internal/core/adt: fix closedness issue related to sharing

A closedness issue could arise when a vertex
was unified multiple times within the same
node if this vertex was (initially) shared.
Basically, the when a shared node was
added again, it was dropped based on pointer
equivalence. If then later this node was
unshared, this node was no longer added,
meaning that arcs could be missing from
closedness information, causing arcs to be
incorrectly defined as missing.

We now ensure the node is added multiple
times, just like when structure sharing is off,
to ensure closedness data is correctly
computed.

Note that this also fixes issue 3527. This
was not marked as problematic, as the
original CL just fixed the panic. But the
result was still incorrect.

Fixes #3641
Fixes #3546
Issue #3527

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: I9c248b0448ae1bcfcf4e4c1832397d73e8b02705
Dispatch-Trailer: {"type":"trybot","CL":1206823,"patchset":1,"ref":"refs/changes/23/1206823/1","targetBranch":"master"}

cue/testdata/cycle/issue3527.txtar

@@ -30,44 +30,29 @@ Unifications: 35
 Conjuncts:    82
 Disjuncts:    39
 -- out/evalalpha --
-Errors:
-assert.res._computed.in.taxPayer.self: field not allowed:
-    ./in.cue:5:22
-    ./in.cue:18:12
-
-Result:
-(_|_){
-  // [eval]
+(struct){
   _taxPayer(:p): (struct){
     self: (struct){
     }
   }
   let sam#1 = (struct){
     taxPayer: ~(_taxPayer(:p))
   }
-  assert: (_|_){
-    // [eval]
+  assert: (struct){
     invoke: (struct){
       taxPayer: ~(_taxPayer(:p))
     }
-    res: (_|_){
-      // [eval]
+    res: (struct){
       in: (#struct){
         taxPayer: (#struct){
           self: (#struct){
           }
         }
       }
-      _computed(:p): (_|_){
-        // [eval]
-        in: (_|_){
-          // [eval]
-          taxPayer: (_|_){
-            // [eval]
-            self: (_|_){
-              // [eval] assert.res._computed.in.taxPayer.self: field not allowed:
-              //     ./in.cue:5:22
-              //     ./in.cue:18:12
+      _computed(:p): (#struct){
+        in: (#struct){
+          taxPayer: (#struct){
+            self: (#struct){
             }
           }
         }
@@ -94,70 +79,42 @@ Result:
 diff old new
 --- old
 +++ new
-@@ -1,32 +1,41 @@
--(struct){
-+Errors:
-+assert.res._computed.in.taxPayer.self: field not allowed:
-+    ./in.cue:5:22
-+    ./in.cue:18:12
-+
-+Result:
-+(_|_){
-+  // [eval]
-   _taxPayer(:p): (struct){
-     self: (struct){
+@@ -4,22 +4,16 @@
      }
    }
    let sam#1 = (struct){
 -    taxPayer: (struct){
 -      self: (struct){
 -      }
 -    }
--  }
--  assert: (struct){
 +    taxPayer: ~(_taxPayer(:p))
-+  }
-+  assert: (_|_){
-+    // [eval]
+   }
+   assert: (struct){
      invoke: (struct){
 -      taxPayer: (struct){
 -        self: (struct){
 -        }
 -      }
--    }
--    res: (struct){
 +      taxPayer: ~(_taxPayer(:p))
-+    }
-+    res: (_|_){
-+      // [eval]
+     }
+     res: (struct){
        in: (#struct){
          taxPayer: (#struct){
 -          self: (struct){
--          }
--        }
--      }
--      _computed(:p): (#struct){
--        in: (#struct){
--          taxPayer: (#struct){
--            self: (struct){
 +          self: (#struct){
-+          }
-+        }
-+      }
-+      _computed(:p): (_|_){
-+        // [eval]
-+        in: (_|_){
-+          // [eval]
-+          taxPayer: (_|_){
-+            // [eval]
-+            self: (_|_){
-+              // [eval] assert.res._computed.in.taxPayer.self: field not allowed:
-+              //     ./in.cue:5:22
-+              //     ./in.cue:18:12
+           }
+         }
+       }
+@@ -26,7 +20,7 @@
+       _computed(:p): (#struct){
+         in: (#struct){
+           taxPayer: (#struct){
+-            self: (struct){
++            self: (#struct){
              }
            }
          }
-@@ -34,30 +43,16 @@
+@@ -34,30 +28,16 @@
      }
    }
    subject: (struct){