Golang Creating local variable via {} in if statement breaks Go.

Go version

1.24.4

Output of go env in your module/workspace:

AR='ar'
CC='clang'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='clang++'
GCCGO='gccgo'
GO111MODULE=''
GOARCH='arm64'
GOARM64='v8.0'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/Users/felipe/Library/Caches/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/Users/felipe/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/fr/61jb9pj52bs_r6q_5dlc4bxw0000gp/T/go-build3339825284=/tmp/go-build -gno-record-gcc-switches -fno-common'
GOHOSTARCH='arm64'
GOHOSTOS='darwin'
GOINSECURE=''
GOMOD='/Users/felipe/go/src/github.com/10gen/custom-mongomirror/go.mod'
GOMODCACHE='/Users/felipe/go/pkg/mod'
GONOPROXY='github.com/10gen/'
GONOSUMDB='github.com/10gen/'
GOOS='darwin'
GOPATH='/Users/felipe/go'
GOPRIVATE='github.com/10gen/'
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/Users/felipe/Library/Application Support/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go/pkg/tool/darwin_arm64'
GOVCS=''
GOVERSION='go1.24.4'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

Try to compile this:

package main

import (
    "errors"
    "fmt"
)

type MyError struct{}

func (m MyError) Error() string {
    return ""
}

var _ error = MyError{}

func makeError() error {
    return nil
}

func main() {
    err := fmt.Errorf("")

    if my := MyError{}; errors.As(err, &my) {
        fmt.Printf("match")
    }
}

What did you see happen?

error: expected boolean expression, found assignment (missing parentheses around composite literal?)

What did you expect to see?

It should compile & run.

It seems the braces might be confusing something. If I replace MyError{} with *new(MyError) then it works.

Comment From: FGasper

For posterity: it seems this is just a limitation of Go’s parser. (It’d be sweet, though, if a more helpful error message could be given.)

Comment From: ianlancetaylor

Search for "parsing ambiguity" in https://go.dev/ref/spec#Composite_literals.

The error message tells you exactly how to fix the problem. Do you have any suggestions for how to improve it? Thanks.