Show synthesizd tree for type mismatch errors in CC

In capture checking, a type mismatch might happen for a synthesized tree
(say from implicit search). It can be helpful to show this tree. Before
capture checking a helpful message is less likely since we don't synthesize
implicit trees that then don't match under the standard typing rules.

Author: odersky

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

@@ -1581,7 +1581,7 @@ class CheckCaptures extends Recheck, SymTransformer:
      *  where local capture roots are instantiated to root variables.
      */
     override def checkConformsExpr(actual: Type, expected: Type, tree: Tree, notes: List[Note])(using Context): Type =
-      try testAdapted(actual, expected, tree, notes: List[Note])(err.typeMismatch)
+      try testAdapted(actual, expected, tree, notes)(err.typeMismatch)
       catch case ex: AssertionError =>
         println(i"error while checking $tree: $actual against $expected")
         throw ex

compiler/src/dotty/tools/dotc/typer/ErrorReporting.scala

@@ -14,6 +14,7 @@ import config.Feature
 import reporting.*
 import Message.Note
 import collection.mutable
+import cc.isCaptureChecking
 
 object ErrorReporting {
 
@@ -190,7 +191,13 @@ object ErrorReporting {
         case _ =>
           Nil
 
-      errorTree(tree, TypeMismatch(treeTp, expectedTp, Some(tree), notes ++ missingElse))
+      def badTreeNote =
+        val span = tree.span
+        if tree.span.isZeroExtent && isCaptureChecking then
+          Note(i"\n\nThe error occurred for a synthesized tree:  $tree") :: Nil
+        else Nil
+
+      errorTree(tree, TypeMismatch(treeTp, expectedTp, Some(tree), notes ++ missingElse ++ badTreeNote))
     }
 
     /** A subtype log explaining why `found` does not conform to `expected` */

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

@@ -45,6 +45,8 @@
    |
    |  Note that capability cap is not included in capture set {cap²}.
    |
+   |  The error occurred for a synthesized tree:  _
+   |
    |  where:    ^ and cap² refer to the universal root capability
    |            ^² and cap refer to a fresh root capability created in package <empty>
  6 |      boundary[Unit]: l2 ?=>