cmd/compile: found illegal assignment FUNC-func(int) error -> main.Fn[int];

[gus]

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

$ go version
go version go1.18beta1 linux/amd64

Does this issue reproduce with the latest release?

Yes, assuming the latest version is gotip.

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/dev/.cache/go-build"
GOENV="/home/dev/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/dev/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/dev/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/dev/sdk/go1.18beta1"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/dev/sdk/go1.18beta1/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.18beta1"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/workspace/dev/go.mod"
GOWORK=""
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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build929647144=/tmp/go-build -gno-record-gcc-switches"

What did you do?

This came up as I was trying to write some table driven unit tests for some functions that allow for generics. I've reduced the problem to this:

https://gotipplay.golang.org/p/0FYmUq0bzvr

The issue appears only if the a type is declared with the same name within multiple functions of a package file, and there is a field on the types with the same name, but different signature.

If I rename one of the types in the caller* functions, I do not see the issue.

What did you expect to see?

Compiled code.

What did you see instead?

# command-line-arguments
./issue.go:22:14: internal compiler error: found illegal assignment FUNC-func(int) error -> main.Fn[int]; 

Please file a bug report including a short program that triggers the error.
https://golang.org/issue/new

[ianlancetaylor]

Thanks. Can you please include a short program that triggers the error?

[gus]

Thanks. Can you please include a short program that triggers the error?

Sorry, accidentally hit tab+enter prematurely. Issue updated with more details.

[ianlancetaylor]

CC @danscales @randall77

(Compilation succeeds with GOEXPERIMENT=unified)

foo.go:22:14: internal compiler error: found illegal assignment FUNC-func(int) error -> main.Fn[int]; 

goroutine 1 [running]:
runtime/debug.Stack()
	/home/iant/go/src/runtime/debug/stack.go:24 +0x65
cmd/compile/internal/base.FatalfAt({0x0?, 0x0?}, {0xd36a84, 0x27}, {0xc000436c30, 0x3, 0x3})
	/home/iant/go/src/cmd/compile/internal/base/print.go:227 +0x1ca
cmd/compile/internal/base.Fatalf(...)
	/home/iant/go/src/cmd/compile/internal/base/print.go:196
cmd/compile/internal/noder.assignconvfn({0xe91250, 0xc0004277c0}, 0xc000439ab0)
	/home/iant/go/src/cmd/compile/internal/noder/transform.go:426 +0x1b2
cmd/compile/internal/noder.transformCompLit(0xc00013b300)
	/home/iant/go/src/cmd/compile/internal/noder/transform.go:980 +0x9f1
cmd/compile/internal/noder.(*irgen).compLit(0xc000152240, {0xe8d0b0, 0xc00013af80}, 0xc000407c70)
	/home/iant/go/src/cmd/compile/internal/noder/expr.go:372 +0x276
cmd/compile/internal/noder.(*irgen).expr0(0xc000152240, {0xe8d0b0, 0xc00013af80}, {0xe8e008?, 0xc000407c70?})
	/home/iant/go/src/cmd/compile/internal/noder/expr.go:107 +0x585
cmd/compile/internal/noder.(*irgen).expr(0xc000152240, {0xe8e008?, 0xc000407c70?})
	/home/iant/go/src/cmd/compile/internal/noder/expr.go:81 +0x5e8
cmd/compile/internal/noder.(*irgen).exprs(0xc000437360?, {0xc000437350, 0x1, 0xc00006cc30?})
	/home/iant/go/src/cmd/compile/internal/noder/expr.go:329 +0x8e
cmd/compile/internal/noder.(*irgen).exprList(0x0?, {0xe8e008?, 0xc000407c70?})
	/home/iant/go/src/cmd/compile/internal/noder/expr.go:312 +0x85
cmd/compile/internal/noder.(*irgen).stmt(0xc000152240, {0xe8de88?, 0xc000035c40?})
	/home/iant/go/src/cmd/compile/internal/noder/stmt.go:79 +0x418
cmd/compile/internal/noder.(*irgen).stmts(0xc000438a80?, {0xc000035cc0?, 0x3, 0xc00006d4a0?})
	/home/iant/go/src/cmd/compile/internal/noder/stmt.go:19 +0xaf
cmd/compile/internal/noder.(*irgen).funcBody(0xc000152240, 0xc000413080, 0xb4d7a0?, 0xc000035ac0, 0xc000035b00)
	/home/iant/go/src/cmd/compile/internal/noder/func.go:45 +0x25f
cmd/compile/internal/noder.(*irgen).funcDecl.func1()
	/home/iant/go/src/cmd/compile/internal/noder/decl.go:145 +0xf5
cmd/compile/internal/noder.(*irgen).generate(0xc000152240, {0xc00000e430, 0x1, 0x203000?})
	/home/iant/go/src/cmd/compile/internal/noder/irgen.go:280 +0x227
cmd/compile/internal/noder.check2({0xc00000e430, 0x1, 0x1})
	/home/iant/go/src/cmd/compile/internal/noder/irgen.go:92 +0x16d
cmd/compile/internal/noder.LoadPackage({0xc00001e1f0, 0x1, 0x0?})
	/home/iant/go/src/cmd/compile/internal/noder/noder.go:90 +0x335
cmd/compile/internal/gc.Main(0xd4a7e8)
	/home/iant/go/src/cmd/compile/internal/gc/main.go:191 +0xb13
main.main()
	/home/iant/go/src/cmd/compile/main.go:55 +0xdd

[randall77]

I don't think this will work currently. We don't allow local type declarations (or functions for that matter) with generic parameters.
Or at least, we shouldn't. Definitely needs a better error message.

Somewhat related to #47631 except it's a generic type declaration in a non-generic function, instead of a non-generic type declaration in a generic function.

[gus]

Thanks, @randall77 . More than happy to be told not to do this as I didn't really like the way the test was looking anyway; but gopls wasn't complaining, so I forged ahead. I will add that if the signature for fn in both X types is the same, then the code compiles (and works if you add a main function to the example).

[randall77]

@gri @ianlancetaylor @findleyr @danscales

I guess we need to decide whether we support local declarations of generic types in non-generic functions. Maybe we could? I'm not sure if this doesn't work because we never tried it, or because it's not supposed to work, or because it should work but doesn't. Opinions welcome.

I have a CL ready to officially make this an error like for #47631, but that's only if we decide it is the right thing to do.

[gopherbot]

Change https://golang.org/cl/372154 mentions this issue: cmd/compile: don't allow generic type declarations inside functions

[danscales]

It looks like we are confusing the two local named types X incorrectly. If you rename the second local type to Y, things work fine. We don't have the same confusion for non-generic local types, so must be some generic-only state. I will take a further look.

However, I would say, for this release, it is probably less confusing to the user and reduces the number of possible unusual cases if we disallow local generic type declarations in non-generic functions.

[ianlancetaylor]

I think that eventually we should support local declarations of generic types in non-generic functions, but I think it's fine if we don't support that in 1.18.

[gopherbot]

Change https://golang.org/cl/372654 mentions this issue: cmd/compile: pop instantiations of local types when leaving scope