SI-9041 Avoid unreported type error with overloading, implicits

If `qual.foo(args)` fails to typecheck, we fall back to
`ImplicitView(qual).foo(args)`. However, if the original type error
stemmed from an overload ambiguity, the tree `Select(qual, 'foo')`
holds onto an error symbol. The fall back attempt just returns an
`Apply` tree containing the erroneous qualifier, as it does not
want to issue cascading type errors.

This commit replaces the error symbol with a `NoSymbol`, which
triggers the second try typechecking to perform overload resolution
again.

A more principled fix might be to more pervasively duplicate trees
before mutating their types and symbols, that this is beyond the
scope of this bug fix.

1 files changed, 10 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index e6fa9a0142..fb47f45bc7 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -825,6 +825,16 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
               }
               orElse { _ =>
                 val resetTree = resetAttrs(original)
+                resetTree match {
+                  case treeInfo.Applied(fun, targs, args) =>
+                    if (fun.symbol != null && fun.symbol.isError)
+                      // SI-9041 Without this, we leak error symbols past the typer!
+                      // because the fallback typechecking notices the error-symbol,
+                      // refuses to re-attempt typechecking, and presumes that someone
+                      // else was responsible for issuing the related type error!
+                      fun.setSymbol(NoSymbol)
+                  case _ =>
+                }
                 debuglog(s"fallback on implicits: ${tree}/$resetTree")
                 val tree1 = typed(resetTree, mode)
                 // Q: `typed` already calls `pluginsTyped` and `adapt`. the only difference here is that