runtime: Generic Type parameter loses context/decays

[vivere-dally]

Go version

go version go1.24.2 darwin/arm64

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=''
GOARCH='arm64'
GOARM64='v8.0'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/Users/s-ved/Library/Caches/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/Users/s-ved/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/25/n9yj6g1n2wqbvrk2__2sv8840000gr/T/go-build1621757110=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD='/Users/s-ved/repos/tests/go_just_main/go.mod'
GOMODCACHE='/Users/s-ved/go/pkg/mod'
GOOS='darwin'
GOPATH='/Users/s-ved/go'
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/Users/s-ved/homebrew/Cellar/go/1.24.2/libexec'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/s-ved/Library/Application Support/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/Users/s-ved/homebrew/Cellar/go/1.24.2/libexec/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.24.2'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

type Printable interface {
	Print()
}

type Printers struct {
	printers []*PrinterWithInk[Printable]
}

type PrinterWithInk[T Printable] struct {
	printer T
	ink     uint
}

func NewPrinterWithInk[T Printable](printer T, ink uint) *PrinterWithInk[T] {
	return &PrinterWithInk[T]{printer: printer, ink: ink}
}

func NewPrinterWithInkTracked[T Printable](ps Printers, printer T, ink uint) *PrinterWithInk[T] {
	p := NewPrinterWithInk[T](printer, ink)

	ps.printers = append(ps.printers, p)

	return p
}

What did you see happen?

Error at line ps.printers = append(ps.printers, p):

1. cannot use p (variable of type *PrinterWithInk[T]) as *PrinterWithInk[Printable] value in argument to append [IncompatibleAssign]

What did you expect to see?

No error, T is constrained to be Printable in the function declaration.

[seankhliao]

I believe this is working as intended.

[]*PrinterWithInk[Printable] is a slice where the struct field printer T is an interface value.
NewPrinterWithInk[T] creates a struct where the field is type that implements the interface, but the field isn't the interface value.

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[gabyhelp]

Related Issues

• runtime: function-scoped generic types with the same name and parameters are treated as the same in interface, causing segfault #62134 (closed)
• Cannot use pointer to generic type as interface without explicit conversion #52507 (closed)
• Generics: Cannot pass instance of type param to function with interface parameter #50487 (closed)
• cmd/compile: generic function argument causes escape to heap #48849
• x/tools/gopls: completion gives wrong lambda suggestion for pointer receivers of a generic type #61189 (closed)
• cmd/compile: internal compiler error: want STRUCT, but have PTR #48056 (closed)
• cmd/compile: invalid typeparameters code can be compiled #49059 (closed)

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