blackbox restriction #2: can't guide type inference

When an application of a blackbox macro still has undetermined type
parameters after Scala’s type inference algorithm has finished working,
these type parameters are inferred forcedly, in exactly the same manner
as type inference happens for normal methods.

This makes it impossible for blackbox macros to influence type inference,
prohibiting fundep materialization.

Author: xeno-by

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

@@ -678,17 +678,20 @@ trait Macros extends FastTrack with MacroRuntimes with Traces with Helpers {
         // e.g. for Foo it will be Int :: String :: Boolean :: HNil), there's no way to convey this information
         // to the typechecker. Therefore the typechecker will infer Nothing for L, which is hardly what we want.
         //
-        // =========== THE SOLUTION ===========
+        // =========== THE SOLUTION (ENABLED ONLY FOR WHITEBOX MACROS) ===========
         //
         // To give materializers a chance to say their word before vanilla inference kicks in,
         // we infer as much as possible (e.g. in the example above even though L is hopeless, C still can be inferred to Foo)
         // and then trigger macro expansion with the undetermined type parameters still there.
         // Thanks to that the materializer can take a look at what's going on and react accordingly.
         val shouldInstantiate = typer.context.undetparams.nonEmpty && !mode.inPolyMode
         if (shouldInstantiate) {
-          forced += delayed
-          typer.infer.inferExprInstance(delayed, typer.context.extractUndetparams(), pt, keepNothings = false)
-          macroExpandApply(typer, delayed, mode, pt)
+          if (isBlackbox(expandee)) typer.instantiatePossiblyExpectingUnit(delayed, mode, pt)
+          else {
+            forced += delayed
+            typer.infer.inferExprInstance(delayed, typer.context.extractUndetparams(), pt, keepNothings = false)
+            macroExpandApply(typer, delayed, mode, pt)
+          }
         } else delayed
       }
     }

test/files/neg/macro-blackbox-fundep-materialization.check

@@ -0,0 +1,12 @@
+Test_2.scala:7: Iso.materializeIso is not a valid implicit value for Iso[Test.Foo,L] because:
+hasMatchingSymbol reported error: type mismatch;
+ found   : Iso[Test.Foo,(Int, String, Boolean)]
+ required: Iso[Test.Foo,Nothing]
+Note: (Int, String, Boolean) >: Nothing, but trait Iso is invariant in type U.
+You may wish to define U as -U instead. (SLS 4.5)
+    val equiv = foo(Foo(23, "foo", true))
+                   ^
+Test_2.scala:7: error: could not find implicit value for parameter iso: Iso[Test.Foo,L]
+    val equiv = foo(Foo(23, "foo", true))
+                   ^
+one error found

test/files/neg/macro-blackbox-fundep-materialization.flags

@@ -0,0 +1 @@
+-Xlog-implicits

test/files/neg/macro-blackbox-fundep-materialization/Macros_1.scala

@@ -0,0 +1,39 @@
+import scala.language.experimental.macros
+import scala.reflect.macros.BlackboxContext
+
+trait Iso[T, U] {
+  def to(t : T) : U
+  // def from(u : U) : T
+}
+
+object Iso {
+  implicit def materializeIso[T, U]: Iso[T, U] = macro impl[T, U]
+  def impl[T: c.WeakTypeTag, U: c.WeakTypeTag](c: BlackboxContext): c.Expr[Iso[T, U]] = {
+    import c.universe._
+    import definitions._
+    import Flag._
+
+    val sym = c.weakTypeOf[T].typeSymbol
+    if (!sym.isClass || !sym.asClass.isCaseClass) c.abort(c.enclosingPosition, s"$sym is not a case class")
+    val fields = sym.typeSignature.declarations.toList.collect{ case x: TermSymbol if x.isVal && x.isCaseAccessor => x }
+
+    def mkTpt() = {
+      val core = Ident(TupleClass(fields.length) orElse UnitClass)
+      if (fields.length == 0) core
+      else AppliedTypeTree(core, fields map (f => TypeTree(f.typeSignature)))
+    }
+
+    def mkFrom() = {
+      if (fields.length == 0) Literal(Constant(Unit))
+      else Apply(Ident(newTermName("Tuple" + fields.length)), fields map (f => Select(Ident(newTermName("f")), newTermName(f.name.toString.trim))))
+    }
+
+    val evidenceClass = ClassDef(Modifiers(FINAL), newTypeName("$anon"), List(), Template(
+      List(AppliedTypeTree(Ident(newTypeName("Iso")), List(Ident(sym), mkTpt()))),
+      emptyValDef,
+      List(
+        DefDef(Modifiers(), nme.CONSTRUCTOR, List(), List(List()), TypeTree(), Block(List(Apply(Select(Super(This(tpnme.EMPTY), tpnme.EMPTY), nme.CONSTRUCTOR), List())), Literal(Constant(())))),
+        DefDef(Modifiers(), newTermName("to"), List(), List(List(ValDef(Modifiers(PARAM), newTermName("f"), Ident(sym), EmptyTree))), TypeTree(), mkFrom()))))
+    c.Expr[Iso[T, U]](Block(List(evidenceClass), Apply(Select(New(Ident(newTypeName("$anon"))), nme.CONSTRUCTOR), List())))
+  }
+}

test/files/run/t5923c/Test_2.scala renamed to test/files/neg/macro-blackbox-fundep-materialization/Test_2.scala

@@ -0,0 +1,3 @@
+C(Int)
+C(String)
+C(Nothing)

test/files/run/macro-blackbox-materialization.check

@@ -0,0 +1,16 @@
+// For the full version of the test, take a look at run/t5923a
+
+import scala.reflect.macros.BlackboxContext
+import language.experimental.macros
+
+case class C[T](t: String)
+object C {
+  implicit def foo[T]: C[T] = macro Macros.impl[T]
+}
+
+object Macros {
+  def impl[T: c.WeakTypeTag](c: BlackboxContext) = {
+    import c.universe._
+    reify(C[T](c.literal(weakTypeOf[T].toString).splice))
+  }
+}

test/files/run/macro-blackbox-materialization/Macros_1.scala

@@ -0,0 +1,5 @@
+object Test extends App {
+  println(implicitly[C[Int]])
+  println(implicitly[C[String]])
+  println(implicitly[C[Nothing]])
+}

test/files/run/macro-blackbox-materialization/Test_2.scala

@@ -1,4 +1,4 @@
-import language.experimental.macros
+import scala.language.experimental.macros
 import scala.reflect.macros.WhiteboxContext
 
 trait Iso[T, U] {