SI-3971 error message carat mispoints at curried methods.

Point at the beginning of the first argument list when
reporting an error, as this is most easily associated with
the application taking place (which may involve multiple
applies in succession.)

Thanks to retronym for figuring out why issuing a better
error message broke the compiler on non-erroneous compile runs.
The changes to "treesInResult" are the consequence.

1 files changed, 11 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/ContextErrors.scala b/src/compiler/scala/tools/nsc/typechecker/ContextErrors.scala
index 1f4d5cbac2..f893e4f0ff 100644
--- a/src/compiler/scala/tools/nsc/typechecker/ContextErrors.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/ContextErrors.scala
@@ -157,6 +157,16 @@ trait ContextErrors {
       }
 
       def AdaptTypeError(tree: Tree, found: Type, req: Type) = {
+        // SI-3971 unwrapping to the outermost Apply helps prevent confusion with the
+        // error message point.
+        def callee = {
+          def unwrap(t: Tree): Tree = t match {
+            case Apply(app: Apply, _) => unwrap(app)
+            case _                    => t
+          }
+          unwrap(tree)
+        }
+
         // If the expected type is a refinement type, and the found type is a refinement or an anon
         // class, we can greatly improve the error message by retyping the tree to recover the actual
         // members present, then display along with the expected members. This is done here because
@@ -181,7 +191,7 @@ trait ContextErrors {
         }
         assert(!foundType.isErroneous && !req.isErroneous, (foundType, req))
 
-        issueNormalTypeError(tree, withAddendum(tree.pos)(typeErrorMsg(foundType, req)))
+        issueNormalTypeError(callee, withAddendum(callee.pos)(typeErrorMsg(foundType, req)))
         infer.explainTypes(foundType, req)
       }