changes the order of whitebox typechecks. yes, again.

My first attempt at SI-6992 was about having whitebox expansions first
typecheck against outerPt and only then verify that the result is compatible
with innerPt.

That was a nice try, but soon after it went live in 2.11.0-M8, we've got
multiple reports with problems - both shapeless and then in a week specs2
started having issues with their whitebox macros.

In shapeless, typecheck against outerPt screwed up type inference, which
was more or less fixable by explicit type annotations, so I decided to
wait a bit before jumping to conclusions.

However, in specs2 the problem was more insidious. After being typechecked
against outerPt, expansions were being implicitly converted to a type
that became incompatible with innerPt. This revealed a fatal flaw of the
implemented approach - if allowed to typecheck against outerPt first,
whitebox macros could never be robust.

Now realizing that "outerPt > innerPt" doesn't work, I nevertheless wasn't
looking forward to rolling that back to "innerPt > outerPt", because that
would revive SI-6992 and SI-8048 that are highly unintuitive, especially
the latter one.

Therefore, this commit combines the permissiveness of "... > innerPt"
approaches with the robustness of "innerPt > outerPt", introducing
"WildcardType > innerPt > outerPt".

7 files changed, 47 insertions, 3 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Macros.scala b/src/compiler/scala/tools/nsc/typechecker/Macros.scala
index cf82d6baac..677c94e063 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Macros.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Macros.scala
@@ -620,9 +620,11 @@ trait Macros extends FastTrack with MacroRuntimes with Traces with Helpers {
         val expanded1 = atPos(enclosingMacroPosition.makeTransparent)(Typed(expanded0, TypeTree(innerPt)))
         typecheck("blackbox typecheck", expanded1, outerPt)
       } else {
-        val expanded1 = expanded0
-        val expanded2 = typecheck("whitebox typecheck #1", expanded1, outerPt)
-        typecheck("whitebox typecheck #2", expanded2, innerPt)
+        // whitebox expansions need to be typechecked against WildcardType first in order to avoid SI-6992 and SI-8048
+        // then we typecheck against innerPt, not against outerPt in order to prevent SI-8209
+        val expanded1 = typecheck("whitebox typecheck #0", expanded0, WildcardType)
+        val expanded2 = typecheck("whitebox typecheck #1", expanded1, innerPt)
+        typecheck("whitebox typecheck #2", expanded2, outerPt)
       }
     }
     override def onDelayed(delayed: Tree) = {
diff --git a/test/files/pos/t8209a.check b/test/files/pos/t8209a.check
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/test/files/pos/t8209a.check
diff --git a/test/files/pos/t8209a/Macros_1.scala b/test/files/pos/t8209a/Macros_1.scala
new file mode 100644
index 0000000000..17014b4744
--- /dev/null
+++ b/test/files/pos/t8209a/Macros_1.scala
@@ -0,0 +1,17 @@
+import scala.language.experimental.macros
+import scala.language.implicitConversions
+import scala.reflect.macros.blackbox.Context
+
+class A
+object A { implicit def a2b(a: A): B = ??? }
+class B
+class C extends A
+
+object Macros {
+  def impl(c: Context) = {
+    import c.universe._
+    q"new C"
+  }
+
+  def foo: A = macro impl
+}
\ No newline at end of file
diff --git a/test/files/pos/t8209a/Test_2.scala b/test/files/pos/t8209a/Test_2.scala
new file mode 100644
index 0000000000..e19d572f55
--- /dev/null
+++ b/test/files/pos/t8209a/Test_2.scala
@@ -0,0 +1,4 @@
+object Test extends App {
+  val a: A = Macros.foo
+  val b: B = Macros.foo
+}
\ No newline at end of file
diff --git a/test/files/pos/t8209b.check b/test/files/pos/t8209b.check
new file mode 100644
index 0000000000..e69de29bb2
--- /dev/null
+++ b/test/files/pos/t8209b.check
diff --git a/test/files/pos/t8209b/Macros_1.scala b/test/files/pos/t8209b/Macros_1.scala
new file mode 100644
index 0000000000..705f7d630c
--- /dev/null
+++ b/test/files/pos/t8209b/Macros_1.scala
@@ -0,0 +1,17 @@
+import scala.language.experimental.macros
+import scala.language.implicitConversions
+import scala.reflect.macros.whitebox.Context
+
+class A
+object A { implicit def a2b(a: A): B = ??? }
+class B
+class C extends A
+
+object Macros {
+  def impl(c: Context) = {
+    import c.universe._
+    q"new C"
+  }
+
+  def foo: A = macro impl
+}
\ No newline at end of file
diff --git a/test/files/pos/t8209b/Test_2.scala b/test/files/pos/t8209b/Test_2.scala
new file mode 100644
index 0000000000..e19d572f55
--- /dev/null
+++ b/test/files/pos/t8209b/Test_2.scala
@@ -0,0 +1,4 @@
+object Test extends App {
+  val a: A = Macros.foo
+  val b: B = Macros.foo
+}
\ No newline at end of file