cmd/compile: internal compiler error: found illegal assignment PTR-*Foo.T -> bang.Foo.U;

[mcy]

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

compile version devel go1.18-088bb4b Tue Nov 2 06:25:39 2021 +0000

Does this issue reproduce with the latest release?

This issue is only reproducible at head.

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

GOHOSTARCH="amd64"
GOHOSTOS="linux"

What did you do?

package bang

func Foo[T any, U interface{~*T}](x T) {
  var _ U = &x
}

https://godbolt.org/z/GocK9493c

What did you expect to see?

Some kind of compilation error specifying that a conversion was not permitted.

What did you see instead?

ICE:

./example.go:4:14: internal compiler error: found illegal assignment PTR-*Foo.T -> bang.Foo.U; 

goroutine 1 [running]:
runtime/debug.Stack()
	/root/master/go/src/runtime/debug/stack.go:24 +0x65
cmd/compile/internal/base.FatalfAt({0x0?, 0x0?}, {0xd262b1, 0x27}, {0xc0000c4fb8, 0x3, 0x3})
	/root/master/go/src/cmd/compile/internal/base/print.go:227 +0x1ca
cmd/compile/internal/base.Fatalf(...)
	/root/master/go/src/cmd/compile/internal/base/print.go:196
cmd/compile/internal/noder.assignconvfn({0xe7d130, 0xc0003be190}, 0xc000396af0)
	/root/master/go/src/cmd/compile/internal/noder/transform.go:423 +0x1b2
cmd/compile/internal/noder.transformAssign({0xe7d450?, 0xc0003be1e0?}, {0xc0000c5200?, 0x1, 0x8?}, {0xc0000c51f0, 0x1, 0xc0000c51f0?})
	/root/master/go/src/cmd/compile/internal/noder/transform.go:369 +0x4b3
cmd/compile/internal/noder.(*irgen).varDecl.func1()
	/root/master/go/src/cmd/compile/internal/noder/decl.go:284 +0x5c5
cmd/compile/internal/noder.(*irgen).varDecl(0xc0000d4300, 0xc0000b3ae8, 0xc000087b60)
	/root/master/go/src/cmd/compile/internal/noder/decl.go:304 +0x331
cmd/compile/internal/noder.(*irgen).decls(0x1?, 0x0?, {0xc000080bc0?, 0x1, 0x0?})
	/root/master/go/src/cmd/compile/internal/noder/decl.go:34 +0xfd
cmd/compile/internal/noder.(*irgen).stmt(0xc0000d4300, {0xe7a960?, 0xc000385080?})
	/root/master/go/src/cmd/compile/internal/noder/stmt.go:50 +0x136
cmd/compile/internal/noder.(*irgen).stmts(0xc000396cb0?, {0xc000080bd0?, 0x1, 0x0?})
	/root/master/go/src/cmd/compile/internal/noder/stmt.go:19 +0xaf
cmd/compile/internal/noder.(*irgen).funcBody(0xc0000d4300, 0xc0003bcdc0, 0xb45c80?, 0xc0000b3780, 0xc0000b37c0)
	/root/master/go/src/cmd/compile/internal/noder/func.go:45 +0x25f
cmd/compile/internal/noder.(*irgen).funcDecl.func1()
	/root/master/go/src/cmd/compile/internal/noder/decl.go:143 +0xf5
cmd/compile/internal/noder.(*irgen).generate(0xc0000d4300, {0xc0000b0458, 0x1, 0x203000?})
	/root/master/go/src/cmd/compile/internal/noder/irgen.go:280 +0x227
cmd/compile/internal/noder.check2({0xc0000b0458, 0x1, 0x1})
	/root/master/go/src/cmd/compile/internal/noder/irgen.go:92 +0x16d
cmd/compile/internal/noder.LoadPackage({0xc0000b4100, 0x1, 0x0?})
	/root/master/go/src/cmd/compile/internal/noder/noder.go:90 +0x335
cmd/compile/internal/gc.Main(0xd3a138)
	/root/master/go/src/cmd/compile/internal/gc/main.go:191 +0xb13
main.main()
	/root/master/go/src/cmd/compile/main.go:55 +0xdd

Compiler returned: 2

[cherrymui]

cc @griesemer @danscales @mdempsky

[griesemer]

Simpler reproducer:

func _[P ~*int](x int) {
	var _ P = &x
}

This works:

func _[P ~*int](x int) {
	var _ P = P(&x)
}

[mdempsky]

FWIW, the test cases work with GOEXPERIMENT=unified, so I don't think it's a types2 issue.

[griesemer]

Agreed. They type-check fine and I believe the examples are valid.

[danscales]

This is related to the structural type constraint. It is why we need to do all the transformations on instantiations, rather than generic functions, so the structural constraints will be available to the compiler from the shape arguments supplied. But I missed delaying assignment transformations in varDecl(). Fix out shortly.

I'll include Robert (as well as Keith) on the reviewers to decide if the fix should go in for beta1. It's a fairly simple fix, but it is last minute...

[gopherbot]

Change https://golang.org/cl/371534 mentions this issue: cmd/compile: fix case where we didn't delay transformAssign in varDecl