Allow local exclusive caps in lazy val initializer

Author: bracevac

compiler/src/dotty/tools/dotc/cc/CheckCaptures.scala

@@ -746,14 +746,18 @@ class CheckCaptures extends Recheck, SymTransformer:
         checkUpdate(qualType, tree.srcPos):
           i"Cannot call update ${tree.symbol} of ${qualType.showRef}"
 
-        // Additionally, lazy val initializers should not call update methods
+        // Additionally, lazy val initializers should not call update methods on
+        // non-local exclusive capabilities (capabilities defined outside the lazy val)
         if curEnv.owner.is(Lazy) then
           qualType.captureSet.elems.foreach: elem =>
             if elem.isExclusive then
-              report.error(
-                em"""Lazy val initializer calls update method on exclusive capability $elem;
-                    |lazy val initializers should only access capabilities in a read-only fashion.""",
-                tree.srcPos)
+              val owner = elem.pathOwner
+              // Allow update methods on local capabilities (owned by the lazy val or nested within it)
+              if !owner.isContainedIn(curEnv.owner) && owner != curEnv.owner then
+                report.error(
+                  em"""Lazy val initializer calls update method on non-local exclusive capability $elem;
+                      |lazy val initializers should only access non-local capabilities in a read-only fashion.""",
+                  tree.srcPos)
 
       // If selecting a lazy val member, charge the qualifier since accessing
       // the lazy val can trigger initialization that uses the qualifier's capabilities

tests/neg-custom-args/captures/lazyvals-sep.scala

@@ -93,8 +93,21 @@ class WrapperRd(val ref: Ref^{cap.rd}):
       case _: Exception => 0
     () => r.get() + x
 
-  // // Test case 12: Local exclusive access - should be OK
-  // lazy val lazyVal12: () => Int =
-  //   val r3: Ref^ = Ref(10)
-  //   r3.set(100) FIXME should work
-  //   () => r3.get()
+  // Test case 12: Local exclusive access - should be OK
+  lazy val lazyVal12: () => Int =
+    val r3: Ref^ = Ref(10)
+    r3.set(100)  // ok
+    val i = r3.get()
+    () => i
+
+  // Test case 13: Local exclusive access - should be OK
+  lazy val lazyVal13: () => Int =
+    val r3: Ref^ = Ref(10)
+    r3.set(100)  // ok
+    () => r3.get()
+
+  // Test case 14: Local exclusive access - should be OK
+  lazy val lazyVal14: () => Ref^ =
+    val r3: Ref^ = Ref(10)
+    r3.set(100)  // ok
+    () => r3