go/format: any input to format.Source panics

[FiloSottile]

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

$ go version
go version devel +91b7619310 Thu Oct 22 15:30:02 2020 +0000 darwin/amd64

Does this issue reproduce with the latest release?

It does not reproduce with Go 1.15.

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/valsorda/Library/Caches/go-build"
GOENV="/Users/valsorda/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/valsorda/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/valsorda"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/valsorda/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/valsorda/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
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/df/mrk3bfz149n8zb5h5p1vp_1m00hbbm/T/go-build525898641=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Pass any input to format.Source.

https://play.golang.org/p/kZcA7s59sam

What did you expect to see?

Formatted output.

What did you see instead?

panic: unreachable

goroutine 1 [running]:
go/printer.(*printer).expr1(0xc000167ab0, 0x29712dc0, 0xc0004f6100, 0x0, 0x1)
	/Users/valsorda/go/src/go/printer/nodes.go:973 +0x274e
go/printer.(*printer).expr(...)
	/Users/valsorda/go/src/go/printer/nodes.go:1067
go/printer.(*printer).printNode(0xc000167ab0, 0x12bb140, 0xc0004f6100, 0xc00037a170, 0x100fe78)
	/Users/valsorda/go/src/go/printer/printer.go:1128 +0x385
go/printer.(*Config).fprint(0xc000167d20, 0x1363640, 0xc0001ba3c0, 0xc0002e6000, 0x12bb140, 0xc0004f6100, 0xc0001ba3f0, 0xc0002e6000, 0x0)
	/Users/valsorda/go/src/go/printer/printer.go:1309 +0x170
go/printer.(*Config).Fprint(...)
	/Users/valsorda/go/src/go/printer/printer.go:1367
go/format.format(0xc0002e6000, 0xc0004f6100, 0x0, 0x0, 0xc000828000, 0x49d49, 0x92e01, 0x40000006, 0x8, 0x0, ...)
	/Users/valsorda/go/src/go/format/internal.go:105 +0x671
go/format.Source(0xc000828000, 0x49d49, 0x92e01, 0x1, 0x0, 0x0, 0x0, 0x1)
	/Users/valsorda/go/src/go/format/format.go:116 +0x127

go/src/go/printer/nodes.go

Line 973 in 1af388f

panic("unreachable")

This is reached because the type of expr is *ast.File, which is not handled by the type switch.

[ianlancetaylor]

I can't reproduce this.

[ianlancetaylor]

The type *ast.File does not implement the interface ast.Expr, so the code should never get to the point of the panic (and, for me, it doesn't).

[FiloSottile]

Indeed, neither does it for me after trying to bisect it. I suppose I was using a cmd/go out of sync with the Go tree and weird things were happening. I do wish there some level of safety against that. make.bash is fast but not so fast I run it on every git checkout, and I never know if I skipped over some change that will cause random compiler behaviors.