internal/core/export: fix dropping of optional field

Lookup dereferences all vertex indirections, including those
for structure-sharing. If indirection happens too
early it may lead to incorrect arc types. This changes
postpones the dereference.

Issue #3060

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: Iae18d29d4d32b2acc762df68d63f46e99e36da56
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1194402
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

internal/core/export/testdata/main/alias.txtar

@@ -206,7 +206,6 @@ was known to compile and is known to be correct.
 		simple: {
 			"a-b": 4
 			foo:   4
-			bar:   5
 			"a-c": 5
 		}
 		cross: {
@@ -271,7 +270,7 @@ was known to compile and is known to be correct.
 		simple: {
 			"a-b": 4
 			foo:   4
-			bar:   5
+			bar?:  5
 			"a-c": 5
 		}
 		cross: {
@@ -340,7 +339,6 @@ was known to compile and is known to be correct.
 		simple: {
 			"a-b": 4
 			foo:   4
-			bar:   5
 			"a-c": 5
 		}
 		cross: {
@@ -392,7 +390,7 @@ was known to compile and is known to be correct.
 		simple: {
 			"a-b": 4
 			foo:   4
-			bar:   5
+			bar?:  5
 			"a-c": 5
 		}
 		cross: {
@@ -461,7 +459,7 @@ was known to compile and is known to be correct.
 		simple: {
 			"a-b": 4
 			foo:   4
-			bar:   5
+			bar?:  5
 			"a-c": 5
 		}
 		cross: {
@@ -513,107 +511,17 @@ was known to compile and is known to be correct.
 diff old new
 --- old
 +++ new
-@@ -4,6 +4,7 @@
- 		simple: {
- 			"a-b": 4
- 			foo:   4
-+			bar:   5
- 			"a-c": 5
+@@ -164,7 +164,7 @@
  		}
- 		cross: {
-@@ -68,7 +69,7 @@
- 		simple: {
- 			"a-b": 4
- 			foo:   4
--			bar?:  5
-+			bar:   5
- 			"a-c": 5
- 		}
- 		cross: {
-@@ -137,34 +138,35 @@
- 		simple: {
- 			"a-b": 4
- 			foo:   4
--			"a-c": 5
--		}
--		cross: {
--			baz: 3
--			"d-2": {}
--		}
--	}
--	valueAlias: {
--		merge: {
--			value: {
--				b: 2
--				v: {
--					X: 3
--				}
--			}
--		}
--		selfRef: {
--			struct: {
--				a: {
--					b: {
--						b: 2
--					}
--				}
--			}
--		}
--		selfRefValue: {
--			struct: {
+ 		selfRefValue: {
+ 			struct: {
 -				a: _|_ // cycle error
-+			bar:   5
-+			"a-c": 5
-+		}
-+		cross: {
-+			baz: 3
-+			"d-2": {}
-+		}
-+	}
-+	valueAlias: {
-+		merge: {
-+			value: {
-+				b: 2
-+				v: {
-+					X: 3
-+				}
-+			}
-+		}
-+		selfRef: {
-+			struct: {
-+				a: {
-+					b: {
-+						b: 2
-+					}
-+				}
-+			}
-+		}
-+		selfRefValue: {
-+			struct: {
 +				a: _|_ // valueAlias.selfRefValue.struct.a: incomplete list: _ (and 3 more errors)
  			}
  			pattern: {
  				a: {}
-@@ -188,7 +190,7 @@
- 		simple: {
- 			"a-b": 4
- 			foo:   4
--			bar?:  5
-+			bar:   5
- 			"a-c": 5
- 		}
- 		cross: {
-@@ -257,7 +259,7 @@
- 		simple: {
- 			"a-b": 4
- 			foo:   4
--			bar?:  5
-+			bar:   5
- 			"a-c": 5
- 		}
- 		cross: {
--- diff/value/todo/p0 --
-Dropped optional field marker.
+-- diff/value/explanation --
+Improved error message.
 Reorderings?
 -- out/value --
 == Simplified

internal/core/export/value.go

@@ -432,7 +432,7 @@ func (e *exporter) structComposite(v *adt.Vertex, attrs []*ast.Attribute) ast.Ex
 			e.inDefinition++
 		}
 
-		arc := v.Lookup(label)
+		arc := v.LookupRaw(label)
 		if arc == nil {
 			continue
 		}
@@ -446,6 +446,7 @@ func (e *exporter) structComposite(v *adt.Vertex, attrs []*ast.Attribute) ast.Ex
 
 		internal.SetConstraint(f, arc.ArcType.Token())
 
+		arc = arc.DerefValue()
 		f.Value = e.vertex(arc)
 
 		if label.IsDef() {

internal/core/export/value_test.go

@@ -20,11 +20,13 @@ import (
 
 	"cuelang.org/go/cue/ast"
 	"cuelang.org/go/cue/errors"
+	"cuelang.org/go/internal"
 	"cuelang.org/go/internal/core/adt"
 	"cuelang.org/go/internal/core/compile"
 	"cuelang.org/go/internal/core/eval"
 	"cuelang.org/go/internal/core/export"
 	"cuelang.org/go/internal/core/runtime"
+	"cuelang.org/go/internal/cuedebug"
 	"cuelang.org/go/internal/cuetdtest"
 	"cuelang.org/go/internal/cuetxtar"
 	"golang.org/x/tools/txtar"
@@ -96,6 +98,9 @@ func TestValueX(t *testing.T) {
 	a := cuetxtar.Load(archive, t.TempDir())
 
 	r := runtime.New()
+	(*runtime.Runtime)(r).SetVersion(internal.DevVersion)
+	(*runtime.Runtime)(r).SetDebugOptions(&cuedebug.Config{Sharing: true})
+
 	v, errs := compile.Files(nil, r, "", a[0].Files...)
 	if errs != nil {
 		t.Fatal(errs)