evalv3: nil pointer dereference with a unification involving optional fields and defaults

[mvdan]

What version of CUE are you using (cue version)?

$ cue version
cue version v0.11.0-0.dev.0.20240830123612-3deadcefdda8

go version devel go1.24-9e8ea567c8 2024-08-28 21:03:29 +0000
      -buildmode exe
       -compiler gc
  DefaultGODEBUG asynctimerchan=1,gotypesalias=0,httpservecontentkeepheaders=1,randseednop=0,tls3des=1,tlskyber=0,x509keypairleaf=0,x509negativeserial=1
     CGO_ENABLED 1
          GOARCH amd64
            GOOS linux
         GOAMD64 v3
             vcs git
    vcs.revision 3deadcefdda82d7347116c5fdda886f87c2ba169
        vcs.time 2024-08-30T12:36:12Z
    vcs.modified false
cue.lang.version v0.11.0

Does this issue reproduce with the latest stable release?

It reproduces on master.

What did you do?

env CUE_EXPERIMENT=
exec cue export input.cue
cmp stdout export.stdout

env CUE_EXPERIMENT=evalv3
exec cue export input.cue
cmp stdout export.stdout

-- input.cue --
x: y?: [...string]
x: {y: ["foo"]} | *{y: ["bar"]}
-- export.stdout --
{
    "x": {
        "y": [
            "bar"
        ]
    }
}

What did you expect to see?

The same behavior with the old and new evaluators.

What did you see instead?

> env CUE_EXPERIMENT=evalv3
> exec cue export input.cue
[stderr]
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
	panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x30 pc=0x6dd639]

goroutine 1 gp=0xc000002380 m=4 mp=0xc000083508 [running]:
panic({0xd20060?, 0x1501540?})
	/home/mvdan/tip/src/runtime/panic.go:804 +0x168 fp=0xc00038fdc0 sp=0xc00038fd10 pc=0x472788
cuelang.org/go/internal/core/adt.runTask.func1()
	/home/mvdan/src/c/cue/internal/core/adt/sched.go:695 +0x76 fp=0xc00038fde8 sp=0xc00038fdc0 pc=0x727db6
panic({0xd20060?, 0x1501540?})
	/home/mvdan/tip/src/runtime/panic.go:785 +0x132 fp=0xc00038fe98 sp=0xc00038fde8 pc=0x472752
runtime.panicmem(...)
	/home/mvdan/tip/src/runtime/panic.go:262
runtime.sigpanic()
	/home/mvdan/tip/src/runtime/signal_unix.go:900 +0x359 fp=0xc00038fef8 sp=0xc00038fe98 pc=0x4749d9
cuelang.org/go/internal/core/adt.(*Vertex).updateArcType(0xc00018b180, 0x3?)
	/home/mvdan/src/c/cue/internal/core/adt/composite.go:332 +0x79 fp=0xc00038ff60 sp=0xc00038fef8 pc=0x6dd639
cuelang.org/go/internal/core/adt.(*nodeContext).getArc(0x72c4f4?, 0x4f?, 0x0?)
	/home/mvdan/src/c/cue/internal/core/adt/fields.go:332 +0x59 fp=0xc00038fff0 sp=0xc00038ff60 pc=0x71efd9
cuelang.org/go/internal/core/adt.(*nodeContext).insertArcCC(0xc0001ac608, 0x4c1, 0x0, {0xc000454aa0, {0xf72a78, 0xc0004542c0}, {0x0, 0xc0001910e0, 0x0, 0x0, ...}}, ...)
	/home/mvdan/src/c/cue/internal/core/adt/fields.go:800 +0xb3 fp=0xc000390140 sp=0xc00038fff0 pc=0x720f13
cuelang.org/go/internal/core/adt.(*nodeContext).insertArc(...)
	/home/mvdan/src/c/cue/internal/core/adt/fields.go:779
cuelang.org/go/internal/core/adt.(*nodeContext).scheduleStruct(0xc0001ac608, 0xc000454320, 0xc000347440, {0x0, 0xc0001910e0, 0x0, 0x0, 0x0, 0x0, {0x0, ...}})
	/home/mvdan/src/c/cue/internal/core/adt/conjunct.go:298 +0xce5 fp=0xc0003902f0 sp=0xc000390140 pc=0x6e7845
cuelang.org/go/internal/core/adt.(*nodeContext).scheduleConjunct(0xc0001ac608, {0xc000454320, {0xf728f8, 0xc000347440}, {0x0, 0xc0001910e0, 0x0, 0x0, 0x0, 0x0, ...}}, ...)
	/home/mvdan/src/c/cue/internal/core/adt/conjunct.go:172 +0x996 fp=0xc000390558 sp=0xc0003902f0 pc=0x6e5db6
cuelang.org/go/internal/core/adt.(*nodeContext).doDisjunct(0xc00018f808, {0xc000454320, {0xf728f8, 0xc000347440}, {0x0, 0xc0001910e0, 0x0, 0x0, 0x0, 0x0, ...}}, ...)
	/home/mvdan/src/c/cue/internal/core/adt/disjunct2.go:436 +0x4f8 fp=0xc000390768 sp=0xc000390558 pc=0x6ff578
cuelang.org/go/internal/core/adt.(*nodeContext).crossProduct(0xc00018f808, {0xc0003909d8?, 0xc0001903c0?, 0xc000192480?}, {0xc000390a70, 0x1, 0x8?}, 0xc0004242a0, 0x4)
	/home/mvdan/src/c/cue/internal/core/adt/disjunct2.go:345 +0x3c5 fp=0xc000390920 sp=0xc000390768 pc=0x6fecc5
cuelang.org/go/internal/core/adt.(*nodeContext).processDisjunctions(0xc00018f808)
	/home/mvdan/src/c/cue/internal/core/adt/disjunct2.go:279 +0x165 fp=0xc000390a90 sp=0xc000390920 pc=0x6fe565
cuelang.org/go/internal/core/adt.processDisjunctions(0x0?, 0xc00018abe0, 0xa5?)
	/home/mvdan/src/c/cue/internal/core/adt/tasks.go:189 +0x1c fp=0xc000390ad0 sp=0xc000390a90 pc=0x72a73c
cuelang.org/go/internal/core/adt.runTask(0xc00018abe0, 0x4)
	/home/mvdan/src/c/cue/internal/core/adt/sched.go:715 +0x545 fp=0xc000390ce8 sp=0xc000390ad0 pc=0x727925
cuelang.org/go/internal/core/adt.(*scheduler).process(0xc00018f9d8, 0x7eff, 0x4)
	/home/mvdan/src/c/cue/internal/core/adt/sched.go:408 +0x21f fp=0xc000390db8 sp=0xc000390ce8 pc=0x726bff
cuelang.org/go/internal/core/adt.(*Vertex).unify(0xc00018aaa0, 0xc000192480, 0x7fff, 0x4)
	/home/mvdan/src/c/cue/internal/core/adt/unify.go:160 +0x2bf fp=0xc000391000 sp=0xc000390db8 pc=0x72c5ff
cuelang.org/go/internal/core/adt.(*nodeContext).completeAllArcs(0xc00018f208, 0x7fff, 0x4)
	/home/mvdan/src/c/cue/internal/core/adt/unify.go:504 +0x22c fp=0xc0003910a8 sp=0xc000391000 pc=0x72dd8c
cuelang.org/go/internal/core/adt.(*Vertex).unify(0xc00018aa00, 0xc000192480, 0x7fff, 0x4)
	/home/mvdan/src/c/cue/internal/core/adt/unify.go:227 +0x8a6 fp=0xc0003912f0 sp=0xc0003910a8 pc=0x72cbe6
cuelang.org/go/internal/core/adt.(*OpContext).unify(0x90?, 0xf728f8?, 0x3915f0?)
	/home/mvdan/src/c/cue/internal/core/adt/eval.go:158 +0x339 fp=0xc000391540 sp=0xc0003912f0 pc=0x7047d9
cuelang.org/go/internal/core/adt.(*Vertex).Finalize(0x0?, 0xc000192480)
	/home/mvdan/src/c/cue/internal/core/adt/composite.go:821 +0x59 fp=0xc000391570 sp=0xc000391540 pc=0x6df419
cuelang.org/go/cue.newVertexRoot(0xc0002693c0, 0x1?, 0xc00018aa00)
	/home/mvdan/src/c/cue/cue/types.go:602 +0x25 fp=0xc000391598 sp=0xc000391570 pc=0x94c0c5
cuelang.org/go/cue.newValueRoot(0x0?, 0xc0003916b0?, {0xf7aa60?, 0xc00018aa00?})
	/home/mvdan/src/c/cue/cue/types.go:611 +0x3e fp=0xc000391670 sp=0xc000391598 pc=0x94c15e
cuelang.org/go/cue.(*Context).make(0xc0002693c0, 0xc00018aa00)
	/home/mvdan/src/c/cue/cue/context.go:252 +0x79 fp=0xc0003916d8 sp=0xc000391670 pc=0x93b439
cuelang.org/go/cue.(*Context).BuildInstances(0xc0002693c0, {0xc000391980, 0x1, 0x1e0?})
	/home/mvdan/src/c/cue/cue/context.go:150 +0x1e5 fp=0xc000391788 sp=0xc0003916d8 pc=0x93a745
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