Improve error message for escaping capabilities

Author: odersky

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

@@ -303,9 +303,9 @@ extends tpd.TreeTraverser:
           val polyType = fn.tpe.widen.asInstanceOf[TypeLambda]
           for case (arg: TypeTree, pinfo, pname) <- args.lazyZip(polyType.paramInfos).lazyZip((polyType.paramNames)) do
             if pinfo.bounds.hi.hasAnnotation(defn.Caps_SealedAnnot) then
-              def where = if fn.symbol.exists then i" in the body of ${fn.symbol}" else ""
+              def where = if fn.symbol.exists then i" in an argument of ${fn.symbol}" else ""
               CheckCaptures.disallowRootCapabilitiesIn(arg.knownType,
-                i"Sealed type variable $pname", " be instantiated to",
+                i"Sealed type variable $pname", "be instantiated to",
                 i"This is often caused by a local capability$where\nleaking as part of its result.",
                 tree.srcPos)
       case _ =>
@@ -346,7 +346,7 @@ extends tpd.TreeTraverser:
               if prevLambdas.isEmpty then restp
               else SubstParams(prevPsymss, prevLambdas)(restp)
 
-        if tree.tpt.hasRememberedType && !sym.isConstructor then
+        if sym.exists && tree.tpt.hasRememberedType && !sym.isConstructor then
           val newInfo = integrateRT(sym.info, sym.paramSymss, Nil, Nil)
             .showing(i"update info $sym: ${sym.info} --> $result", capt)
           if newInfo ne sym.info then

tests/neg-custom-args/captures/try.check

@@ -1,9 +1,9 @@
 -- Error: tests/neg-custom-args/captures/try.scala:23:16 ---------------------------------------------------------------
 23 |  val a = handle[Exception, CanThrow[Exception]] { // error
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |          Sealed type variable R cannot  be instantiated to box CT[Exception]^ since
+   |          Sealed type variable R cannot be instantiated to box CT[Exception]^ since
    |          that type captures the root capability `cap`.
-   |          This is often caused by a local capability in the body of method handle
+   |          This is often caused by a local capability in an argument of method handle
    |          leaking as part of its result.
 -- [E007] Type Mismatch Error: tests/neg-custom-args/captures/try.scala:29:43 ------------------------------------------
 29 |  val b = handle[Exception, () -> Nothing] {  // error
@@ -31,7 +31,7 @@
 -- Error: tests/neg-custom-args/captures/try.scala:35:11 ---------------------------------------------------------------
 35 |  val xx = handle {  // error
    |           ^^^^^^
-   |           Sealed type variable R cannot  be instantiated to box () => Int since
+   |           Sealed type variable R cannot be instantiated to box () => Int since
    |           that type captures the root capability `cap`.
-   |           This is often caused by a local capability in the body of method handle
+   |           This is often caused by a local capability in an argument of method handle
    |           leaking as part of its result.

tests/neg-custom-args/captures/usingLogFile-alt.check

@@ -1,7 +1,7 @@
 -- Error: tests/neg-custom-args/captures/usingLogFile-alt.scala:18:2 ---------------------------------------------------
 18 |  usingFile(  // error
    |  ^^^^^^^^^
-   |  Sealed type variable T cannot  be instantiated to box () => Unit since
+   |  Sealed type variable T cannot be instantiated to box () => Unit since
    |  that type captures the root capability `cap`.
-   |  This is often caused by a local capability in the body of method usingFile
+   |  This is often caused by a local capability in an argument of method usingFile
    |  leaking as part of its result.

tests/neg-custom-args/captures/usingLogFile.check

@@ -13,16 +13,16 @@
 -- Error: tests/neg-custom-args/captures/usingLogFile.scala:23:14 ------------------------------------------------------
 23 |  val later = usingLogFile { f => () => f.write(0) } // error
    |              ^^^^^^^^^^^^
-   |              Sealed type variable T cannot  be instantiated to box () => Unit since
+   |              Sealed type variable T cannot be instantiated to box () => Unit since
    |              that type captures the root capability `cap`.
-   |              This is often caused by a local capability in the body of method usingLogFile
+   |              This is often caused by a local capability in an argument of method usingLogFile
    |              leaking as part of its result.
 -- Error: tests/neg-custom-args/captures/usingLogFile.scala:28:23 ------------------------------------------------------
 28 |  private val later2 = usingLogFile { f => Cell(() => f.write(0)) } // error
    |                       ^^^^^^^^^^^^
-   |                       Sealed type variable T cannot  be instantiated to box Test2.Cell[() => Unit]^? since
+   |                       Sealed type variable T cannot be instantiated to box Test2.Cell[() => Unit]^? since
    |                       the part () => Unit of that type captures the root capability `cap`.
-   |                       This is often caused by a local capability in the body of method usingLogFile
+   |                       This is often caused by a local capability in an argument of method usingLogFile
    |                       leaking as part of its result.
 -- Error: tests/neg-custom-args/captures/usingLogFile.scala:47:6 -------------------------------------------------------
 47 |  val later = usingLogFile { f => () => f.write(0) } // error
@@ -34,14 +34,14 @@
 -- Error: tests/neg-custom-args/captures/usingLogFile.scala:62:16 ------------------------------------------------------
 62 |    val later = usingFile("out", f => (y: Int) => xs.foreach(x => f.write(x + y))) // error
    |                ^^^^^^^^^
-   |                Sealed type variable T cannot  be instantiated to box (x$0: Int) => Unit since
+   |                Sealed type variable T cannot be instantiated to box (x$0: Int) => Unit since
    |                that type captures the root capability `cap`.
-   |                This is often caused by a local capability in the body of method usingFile
+   |                This is often caused by a local capability in an argument of method usingFile
    |                leaking as part of its result.
 -- Error: tests/neg-custom-args/captures/usingLogFile.scala:71:16 ------------------------------------------------------
 71 |    val later = usingFile("logfile",            // error
    |                ^^^^^^^^^
-   |                Sealed type variable T cannot  be instantiated to box () => Unit since
+   |                Sealed type variable T cannot be instantiated to box () => Unit since
    |                that type captures the root capability `cap`.
-   |                This is often caused by a local capability in the body of method usingFile
+   |                This is often caused by a local capability in an argument of method usingFile
    |                leaking as part of its result.

tests/neg-custom-args/captures/vars.check

@@ -20,7 +20,7 @@
 -- Error: tests/neg-custom-args/captures/vars.scala:32:2 ---------------------------------------------------------------
 32 |  local { cap3 => // error
    |  ^^^^^
-   |  Sealed type variable T cannot  be instantiated to box (x$0: String) => String since
+   |  Sealed type variable T cannot be instantiated to box (x$0: String) => String since
    |  that type captures the root capability `cap`.
-   |  This is often caused by a local capability in the body of method local
+   |  This is often caused by a local capability in an argument of method local
    |  leaking as part of its result.