cmd/compile: internal compiler error: evconst changed Type

[dsnet]

Was playing around with #23413 and managed to get this compiler error.

var s = struct {
	t1 struct{}
	_  int
	t2 struct{}
}{}
print(&s.t1 == &s.t2)

This crashes on Go1.9 and on:

# command-line-arguments
<autogenerated>:1:0: internal compiler error: evconst changed Type: true && true had type bool, now untyped bool

goroutine 1 [running]:
runtime/debug.Stack(0x0, 0x0, 0x0)
	/usr/local/go.tip/src/runtime/debug/stack.go:24 +0xa7
cmd/compile/internal/gc.Fatalf(0xc26e98, 0x2c, 0xc42037cc80, 0x3, 0x3)
	/usr/local/go.tip/src/cmd/compile/internal/gc/subr.go:182 +0x1f2
cmd/compile/internal/gc.walkexpr(0xc42033e600, 0xc42033e690, 0xc42050cfc0)
	/usr/local/go.tip/src/cmd/compile/internal/gc/walk.go:1745 +0xae0
cmd/compile/internal/gc.walkexprlist(0xc420514278, 0x1, 0x1, 0xc42033e690)
	/usr/local/go.tip/src/cmd/compile/internal/gc/walk.go:378 +0x50
cmd/compile/internal/gc.walkstmt(0xc42033e680, 0xc42037cf01)
	/usr/local/go.tip/src/cmd/compile/internal/gc/walk.go:292 +0x11f
cmd/compile/internal/gc.walkstmtlist(0xc420514288, 0x1, 0x1)
	/usr/local/go.tip/src/cmd/compile/internal/gc/walk.go:78 +0x46
cmd/compile/internal/gc.walk(0xc4204b9500)
	/usr/local/go.tip/src/cmd/compile/internal/gc/walk.go:62 +0x3a1
cmd/compile/internal/gc.compile(0xc4204b9500)
	/usr/local/go.tip/src/cmd/compile/internal/gc/pgen.go:203 +0xae
cmd/compile/internal/gc.funccompile(0xc4204b9500)
	/usr/local/go.tip/src/cmd/compile/internal/gc/dcl.go:1040 +0xad
cmd/compile/internal/gc.geneq(0xc4205326c0, 0xc420374fc0)
	/usr/local/go.tip/src/cmd/compile/internal/gc/alg.go:503 +0x790
cmd/compile/internal/gc.dalgsym(0xc420374fc0, 0xffffffffffffffff)
	/usr/local/go.tip/src/cmd/compile/internal/gc/reflect.go:1685 +0x5c4
cmd/compile/internal/gc.dcommontype(0xc42053a230, 0x0, 0xc420374fc0, 0xc420333418)
	/usr/local/go.tip/src/cmd/compile/internal/gc/reflect.go:856 +0xa3
cmd/compile/internal/gc.dtypesym(0xc420374fc0, 0xc420343de0)
	/usr/local/go.tip/src/cmd/compile/internal/gc/reflect.go:1360 +0xd19
cmd/compile/internal/gc.dumpsignats()
	/usr/local/go.tip/src/cmd/compile/internal/gc/reflect.go:1511 +0x341
cmd/compile/internal/gc.dumpobj1(0x7fff19d0ace3, 0x23, 0x3)
	/usr/local/go.tip/src/cmd/compile/internal/gc/obj.go:139 +0x2a1
cmd/compile/internal/gc.dumpobj()
	/usr/local/go.tip/src/cmd/compile/internal/gc/obj.go:50 +0x51
cmd/compile/internal/gc.Main(0xc2e5f8)
	/usr/local/go.tip/src/cmd/compile/internal/gc/main.go:683 +0x2b0e
main.main()
	/usr/local/go.tip/src/cmd/compile/main.go:49 +0x89

\cc @randall77 @mdempsky

[dsnet]

Since this is broken on Go1.9, the fix can probably wait till Go1.11.

[bradfitz]

Works with Go 1.8, though.

[namusyaka]

I guess the issue has been occured since 26be4b9.
cc @josharian

[gopherbot]

Change https://golang.org/cl/91316 mentions this issue: cmd/compile: avoid unexpected changing untyped bool by evconst

[mdempsky]

The test case can be further minimized to just:

package p

type t struct {
        t1 struct{}
        _  int
        t2 struct{}
}

The problem is this is generating a function like

func eq.t(p, q *t) bool {
    return p.t1 == q.t1 && p.t2 == q.t2
}

This is getting optimized to

    return true && true

but for some reason the true nodes have type untyped bool, but the overall true && true expression has type bool. When we apply evconst to this expression, its type changes from bool to untyped bool.

[gopherbot]

Change https://golang.org/cl/94022 mentions this issue: cmd/compile: fix typechecking of untyped boolean expressions