cmd/go2go: type checking for generic constraints doesn't work correctly

[tdakkota]

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

$ go version
go version devel +0a030888da Sat Jun 20 04:46:40 2020 +0000 windows/amd64

Does this issue reproduce with the latest release?

n/a

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

• windows/amd64 with 0a03088
• go2go playground

What did you do?

https://go2goplay.golang.org/p/QLrCSQoyvVb

What did you expect to see?

go2go's typechecker error for line 57.

What did you see instead?

Successful compilation.

[tdakkota]

But

	_ = F2(string, Optional(int), Result(int))(OptionalFrom(int)(0), func(o Optional(int)) (r Result(int)) {
		return
	})

fails with

type checking failed for main
prog.go2:51:17: Optional(int) does not satisfy Box(V) (missing method Val)

[tdakkota]

Another example
https://go2goplay.golang.org/p/Juv92RyNgwh

crashes on 0a03088 and runs on playground.

Partial stack trace:

runtime: goroutine stack exceeds 1000000000-byte limit
runtime: sp=0xc0201e1580 stack=[0xc0201e0000, 0xc0401e0000]
fatal error: stack overflow

runtime stack:
runtime.throw(0xd3743b, 0xe)
	G:/workspace/exprm/go/src/runtime/panic.go:1116 +0x79
runtime.newstack()
	G:/workspace/exprm/go/src/runtime/stack.go:1059 +0x80d
runtime.morestack()
	G:/workspace/exprm/go/src/runtime/asm_amd64.s:449 +0x97

goroutine 1 [running]:
go/types.(*Checker).completeInterface(0xc0001665a0, 0x0, 0xc00016e380)
	G:/workspace/exprm/go/src/go/types/typexpr.go:818 +0xaba fp=0xc0201e1590 sp=0xc0201e1588 pc=0xbc2a1a
go/types.(*TypeParam).Bound(0xc0001bc140, 0x0)
	G:/workspace/exprm/go/src/go/types/type.go:811 +0xe5 fp=0xc0201e15e0 sp=0xc0201e1590 pc=0xbb3185
go/types.optype(0xd6cdc0, 0xc0001bc140, 0x0, 0x0)
	G:/workspace/exprm/go/src/go/types/type.go:830 +0x66 fp=0xc0201e1648 sp=0xc0201e15e0 pc=0xbb3226
go/types.(*TypeParam).Interface(0xc0001bc140, 0x0)
	G:/workspace/exprm/go/src/go/types/type.go:848 +0x4c fp=0xc0201e1698 sp=0xc0201e1648 pc=0xbb38ac
go/types.(*TypeParam).Interface(0xc0001bc180, 0x0)
	G:/workspace/exprm/go/src/go/types/type.go:848 +0x6c fp=0xc0201e16e8 sp=0xc0201e1698 pc=0xbb38cc
go/types.(*TypeParam).Interface(0xc0001bc140, 0x0)
	G:/workspace/exprm/go/src/go/types/type.go:848 +0x6c fp=0xc0201e1738 sp=0xc0201e16e8 pc=0xbb38cc
go/types.(*TypeParam).Interface(0xc0001bc180, 0x0)
	G:/workspace/exprm/go/src/go/types/type.go:848 +0x6c fp=0xc0201e1788 sp=0xc0201e1738 pc=0xbb38cc
go/types.(*TypeParam).Interface(0xc0001bc140, 0x0)
	G:/workspace/exprm/go/src/go/types/type.go:848 +0x6c fp=0xc0201e17d8 sp=0xc0201e1788 pc=0xbb38cc
go/types.(*TypeParam).Interface(0xc0001bc180, 0x0)
...

[cagedmantis]

/cc @griesemer @ianlancetaylor

[ianlancetaylor]

Looks like the type checker fails to report an error.

Smaller test case:

package main

type Optional(type T) struct {
	p   T
	set bool
}

func (o Optional(T)) Val() (T, bool) {
	return o.p, true
}

type Box(type T) interface {
	Val() (T, bool)
}

func F1(type V interface{}, A, B Box(V))() {}

func main() {
	F1(int, Optional(int), Optional(string))()
}

This requires that both Optional(int) and Optional(string) implement Box(int). Optional(int) does implement it, but Optional(string) does not. The type checker passes this without error but probably should not.

[ianlancetaylor]

@mmaedel I'm not sure what problem you are running into, but it's a different problem. Please open a separate issue. Thanks.

[tdakkota]

https://go2goplay.golang.org/p/jmaTevSNW0X
Seems related. Type checker can't compare type of result of func New with I generic type

UPD.
This bug is related to optype function.
When type checker tries to assign result of func New to I, optype is called

func optype(typ Type) Type { // typ is I generic 
	if t := typ.TypeParam(); t != nil { // I is a type param
		if u := t.Bound().allTypes; u != nil && u != typ { // t bound only by method list, not by type list, so allTypes is nil
			return u.Under()
		}
		return theTop // here function returns
	}
	return typ
}

Tuple(A, B) becomes T top type, i.e. Any

[gopherbot]

Change https://golang.org/cl/240339 mentions this issue: [dev.go2go] go/types: fix type check skipping for constraints when th…

[tdakkota]

Seems to be fixed. Closing.

[gopherbot]

Change https://golang.org/cl/240478 mentions this issue: [dev.go2go] go/types: fix broken test case