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.

1 files changed, 7 insertions, 4 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/Macros.scala b/src/compiler/scala/tools/nsc/typechecker/Macros.scala
index 57358f72f5..27920dbd74 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Macros.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Macros.scala
@@ -678,7 +678,7 @@ 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)
@@ -686,9 +686,12 @@ trait Macros extends FastTrack with MacroRuntimes with Traces with Helpers {
         // 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
       }
     }