Issue a warning when a global value is declared with a capture set

Author: odersky

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

@@ -1019,9 +1019,20 @@ class CheckCaptures extends Recheck, SymTransformer:
           // function is compiled since we do not propagate expected types into blocks.
           interpolateIfInferred(tree.tpt, sym)
 
+        def declaredCaptures = tree.tpt.nuType.captureSet
         if runInConstructor && savedEnv.owner.isClass then
           curEnv = savedEnv
-          markFree(tree.tpt.nuType.captureSet, tree, addUseInfo = false)
+          markFree(declaredCaptures, tree, addUseInfo = false)
+
+        if sym.owner.isStaticOwner && !declaredCaptures.elems.isEmpty then
+          def where =
+            if sym.effectiveOwner.is(Package) then "top-level definition"
+            else i"member of static ${sym.owner}"
+          report.warning(
+            em"""$sym has a non-empty capture set but will not be added as
+                |a capability to computed capture sets since it is globally accessible
+                |as a $where. Global values cannot be capabilities.""",
+            tree.namePos)
     end recheckValDef
 
     /** Recheck method definitions:

tests/neg-custom-args/captures/class-caps.check

@@ -20,3 +20,9 @@
 30 |    a + b
    |
    | longer explanation available when compiling with `-explain`
+-- Warning: tests/neg-custom-args/captures/class-caps.scala:35:6 -------------------------------------------------------
+35 |  val console: Console^ = Console() // provide capability locally
+   |      ^^^^^^^
+   |      value console has a non-empty capture set but will not be added as
+   |      a capability to computed capture sets since it is globally accessible
+   |      as a member of static object Test2. Global values cannot be capabilities.

tests/neg-custom-args/captures/nonsensical-for-pure.check

@@ -2,3 +2,9 @@
 1 |val x: Int^ = 2  // error
   |       ^^^^
   |       Int is a pure type, it makes no sense to add a capture set to it
+-- Warning: tests/neg-custom-args/captures/nonsensical-for-pure.scala:1:4 ----------------------------------------------
+1 |val x: Int^ = 2  // error
+  |    ^
+  |    value x has a non-empty capture set but will not be added as
+  |    a capability to computed capture sets since it is globally accessible
+  |    as a top-level definition. Global values cannot be capabilities.

tests/pos-custom-args/captures/i23421.scala

@@ -12,5 +12,5 @@ object Collection:
 trait Foo:
   val thunks: Collection[() => Unit] // that's fine
 
-object FooImpl1 extends Foo:
+class FooImpl1 extends Foo:
   val thunks: Collection[() => Unit] = Collection.empty // was error, now ok