internal/core/adt: discard errors in field verification

These errorrs are already handled elsewhere and
leaving them unhandled violates an invariant.

Fixes #799.

Change-Id: Idef317fd873ead27f1836855b7b700c6679db735
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/8903
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cue/testdata/eval/dynamic_field.txtar

@@ -8,14 +8,32 @@ X="\(b)": {
 	a: 1
 }
 c: X
+
+withError: {
+    issue799: {key: int32} & {"\(1&2)": 123}
+}
 -- out/eval --
-(struct){
+Errors:
+invalid interpolation: conflicting values 2 and 1:
+    ./in.cue:12:31
+
+Result:
+(_|_){
+  // [eval]
   a: (string){ "foo" }
   e: (int){ 2 }
   b: (string){ "bar" }
   c: (struct){
     a: (int){ 1 }
   }
+  withError: (_|_){
+    // [eval]
+    issue799: (_|_){
+      // [eval] invalid interpolation: conflicting values 2 and 1:
+      //     ./in.cue:12:31
+      key: (int){ &(>=-2147483648, <=2147483647, int) }
+    }
+  }
   foo: (struct){
     b: (struct){
       c: (struct){
@@ -44,4 +62,11 @@ c: X
     a: 1
   }
   c: 〈0;("\(〈0;b〉)")〉
+  withError: {
+    issue799: ({
+      key: &(int, >=-2147483648, <=2147483647)
+    } & {
+      "\((1 & 2))": 123
+    })
+  }
 }

internal/core/adt/closed.go

@@ -468,6 +468,10 @@ func verifyArc(ctx *OpContext, s *StructInfo, f Feature, label Value) bool {
 		return false
 	}
 
+	// Do not record errors during this validation.
+	errs := ctx.errs
+	defer func() { ctx.errs = errs }()
+
 	if len(o.Dynamic) > 0 && f.IsString() {
 		if label == nil && f.IsString() {
 			label = f.ToValue(ctx)