplied second type parameter takes precedence over any value supplied in the first type parameter as the error expects the type that the user supplied in the second parameter. But if the second argument is not supplied, would it be safe to infer the type? If it is not an explanation as to why the type cannot be inferred or a counter example would be appreciated.

• Second reason: Most surprisingly, functions do not have this limitation. The type inference works as I would expect on Func1 and Func2 in the example.

[gabyhelp]

Related Issues

• spec: interfaces that have different-size method sets never unify (but they possibly could) #57192 (closed)
• cmd/go2go: can't call function with generic type that implements generic interface #40018 (closed)
• Generic type does not match when the type should match #68210 (closed)
• cmd/compile (types2, go/types): cannot infer generic interface types #41176 (closed)
• spec: function type inference ignores type parameter constraints #50272 (closed)
• cmd/compile: Compiler Type Inferencing on Methods With Narrowing Types #60975 (closed)
• proposal: spec: Allow interfaces to be inferrable generic types #69822 (closed)
• go/types, types2: type inference fails if inferred type is generic function #59953 (closed)
• cmd/compile: invalidly inferred type for interface that doesn't use type parameter #60377 (closed)
• go/types, cmd/compile/internal/types2: type parameter inference fails for constraints that succeed individually #66751 (closed)

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[seankhliao]

https://go.dev/ref/spec#Type_inference

inference is only for functions