cmd/compile: internal compiler error: found illegal assignment FUNC-func() T₁ -> main.T₁;

[reusee]

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

$ go version
go version devel go1.18-df4c625d88 Thu Sep 2 01:06:17 2021 +0000 windows/amd64

Does this issue reproduce with the latest release?

No

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

go env Output

$ go env
set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\reus\AppData\Local\go-build
set GOENV=C:\Users\reus\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\reus\go\pkg\mod
set GONOPROXY=github.com/reusee/*
set GONOSUMDB=github.com/reusee/*
set GOOS=windows
set GOPATH=C:\Users\reus\go
set GOPRIVATE=github.com/reusee/*
set GOPROXY=https://goproxy.cn,direct
set GOROOT=C:\Users\reus\gotip
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=C:\Users\reus\gotip\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=devel go1.18-df4c625d88 Thu Sep 2 01:06:17 2021 +0000
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=NUL
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fmessage-length=0 -fdebug-prefix-map=C:\Users\reus\AppData\Local\Temp\go-build4127425866=/tmp/go-build -gno-record-gcc-switches
GOROOT/bin/go version: go version devel go1.18-df4c625d88 Thu Sep 2 01:06:17 2021 +0000 windows/amd64
GOROOT/bin/go tool compile -V: compile version devel go1.18-df4c625d88 Thu Sep 2 01:06:17 2021 +0000

What did you do?

package main

func Foo[
        T interface {
                ~func() T
        },
]() T {
        var t T
        t = func() T {
                return t
        }
        return t
}

func main() {
  type Bar func() Bar
  Foo[Bar]()
}

What did you expect to see?

Compile OK

What did you see instead?

# command-line-arguments
.\a.go:9:2: internal compiler error: found illegal assignment FUNC-func() T₁ -> main.T₁;

goroutine 1 [running]:
runtime/debug.Stack()
        C:/Users/reus/gotip/src/runtime/debug/stack.go:24 +0x65
cmd/compile/internal/base.FatalfAt({0x0, 0x0}, {0x144f237, 0x27}, {0xc0000cd250, 0x3, 0x3})
        C:/Users/reus/gotip/src/cmd/compile/internal/base/print.go:227 +0x154
cmd/compile/internal/base.Fatalf(...)
        C:/Users/reus/gotip/src/cmd/compile/internal/base/print.go:196
cmd/compile/internal/noder.assignconvfn({0x15adc90, 0xc00007a5f0}, 0xc000404620)
        C:/Users/reus/gotip/src/cmd/compile/internal/noder/transform.go:413 +0x165
cmd/compile/internal/noder.transformAssign({0x15ad650, 0xc00007a730}, {0xc0000cd538, 0x1, 0x38}, {0xc0000cd528, 0x1, 0x1b3b503352f})
        C:/Users/reus/gotip/src/cmd/compile/internal/noder/transform.go:363 +0x4b3
cmd/compile/internal/noder.(*irgen).stmt(0xc000410000, {0x15991b8, 0xc0000761c0})
        C:/Users/reus/gotip/src/cmd/compile/internal/noder/stmt.go:109 +0xfbe
cmd/compile/internal/noder.(*irgen).stmts(0xc0004049a0, {0xc000076200, 0x3, 0xc0000061d0})
        C:/Users/reus/gotip/src/cmd/compile/internal/noder/stmt.go:19 +0xaf
cmd/compile/internal/noder.(*irgen).funcBody(0xc000410000, 0xc00007e2c0, 0x12c8bfd, 0xc0000760c0, 0xc000076100)
        C:/Users/reus/gotip/src/cmd/compile/internal/noder/func.go:45 +0x25f
cmd/compile/internal/noder.(*irgen).funcDecl.func1()
        C:/Users/reus/gotip/src/cmd/compile/internal/noder/decl.go:128 +0x68
cmd/compile/internal/noder.(*irgen).generate(0xc000410000, {0xc000088b50, 0x2, 0x1b197c0})
        C:/Users/reus/gotip/src/cmd/compile/internal/noder/irgen.go:265 +0x1df
cmd/compile/internal/noder.check2({0xc000088b50, 0x2, 0x2})
        C:/Users/reus/gotip/src/cmd/compile/internal/noder/irgen.go:92 +0x175
cmd/compile/internal/noder.LoadPackage({0xc0000de100, 0x2, 0x0})
        C:/Users/reus/gotip/src/cmd/compile/internal/noder/noder.go:90 +0x335
cmd/compile/internal/gc.Main(0x1462ac8)
        C:/Users/reus/gotip/src/cmd/compile/internal/gc/main.go:190 +0xaf3
main.main()
        C:/Users/reus/gotip/src/cmd/compile/main.go:55 +0xdd

Compile OK in unified mode.

[danscales]

A slightly different form, that may be more readable:

package main

type Constraint[T any] interface {
	~func() T
}

func Foo[T Constraint[T]]() T {
	var t T

	t = func() T {
		return t
	}
	return t
}

func main() {
	type Bar func() Bar
	Foo[Bar]()
}

[gopherbot]

Change https://golang.org/cl/347915 mentions this issue: cmd/compile: delay transformAssign if lhs/rhs have typeparam