Fix scala#2412: Local error causes spurious errors to be reported

`FunProto#typedArgs` was using the implicit Context from the `FunProto`
constructor, and thus errors weren't stored in the proper context.

Author: smarter

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

@@ -1342,7 +1342,7 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
           if (isDetermined(alts2)) alts2
           else {
             pretypeArgs(alts2, pt)
-            narrowByTrees(alts2, pt.typedArgs, resultType)
+            narrowByTrees(alts2, pt.typedArgs(ctx.addMode(Mode.ImplicitsEnabled)), resultType)
           }
         }
 

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

@@ -227,7 +227,7 @@ object ProtoTypes {
     /** The typed arguments. This takes any arguments already typed using
      *  `typedArg` into account.
      */
-    def typedArgs: List[Tree] = {
+    def typedArgs(implicit ctx: Context): List[Tree] = {
       if (myTypedArgs.size != args.length)
         myTypedArgs = args.mapconserve(cacheTypedArg(_, typer.typed(_)))
       myTypedArgs
@@ -294,7 +294,7 @@ object ProtoTypes {
    *  [](args): resultType, where args are known to be typed
    */
   class FunProtoTyped(args: List[tpd.Tree], resultType: Type, typer: Typer)(implicit ctx: Context) extends FunProto(args, resultType, typer)(ctx) {
-    override def typedArgs = args
+    override def typedArgs(implicit ctx: Context) = args
   }
 
   /** A prototype for implicitly inferred views:

tests/pos/i2412.scala

@@ -0,0 +1,8 @@
+object Test {
+  def test(foo: List[String]): Unit = {
+    foo.filter(f => {
+      iDontExist // error: not found
+      true
+    })
+  }
+}