internal/core/adt: don't cache incomplete errors for let

These may be resolved later.

Issue #1116

Signed-off-by: Marcel van Lohuizen <mpvl@golang.org>

Change-Id: I46b8dda41193b41f5b6b0fd341164a2568697bfc
Signed-off-by: Marcel van Lohuizen <mpvl@golang.org>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/528052
Reviewed-by: Marcel van Lohuizen <mpvl@gmail.com>

Author: mpvl

cue/testdata/references/let.txtar

@@ -63,6 +63,18 @@ incompleteLet: {
         }
     }
 }
+issue1116: {
+  a: {
+    #a: ["a", "b"]
+    let List = #a
+    List[len(List)-1]
+  }
+  b: {
+    let List = #a
+    List[len(List)-1]
+    #a: ["a", "b"]
+  }
+}
 
 -- out/compile --
 --- in.cue
@@ -183,6 +195,22 @@ incompleteLet: {
       if (〈import;list〉.max < 0) {}
     }
   }
+  issue1116: {
+    a: {
+      #a: [
+        "a",
+        "b",
+      ]
+      〈0;let List〉[(len(〈0;let List〉) - 1)]
+    }
+    b: {
+      〈0;let List〉[(len(〈0;let List〉) - 1)]
+      #a: [
+        "a",
+        "b",
+      ]
+    }
+  }
 }
 -- out/eval --
 (struct){
@@ -282,4 +310,20 @@ incompleteLet: {
       //     ./in.cue:61:17
     }
   }
+  issue1116: (struct){
+    a: (string){
+      "b"
+      #a: (#list){
+        0: (string){ "a" }
+        1: (string){ "b" }
+      }
+    }
+    b: (string){
+      "b"
+      #a: (#list){
+        0: (string){ "a" }
+        1: (string){ "b" }
+      }
+    }
+  }
 }

internal/core/adt/composite.go

@@ -142,7 +142,9 @@ func (e *Environment) evalCached(c *OpContext, x Expr) Value {
 		c.e, c.src = e, x.Source()
 		v = c.evalState(x, Partial) // TODO: should this be Finalized?
 		c.e, c.src = env, src
-		e.cache[x] = v
+		if b, ok := v.(*Bottom); !ok || !b.IsIncomplete() {
+			e.cache[x] = v
+		}
 	}
 	return v
 }