internal/reflectlite: Implements erroneously reports that map types do not implement interfaces

[proglottis]

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

$ go version
go version go1.13 darwin/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=""
GOCACHE="/Users/james/Library/Caches/go-build"
GOENV="/Users/james/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/james/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.13/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.13/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/james/src/proglottis/ts/go.mod"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/nn/cm9pxx2x4pzcmc4_brdzc_9m0000gn/T/go-build569203332=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Tried to check if an error was of type schema.MultiError.

https://play.golang.org/p/eAMul1kgxXJ

What did you expect to see?

Preferably it would populate my errMulti := schema.MultiError{} variable so I can inspect the contents for better error handling.

What did you see instead?

A runtime error:

runtime: nameOff 0x74696177 out of range 0xf0000 - 0x161ba6
fatal error: runtime: name offset out of range

[cuonglm]

This is fixed in master, after https://go-review.googlesource.com/c/go/+/191198

[bcmills]

Closing based on @cuonglm's reply.

@proglottis, please let us know if this still isn't working for you using gotip.

[proglottis]

Thought I'd have a go with gotip, and it now panics. This might be fine if maps aren't supported, but the message is not correct as this type does implement error.

$ ./go version
go version devel +1804bba Tue Sep 24 18:20:52 2019 +0000 darwin/amd64

$ ./go run ./issue.go 
panic: errors: *target must be interface or implement error

goroutine 1 [running]:
errors.As(0x10f1ec0, 0xc0000ba040, 0x10b8200, 0xc0000c2000, 0xc000088f50)
	/Users/james/sdk/gotip/src/errors/wrap.go:87 +0x5cc
main.main()
	/Users/james/sdk/gotip/bin/issue.go:17 +0xb0
exit status 2

I wanted to exclude the possibility that there's something going on in the 3rd party library, and also add a compile time error type assertion. So here's a new script that panics as above on tip:

https://play.golang.org/p/ArfXTQ2AeFc

[bcmills]

CC @randall77 @martisch @mpvl

[cuonglm]

@bcmills should cc @neild too, this is errors.As implementation details, not the compiler.

[cuonglm]

Hmm, somehow, e.Implements(errorType) returns false for a map.

[bcmills]

Yes, this is a bug in internal/reflectlite. (It can't be demonstrated in the Playground because internal/reflectlite is internal, but the test in https://golang.org/cl/197179 clearly reproduces the bug.)