cmd/compile: fail to infer type for generic function

[xobotyi]

Go version

1.25

Output of go env in your module/workspace:

AR='ar'
CC='cc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='c++'
GCCGO='gccgo'
GO111MODULE='on'
GOARCH='arm64'
GOARM64='v8.0'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/Users/***/Library/Caches/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/Users/***/Library/Application Support/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -ffile-prefix-map=/var/folders/98/l40qltv14c17n0rm930h2p_h0000gp/T/go-build3566329617=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD='/Users/***/wrk/***/go.mod'
GOMODCACHE='/Users/***/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='darwin'
GOPATH='/Users/***/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/opt/homebrew/opt/go/libexec'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/***/Library/Application Support/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/opt/homebrew/opt/go/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.25.4'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

https://go.dev/play/p/hqNuW-6k49X

This playground explains and displays the issue. In case of usage of type with generics - there is compilation error, in case of usage of similar non-generic type - everything is fine.

What did you see happen?

Substitution error of obviously substitutable types

What did you expect to see?

Everything works =)

[xobotyi]

The only reason i could imagine here, is FilterPredicate[P] creating, during inference, a different type that is not directly compatible with func(P) bool which is.. well.. counterintuitive, at least, since those types are substitutable directly (as it is shown on lines 132 and 133).

[gabyhelp]

Related Issues

• go/types generics type inference issue? #66369 (closed)
• go/types, types2: generic type inference - generic interface input parameter of generic function #62612 (closed)
• cmd/compile: different results between inferred generic type argument and explicit generic type argument #61741 (closed)
• cmd/compile: internal compiler error when using generics #48412 (closed)
• x/tools/gopls: Completion of generic argument resulting in invalid casting to constraints #75469
• cmd/compile: generics cannot infer slice/array of slice/array types #68262 (closed)
• cmd/compile: constraint type inference for generic types is not implemented #54469 (closed)
• cmd/compile: does not match inferred type #60460 (closed)
• cmd/compile: internal compiler error: found illegal assignment main.Comp[.shape.[]string_0] -> main.Ordering[.shape.[]string_0]; : main.Comp[.shape.[]string_0] does not implement main.Ordering[.shape.[]string_0] #48889 (closed)
• cmd/compile: recursive generic interface instantiation error #65362 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[JunyangShao]

@golang/compiler

[griesemer]

Thanks for the report, @xobotyi . Inference related bugs are notoriously difficult to debug and it would really help if you could simplify your example even more, to the bare-bone minimum required to provoke the error message. This includes shortening names, remove unnecessary code, simplifying the program while retaining the error message. This is will be much easier for you to do because you understand the intent of the code. Thanks.

[griesemer]

This is working as intended: the problem is that your FilterPredicate (line 54 in your code) is a type definition, which defines a new type. That new type FilterPredicate does not match against func(T) bool and so inference fails. If you make that line an alias declaration, the code works (playground, see line 54).

You may wonder why it doesn't match: Note that the type parameter declaration FP FilterPredicate[P] is a shortcut for FP interface { FilterPredicate[P] }. The type set of FP thus consists of a single type, the generic named type FilterPredicate. On the other hand, FP func(P) bool is a shortcut for FP interface { func(P) bool }, this type set includes the single type func(P) bool. They (FilterPredicate[P] and func(P) bool) are different and hence don't match.

As an aside, for future reference, this is what I mean when I ask to simplify the code: remove all function bodies (replace with panic(0), that takes care of functions with results), remove all unnecessary types, remove unnecessary parameters, etc. while retaining the error. Rename identifiers and make them short. Remove comments. Get it all on one page. Once you're down to a dozen lines of code or so, it's much easier to see a problem and you may even find the answer yourself. Thanks.