cmd/compile: better error message for ambiguous parse

[smu-ggl]

Go version

go version 1.23

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/smu/.cache/go-build'
GOENV='/home/smu/.config/go/env'
GOEXE=''
GOEXPERIMENT='fieldtrack,boringcrypto'
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/smu/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/smu/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/lib/google-golang'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/lib/google-golang/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.23 [redacted] X:fieldtrack,boringcrypto'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/smu/.config/go/telemetry'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='clang'
CXX='clang++'
CGO_ENABLED='1'
GOMOD='/dev/null'
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 -fno-caret-diagnostics -Qunused-arguments -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1689118444=/tmp/go-build -gno-record-gcc-switches'

What did you do?

I expected

if mErr := &MyError{}; errors.As(err1, &mErr) {
...
}

to be parse correctly. See https://go.dev/play/p/XHkampOkowJ for a reproduction case.

Actual result of that playground snippet was:

./prog.go:23:10: syntax error: cannot use mErr := &MyError as value
./prog.go:26:1: syntax error: non-declaration statement outside function body

Changing the line

if mErr := &MyError{}; errors.As(err1, &mErr) {

into

if mErr := &(MyError{}); errors.As(err1, &mErr) {

or into

mErr := &MyError{}
if errors.As(err1, &mErr) {

makes the code work, but I would expect the first variant to work just as well.

What did you see happen?

./prog.go:23:10: syntax error: cannot use mErr := &MyError as value
./prog.go:26:1: syntax error: non-declaration statement outside function body

What did you expect to see?

A compiled program

[gabyhelp]

Related Issues

• builtin/error: late assignment of custom error type cause nil checking of the error variable not working #53768 (closed)
• everything: fail to check err == nil when assigning explicit type to `error` #56930 (closed)
• x/tools/gopls: Warn UnusedVar for unused assign #56598 (closed)
• Compiler reports an error when trying to assign values from expression that includes function call #66529 (closed)
• Nil-ness of a pointer is lost when converted to an interface #42306 (closed)
• Can not compile this: if err := T{}.F(); err != nil{} #37043 (closed)
• Function nesting calls a function that returns a pointer that implements the error, the returned error is always not equal to nil #43767 (closed)
• Compile error: if-statement with assignment #35932 (closed)
• cmd/compile: named return values produce functionally different code #21049 (closed)
• errors: As can be confusing to use when given a pointer to a type that does not implement error, but a pointer to that type does #37625 (closed)

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[schachmat]

Probably the same as #37043 (comment) ?

[smu-ggl]

This is well known; see https://golang.org/ref/spec#Composite_literals:

A parsing ambiguity arises when a composite literal using the TypeName form of the LiteralType appears as an operand between the keyword and the opening brace of the block of an "if", "for", or "switch" statement, and the composite literal is not enclosed in parentheses, square brackets, or curly braces. In this rare case, the opening brace of the literal is erroneously parsed as the one introducing the block of statements. To resolve the ambiguity, the composite literal must appear within parentheses.

[smu-ggl]

closing this

[ianlancetaylor]

Reopening because I think we can produce a better error message.

package p

type S struct {
	F int
}

func F() {
	if s := S{}; s.F != 0 {
	}
}

The gc compiler says:

foo.go:8:7: syntax error: cannot use s := S as value
foo.go:10:1: syntax error: non-declaration statement outside function body

The gccgo compiler says

foo.go:8:17: error: parentheses required around this composite literal to avoid parsing ambiguity
    8 |         if s := S{}; s.F != 0 {
      |                 ^

CC @griesemer