cmd/compile: type parameter unexpectedly does not implement constraint

[silviucm]

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

$ go version
go version go1.19.3 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOVERSION="go1.19.3"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
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"

What did you do?

Run the issue reported at
https://groups.google.com/g/golang-nuts/c/_-vNSfDmPG8?pli=1
both on the playground
https://go.dev/play/p/GS_x9Y8HOMK
as well as locally with go run [temp go file]

@ianlancetaylor suggested in that Google group thread that there is merit to record this as a bug, to be further looked at.
It felt interesting so I recorded it, despite not being the original submitter from the group (apologies if duplicate).

What did you expect to see?

Potentially the program successfully build

What did you see instead?

command-line-arguments
./test.go:22:41: []T does not implement Scalar|Vector ([]T missing in int | float32 | float64 | []int | []float32 | []float64)
./test.go:24:19: []T does not implement Scalar|Vector ([]T missing in int | float32 | float64 | []int | []float32 | []float64)

[ianlancetaylor]

CC @griesemer @findleyr

[griesemer]

This is working as expected.

Here's a simpler case reporting the same error:

type Container[T []int] struct {
	Val T
}

func Create1[T []int](val T) Container[T] {
	return Container[T]{}
}

func Create2[T int](val T) Container[[]T] { // ERROR
	return Container[[]T]{} // ERROR
}

Create2 uses the type []T to instantiate a type parameter constrained by []int. Note that []T is not []int (even though T is constrained to be just int). When testing constraint satisfaction (which is interface implementation), we simply need to show that the type argument []T (a single type) is an element of the type set of interface{[]int}, which it is not. Hence the instantiation fails. (Note that this works.)