From 2d6911676d27a483b8dcd006073fdc629410569c Mon Sep 17 00:00:00 2001
From: Jason Zaugg <jzaugg@gmail.com>
Date: Mon, 30 Sep 2019 13:42:01 +1000
Subject: [PATCH] Enable -Xlog-implicits by default for whitebox macro
 exceptions

```
scala> import reflect.macros._, language.experimental.macros; class C1; implicit def c1: C1 = macro c1Impl; def c1Impl(c: whitebox.Context) = {import c.universe._; throw null}
import reflect.macros._
import language.experimental.macros
defined class C1
defined term macro c1: C1
c1Impl: (c: scala.reflect.macros.whitebox.Context)Nothing

scala> implicitly[C1]
<console>:18: c1 is not a valid implicit value for C1 because:
exception typechecking implicit candidate (a whitebox macro): exception during macro expansion:
java.lang.NullPointerException
	at .c1Impl(<console>:11)

       implicitly[C1]
                 ^
<console>:18: this.<c1: error> is not a valid implicit value for C1 because:
exception typechecking implicit candidate (a whitebox macro): exception during macro expansion:
java.lang.NullPointerException
	at .c1Impl(<console>:11)

       implicitly[C1]
                 ^
<console>:18: error: could not find implicit value for parameter e: C1
       implicitly[C1]
                 ^

scala> :quit
```
---
 .../scala/tools/nsc/typechecker/Implicits.scala   | 15 ++++++++++++---
 .../scala/tools/nsc/typechecker/Macros.scala      |  5 ++++-
 2 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
index c26da80b0db3..f38a40d6e3a1 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Implicits.scala
@@ -782,9 +782,18 @@ trait Implicits {
           }
         }
 
-        if (context.reporter.hasErrors)
-          fail("hasMatchingSymbol reported error: " + context.reporter.firstError.get.errMsg)
-        else if (itree3.isErroneous)
+        if (context.reporter.hasErrors) {
+          val msg = context.reporter.firstError.get.errMsg
+          val isTopLevelWhiteboxMacro = info.sym.isMacro && !isBlackbox(info.sym) && msg.contains("during macro expansion") && context.openImplicits.lengthCompare(1) == 0
+          if (isTopLevelWhiteboxMacro) {
+            val saved = settings.XlogImplicits.value
+            if (info.sym.isMacro && !isBlackbox(info.sym) && msg.contains("during macro expansion") && context.openImplicits.lengthCompare(1) == 0) settings.XlogImplicits.value = true
+            try fail("exception typechecking implicit candidate (a whitebox macro): " + msg)
+            finally settings.XlogImplicits.value = saved
+          } else {
+            fail("error typechecking implicit candidate: " + msg)
+          }
+        } else if (itree3.isErroneous)
           fail("error typechecking implicit candidate")
         else if (isLocalToCallsite && !hasMatchingSymbol(itree2))
           fail("candidate implicit %s is shadowed by %s".format(
diff --git a/src/compiler/scala/tools/nsc/typechecker/Macros.scala b/src/compiler/scala/tools/nsc/typechecker/Macros.scala
index c72d6f570a49..1f05d4c3f26a 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Macros.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Macros.scala
@@ -868,7 +868,10 @@ trait Macros extends MacroRuntimes with Traces with Helpers {
               case ex: AbortMacroException => MacroGeneratedAbort(expandee, ex)
               case ex: ControlThrowable => throw ex
               case ex: TypeError => MacroGeneratedTypeError(expandee, ex)
-              case NonFatal(_) => MacroGeneratedException(expandee, realex)
+              case NonFatal(_) =>
+                if (typer.context.isSearchingForImplicitParam)
+                  typer.context.openImplicits
+                MacroGeneratedException(expandee, realex)
               case fatal => throw fatal
             }
         } finally {