spec: interfaces that have different-size method sets never unify (but they possibly could)

[testinginprod]

What version of Go are you using (go version)?

$ go version
go version go1.19.3 darwin/arm64

Does this issue reproduce with the latest release?

Yes, tested on playground.

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="arm64"
GOBIN=""
GOCACHE="/Users/*/Library/Caches/go-build"
GOENV="/Users/*/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/*/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/*/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/opt/homebrew/Cellar/go/1.19.3/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/opt/homebrew/Cellar/go/1.19.3/libexec/pkg/tool/darwin_arm64"
GOVCS=""
GOVERSION="go1.19.3"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/cl/x9ccznh92fbc0_57g_7nk2gc0000gn/T/go-build3394357358=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Given GenericInterface1[T any], GenericInterface2[T any] { GenericInterface1[T] }, and a function func F[T any](GI1 GenericInterface1[T](), T is not inferred if we pass to F a GenericInterface2.

https://go.dev/play/p/dsZOHq2Y1Au

What did you expect to see?

Type inference to work since GenericInterface2 is also GenericInterface1, and since passing to F a declared variable of type GenericInterface1[T] works (if it's not explicitly declared, then type inference cannot infer T even for GenericInterface1).

What did you see instead?

Compilation failure.

[ianlancetaylor]

This is expected behavior for the current language spec. Changing this to be a language issue.

[ianlancetaylor]

I would not be surprised if fixing #41176 fixes this one as well.

[griesemer]

There are two issues here:

1. Two interface types, even with the same type set (which includes methods), do not unify if they are defined types with different names. We probably should ignore the names of interface types.

2. Two interface types which don't have identical type sets do not unify. But if one interface's typeset is a superset of the other (e.g., the method set of one interface is a subset of the method set of the other), they probably should unify. More generally, if the intersection of the type sets of two interfaces unifies, they probably should unify.

If we do 1) and 2), this example will work.

Or putting it differently, two interfaces should unify if one of them can be made (through unification) to implement the other.

[gopherbot]

Change https://go.dev/cl/496256 mentions this issue: go/types, types2: more flexible type unification of interface types