generics: incorrect error message with range clause over type sets with multiple underlying types

[perillo]

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

$ go version
go version devel go1.18-59122f85bd Thu Jan 20 16:31:11 2022 +0000 linux/amd64

Does this issue reproduce with the latest release?

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

go env Output

$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN="/home/manlio/.local/bin"
GOCACHE="/home/manlio/.cache/go-build"
GOENV="/home/manlio/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE="*.local"
GOMODCACHE="/home/manlio/.local/lib/go/pkg/mod"
GONOPROXY=""
GONOSUMDB="*.local"
GOOS="linux"
GOPATH="/home/manlio/.local/lib/go:/home/manlio/src/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/manlio/sdk/gotip"
GOSUMDB="sum.golang.org"
GOTMPDIR="/tmp/gotmp"
GOTOOLDIR="/home/manlio/sdk/gotip/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="devel go1.18-59122f85bd Thu Jan 20 16:31:11 2022 +0000"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/manlio/src/go/src/test.localhost/type-params/proposal/go.mod"
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 -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/gotmp/go-build3690629167=/tmp/go-build -gno-record-gcc-switches"
GOROOT/bin/go version: go version devel go1.18-59122f85bd Thu Jan 20 16:31:11 2022 +0000 linux/amd64
GOROOT/bin/go tool compile -V: compile version devel go1.18-59122f85bd Thu Jan 20 16:31:11 2022 +0000
uname -sr: Linux 5.15.13-arch1-1
/usr/lib/libc.so.6: GNU C Library (GNU libc) release release version 2.33.
gdb --version: GNU gdb (GDB) 11.1

What did you do?

https://gotipplay.golang.org/p/muzbkNYCk31

What did you expect to see?

An error message reporting that range over a type with an union constraint with multiple elements is not allowed.

What did you see instead?

./prog.go:15:20: cannot range over it (variable of type S constrained by Iterable[E]) (S has no structural type)

P.S.:
The concept of multiple underlying types used in some CL is not defined in the Type Parameter Proposal and in The Go Programming Language Specification.

Thanks.

[ianlancetaylor]

Thanks. The spec has not yet been updated. The error message is correct for what we expect to implement in 1.18. Closing because there is nothing to change here.

[perillo]

Isn't the message S has no structural type incorrect?

This example: https://gotipplay.golang.org/p/dS-BdrZyZ8z
reports the same error:

prog.go:6:20: cannot range over it (variable of type S constrained by any) (S has no structural type)

with the difference that in this case S really has no structural type (assuming that I'm interpreting the term structural type correctly).

Thanks.

[ianlancetaylor]

In the original example the type of S is either []T or []int. That means that it doesn't have a structural type. A structural type is a single type, but here there are two. It's true that they are both slices, but they are slices of different element types.