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

$ go version
go version go1.20.6 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN="/home/maxb/.local/bin"
GOCACHE="/home/maxb/.cache/go-build"
GOENV="/home/maxb/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/maxb/.cache/go-modules"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/maxb/.cache/go-path"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/opt/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/opt/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.20.6"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/maxb/code/exp/go.mod"
GOWORK=""
CGO_CFLAGS="-O2 -g"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-O2 -g"
CGO_FFLAGS="-O2 -g"
CGO_LDFLAGS="-O2 -g"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build3661324250=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I observed bizarre output from the x/exp/cmd/apidiff CLI. I investigated and produced a test reproducing the behaviour. Append this additional test case to https://github.com/golang/exp/blob/master/apidiff/testdata/tests.go and run the tests :

//////////////// A discovered bug

// old
func BugWithFuncParamFromNamedTypeToBasicType(closer io.ReadCloser) {}

// new
// i BugWithFuncParamFromNamedTypeToBasicType: changed from func(io.ReadCloser) to func(int)
func BugWithFuncParamFromNamedTypeToBasicType(integer int) {}

// both
func UnchangedFuncTakingSameNamedInterface(closer io.ReadCloser) {}

What did you expect to see?

You would reasonably expect the tests to pass, as the expected new message about the incompatible change was added to the test data (the // i line).

What did you see instead?

Not only does the expected message about the incompatibility not get generated, but two bizarre and untrue incompatibilities are now reported:

ReadCloser: changed from interface{io.Reader; io.Closer} to int

apidiff is claiming that the type of the stdlib io.ReadCloser has changed to int!

UnchangedFuncTakingSameNamedInterface: changed from func(io.ReadCloser) to func(io.ReadCloser)

and that this function has changed, despite it not changing!

Investigation reveals this defective code:

https://github.com/golang/exp/blob/613f0c0eb8a17a98ecdb096a7f9f7d5053c1c963/apidiff/correspondence.go#L159-L210

If you look carefully, you'll see that if this function is called with the new input not being a *types.Named, it will falsely assume correspondence with no further checks at all - and here it is being called with a *types.Basic.