go/types,types2: internal compiler error: panic: cmd/compile/internal/types2/check.go:595: assertion failed

[Carrotman42]

Go version

go dev branch (for playground)

Output of go env in your module/workspace:

go playground (sorry, not sure how to get the env from there)

What did you do?

This program crashes the compiler at the current dev branch in the playground: https://go.dev/play/p/3JI9h22Igk8?v=gotip

(sidenote: I was only trying to get this part of the spec to compile :])

What did you see happen?

Compiler crash

What did you expect to see?

Compiler success or standard error message

[i3d]

I'd also request to clarify whether this is indeed an intended syntax [or some sort of typo]

[randall77]

@griesemer

import "fmt"

func recv[T ~bool](ch <-chan T) {
	var x, ok T = <-ch
	fmt.Println(x, ok)
}

func main() {
	ch := make(chan bool, 1)
	ch <- true
	recv(ch)
}

# command-line-arguments
<unknown line number>: internal compiler error: panic: ../../../sandbox/tmp/src/cmd/compile/internal/types2/check.go:596: assertion failed

goroutine 1 [running]:
runtime/debug.Stack()
	../../../sandbox/tmp/src/runtime/debug/stack.go:26 +0x64
cmd/compile/internal/base.FatalfAt({0x300e8?, 0x140?}, {0x10182d7e2, 0x9}, {0x14000030118, 0x1, 0x1})
	../../../sandbox/tmp/src/cmd/compile/internal/base/print.go:225 +0x1fc
cmd/compile/internal/base.Fatalf(...)
	../../../sandbox/tmp/src/cmd/compile/internal/base/print.go:194
cmd/compile/internal/gc.handlePanic()
	../../../sandbox/tmp/src/cmd/compile/internal/gc/main.go:52 +0x90
panic({0x1019d2fa0?, 0x1400002cfa0?})
	../../../sandbox/tmp/src/runtime/panic.go:759 +0x124
cmd/compile/internal/types2.(*Checker).handleBailout(0x1400043c000, 0x14000031020)
	../../../sandbox/tmp/src/cmd/compile/internal/types2/check.go:364 +0x9c
panic({0x1019d2fa0?, 0x1400002cfa0?})
	../../../sandbox/tmp/src/runtime/panic.go:759 +0x124
cmd/compile/internal/types2.assert(0x10?)
	../../../sandbox/tmp/src/cmd/compile/internal/types2/errors.go:25 +0x60
cmd/compile/internal/types2.(*Checker).recordCommaOkTypes(0x1400043c000, {0x101a7d3c0, 0x14000438060}, {0x1400002cf70, 0x2, 0x0?})
	../../../sandbox/tmp/src/cmd/compile/internal/types2/check.go:596 +0x15c
cmd/compile/internal/types2.(*Checker).initVars(0x1400043c000, {0x1400002cf60, 0x2, 0x2}, {0x140000305f0, 0x1, 0x1}, {0x0, 0x0})
	../../../sandbox/tmp/src/cmd/compile/internal/types2/assignments.go:431 +0x430
cmd/compile/internal/types2.(*Checker).varDecl(0x1400043c000, 0x1400043f9d0, {0x1400002cf60, 0x2, 0x2}, {0x101a7d258, 0x14000432a50}, {0x101a7d3c0, 0x14000438060})
	../../../sandbox/tmp/src/cmd/compile/internal/types2/decl.go:466 +0x1f4

It is probably confused by the fact that the ok part of the assignment is ~bool.

I believe the syntax your are using is correct, although weird. Probably better (and solves the crash) to do x, ok := <-ch.
Converting the ok to the type parameter can always be done later.

[go101]

Similar:

package main

import "fmt"

func foo[T ~bool](m map[int]T) {
	var x, ok T = m[0]
	fmt.Println(x, ok)
}

func main() {
	m := map[int]bool{}
	foo(m)
}

[Carrotman42]

Thanks for taking a look, @randall77 . To be clear I don't actually need a workaround here - I filed an issue because the crash message told me to do so :)

(I'm not really writing this code, I was just trying to get this code example from the spec to compile and found this unrelated problem.)

[griesemer]

Minimal reproducer:

func _[T bool](ch chan T) {
	var _, _ T = <-ch
}

[gopherbot]

Change https://go.dev/cl/580075 mentions this issue: go/types, types2: use correct predicate when asserting comma-ok types