Fix of pos/t2429

This was a hard nut to crack. The problem exemplified by t2429 is that in a situation like

     val x: T = foo(...)

where `foo` needs implicit parameters the expected result type `T` is propagated into the
typechecking of foo(...) and consequently also into the implicit parameter search. This
is often necessary, for instance to instantiate type parameters. But it can lead to overconstrained
searches if in fact the right expansion is

     val x: T = viewToT(foo(...)(implicit params))

where `viewToT` is some implicit conversion. The fix handles that case by re-hecking the
application foo(...) with an empty result type, if the implicit parameter search fails.
But the re-checking is very subtle as is explained in the comment to `TyperState#tryWithFallback`.

1 files changed, 11 insertions, 1 deletions

diff --git a/src/dotty/tools/dotc/typer/Namer.scala b/src/dotty/tools/dotc/typer/Namer.scala
index c3f1dcc81..e9195a072 100644
--- a/src/dotty/tools/dotc/typer/Namer.scala
+++ b/src/dotty/tools/dotc/typer/Namer.scala
@@ -404,8 +404,18 @@ class Namer { typer: Typer =>
         }
     }
 
-    final override def complete(denot: SymDenotation)(implicit ctx: Context) =
+    final override def complete(denot: SymDenotation)(implicit ctx: Context) = {
+      if (completions != noPrinter && ctx.typerState != this.ctx.typerState) {
+        completions.println(completions.getClass.toString)
+        def levels(c: Context): Int =
+          if (c.typerState eq this.ctx.typerState) 0
+          else if (c.typerState == null) -1
+          else if (c.outer.typerState == c.typerState) levels(c.outer)
+          else levels(c.outer) + 1
+        completions.println(s"!!!completing ${denot.symbol.showLocated} in buried typerState, gap = ${levels(ctx)}")
+      }
       completeInCreationContext(denot)
+    }
 
     def completeInCreationContext(denot: SymDenotation): Unit =
       denot.info = typeSig(denot.symbol)