Merge pull request scala#4380 from retronym/ticket/9020

SI-9020 Avoid spurious value discarding warning post-typer

Author: retronym

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -1039,11 +1039,11 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
           // to non-continuation types.
           if (tree.tpe <:< AnyTpe) pt.dealias match {
             case TypeRef(_, UnitClass, _) => // (12)
-              if (settings.warnValueDiscard)
+              if (!isPastTyper && settings.warnValueDiscard)
                 context.warning(tree.pos, "discarded non-Unit value")
               return typedPos(tree.pos, mode, pt)(Block(List(tree), Literal(Constant(()))))
             case TypeRef(_, sym, _) if isNumericValueClass(sym) && isNumericSubType(tree.tpe, pt) =>
-              if (settings.warnNumericWiden)
+              if (!isPastTyper && settings.warnNumericWiden)
                 context.warning(tree.pos, "implicit numeric widening")
               return typedPos(tree.pos, mode, pt)(Select(tree, "to" + sym.name))
             case _ =>

test/files/pos/t9020.flags

@@ -0,0 +1 @@
+-Ywarn-value-discard -Xfatal-warnings

test/files/pos/t9020.scala

@@ -0,0 +1,10 @@
+trait ValueDiscard[@specialized U] {
+  def u: U
+}
+/* Was:
+scalac-hash v2.11.5 -Ywarn-value-discard test/files/pos/t9020.scala
+test/files/pos/t9020.scala:2: warning: discarded non-Unit value
+  def u: U
+      ^
+one warning found
+*/