go/types: panic in new logic in recordCommaOkTypes

[findleyr]

Just hit this panic in gopls, using go version devel go1.21-a7fd2fab0e Fri Mar 31 16:05:14 2023 +0000 linux/amd64

panic: /home/rfindley/src/go/src/go/types/check.go:490: assertion failed
        panic: /home/rfindley/src/go/src/go/types/check.go:490: assertion failed [recovered]
        panic: /home/rfindley/src/go/src/go/types/check.go:490: assertion failed

goroutine 756640 [running]:
go/types.(*Checker).handleBailout(0xc000875680, 0xc009a51668)
        /home/rfindley/src/go/src/go/types/check.go:299 +0x88
panic({0xd26e00?, 0xc0123e4eb0?})
        /home/rfindley/src/go/src/runtime/panic.go:913 +0x21f
go/types.assert(0x0?)
        /home/rfindley/src/go/src/go/types/errors.go:28 +0x54
go/types.(*Checker).recordCommaOkTypes(0xc000875680, {0x10fa300, 0xc00eec3350}, {0xc0123e4e80, 0x2, 0xc006c60210?})
        /home/rfindley/src/go/src/go/types/check.go:490 +0x158
go/types.(*Checker).initVars(0xc000875680?, {0xc0123e4e50?, 0x2, 0x2}, {0xc005c16500?, 0x1, 0x1}, {0x0, 0x0})
        /home/rfindley/src/go/src/go/types/assignments.go:391 +0x2a5
go/types.(*Checker).shortVarDecl(0xc000875680, {0x10f5a60, 0xc0121d9aa0}, {0xc007b02f20, 0x2, 0xc007b02da0?}, {0xc005c16500?, 0x1, 0x1})
        /home/rfindley/src/go/src/go/types/assignments.go:528 +0x9d9
go/types.(*Checker).stmt(0xc000875680, 0x0, {0x10f9dc0?, 0xc00474a3c0?})
        /home/rfindley/src/go/src/go/types/stmt.go:476 +0xb05
go/types.(*Checker).stmtList(0x130335f?, 0x0, {0xc00eb15a80?, 0x16631c?, 0x16631c?})
        /home/rfindley/src/go/src/go/types/stmt.go:124 +0x78
go/types.(*Checker).funcBody(0xc000875680, 0xc00aff3f20, {0xe95c6a?, 0xc00007eaa0?}, 0xc0076d9d40, 0xc00eec3410, {0x0, 0x0})
        /home/rfindley/src/go/src/go/types/stmt.go:44 +0x345
go/types.(*Checker).exprInternal.func1()
        /home/rfindley/src/go/src/go/types/expr.go:1362 +0x3b
go/types.(*Checker).processDelayed(0xc000875680, 0xd0b)
        /home/rfindley/src/go/src/go/types/check.go:387 +0x15c
panic({0xd26e00?, 0xc0123e4ca0?})
        /home/rfindley/src/go/src/runtime/panic.go:919 +0x270
go/types.assert(0x0?)
        /home/rfindley/src/go/src/go/types/errors.go:28 +0x54
go/types.(*Checker).recordCommaOkTypes(0xc000875680, {0x10fa300, 0xc00eec3350}, {0xc0123e4c60, 0x2, 0xc0066a27ac?})
        /home/rfindley/src/go/src/go/types/check.go:490 +0x158
go/types.(*Checker).initVars(0xc000875680?, {0xc0123e4c30?, 0x2, 0x2}, {0xc005c16500?, 0x1, 0x1}, {0x0, 0x0})
        /home/rfindley/src/go/src/go/types/assignments.go:391 +0x2a5
go/types.(*Checker).shortVarDecl(0xc000875680, {0x10f5a60, 0xc0121d9a40}, {0xc007b02f20, 0x2, 0xc007b02da0?}, {0xc005c16500?, 0x1, 0x1})
        /home/rfindley/src/go/src/go/types/assignments.go:528 +0x9d9
go/types.(*Checker).stmt(0xc000875680, 0x0, {0x10f9dc0?, 0xc00474a3c0?})
        /home/rfindley/src/go/src/go/types/stmt.go:476 +0xb05
go/types.(*Checker).stmtList(0x6ae32b?, 0x0, {0xc00eb15a80?, 0x64d42c?, 0xc0076d9b00?})
        /home/rfindley/src/go/src/go/types/stmt.go:124 +0x78
go/types.(*Checker).funcBody(0xc000875680, 0xc00aff3f20, {0xe95c6a?, 0xc00007eaa0?}, 0xc0076d9d40, 0xc00eec3410, {0x0, 0x0})
        /home/rfindley/src/go/src/go/types/stmt.go:44 +0x345
go/types.(*Checker).exprInternal.func1()
        /home/rfindley/src/go/src/go/types/expr.go:1362 +0x3b
go/types.(*Checker).processDelayed(0xc000875680, 0xd0b)
        /home/rfindley/src/go/src/go/types/check.go:387 +0x15c
go/types.(*Checker).shortVarDecl(0xc000875680, {0x10f5a60, 0xc0121d99e0}, {0xc005c16320, 0x1, 0xc006f57500?}, {0xc005c16630?, 0x1, 0x1})
        /home/rfindley/src/go/src/go/types/assignments.go:531 +0x9ee
go/types.(*Checker).stmt(0xc000875680, 0x0, {0x10f9dc0?, 0xc00474a6c0?})
        /home/rfindley/src/go/src/go/types/stmt.go:476 +0xb05
go/types.(*Checker).stmtList(0xc009a51468?, 0x0, {0xc007a96c00?, 0x0?, 0x6ae32b?})
        /home/rfindley/src/go/src/go/types/stmt.go:124 +0x78
go/types.(*Checker).funcBody(0xc000875680, 0xc00aff3f20, {0xc0073a82e8?, 0xc007f706c0?}, 0xc00da22d80, 0xc00eec3680, {0x0, 0x0})
        /home/rfindley/src/go/src/go/types/stmt.go:44 +0x345
go/types.(*Checker).funcDecl.func1()
        /home/rfindley/src/go/src/go/types/decl.go:831 +0x39
go/types.(*Checker).processDelayed(0xc000875680, 0x0)
        /home/rfindley/src/go/src/go/types/check.go:387 +0x15c
go/types.(*Checker).checkFiles(0xc000875680, {0xc0069b7300, 0x1c, 0x20})
        /home/rfindley/src/go/src/go/types/check.go:332 +0x16e
go/types.(*Checker).Files(...)

Given that this logic changed in https://go.dev/cl/478218 (and surrounding CLs), it seems likely that this is a new panic in go/types. (though I will caveat that a lot of logic in gopls has changed recently with respect to type checking).

Unfortunately, I was not able to reproduce by replaying my editing session (likely due batching of keystrokes in the replay), so we'll have to stare at this. For starters, we should separate those assertions onto separate lines.

CC @griesemer

[findleyr]

Minimal repro: just ensure that 'ok' has type Typ[Invalid], and we won't get a typed RHS operand.

func _() {
  var ok undef
  var m map[int]int
  _, ok := m[0]
}

[griesemer]

Even simpler repro:

func _() {
	var m map[int]int
	_, ok = m[0]
}

[griesemer]

Analysis: This code was broken with CL 478218. Before that CL, Checker.assignVar used to return the assigned type, or nil, in case of failure. Checker.recordCommaOkTypes used to take two types (not two operands), and if one of those types was nil, it would simply not record. With the changes in CL 478218, we lost that (nil) signal.

[gopherbot]

Change https://go.dev/cl/486135 mentions this issue: go/types, types2: don't panic for invalid assignments of comma-ok expressions