SI-8704 Also warn if effectively multiple implicit

Current semantics are that leading implicit param
turns the parameter section into an implicit section
(though without making other params implicitly
implicit).

Warn if more than one head of a param section is
implicit, since that results in multiple implicit
param sections.

5 files changed, 15 insertions, 0 deletions

diff --git a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala b/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
index 45c177f5b9..440e45c577 100644
--- a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
+++ b/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
@@ -2272,6 +2272,12 @@ self =>
         syntaxError(implicitOffset, "an implicit parameter section must be last")
       if (warnAt != -1)
         syntaxError(warnAt, "multiple implicit parameter sections are not allowed")
+      else if (settings.warnExtraImplicit) {
+        // guard against anomalous class C(private implicit val x: Int)(implicit s: String)
+        val ttl = vds.count { case ValDef(mods, _, _, _) :: _ => mods.isImplicit ; case _ => false }
+        if (ttl > 1)
+          warning(in.offset, s"$ttl parameter sections are effectively implicit")
+      }
       val result = vds.toList
       if (owner == nme.CONSTRUCTOR && (result.isEmpty || (result.head take 1 exists (_.mods.isImplicit)))) {
         in.token match {
diff --git a/src/compiler/scala/tools/nsc/settings/Warnings.scala b/src/compiler/scala/tools/nsc/settings/Warnings.scala
index 839e734abc..87534656f9 100644
--- a/src/compiler/scala/tools/nsc/settings/Warnings.scala
+++ b/src/compiler/scala/tools/nsc/settings/Warnings.scala
@@ -25,6 +25,8 @@ trait Warnings {
   // currently considered too noisy for general use
   val warnUnusedImport     = BooleanSetting("-Ywarn-unused-import", "Warn when imports are unused.")
 
+  val warnExtraImplicit    = BooleanSetting("-Ywarn-extra-implicit", "Warn when more than one implicit parameter section is defined.")
+
   // Experimental lint warnings that are turned off, but which could be turned on programmatically.
   // They are not activated by -Xlint and can't be enabled on the command line because they are not
   // created using the standard factory methods.
diff --git a/test/files/neg/t8704.check b/test/files/neg/t8704.check
index ccd6d30818..b567a8bb17 100644
--- a/test/files/neg/t8704.check
+++ b/test/files/neg/t8704.check
@@ -1,7 +1,11 @@
+t8704.scala:7: warning: 2 parameter sections are effectively implicit
+class D(private implicit val i: Int)(implicit s: String)
+                                                        ^
 t8704.scala:3: error: an implicit parameter section must be last
 class C(i: Int)(implicit j: Int)(implicit k: Int)(n: Int) {
                 ^
 t8704.scala:3: error: multiple implicit parameter sections are not allowed
 class C(i: Int)(implicit j: Int)(implicit k: Int)(n: Int) {
                                  ^
+one warning found
 two errors found
diff --git a/test/files/neg/t8704.flags b/test/files/neg/t8704.flags
new file mode 100644
index 0000000000..f175a06c74
--- /dev/null
+++ b/test/files/neg/t8704.flags
@@ -0,0 +1 @@
+-Ywarn-extra-implicit
diff --git a/test/files/neg/t8704.scala b/test/files/neg/t8704.scala
index cfabd18507..db43bfcaa5 100644
--- a/test/files/neg/t8704.scala
+++ b/test/files/neg/t8704.scala
@@ -3,3 +3,5 @@
 class C(i: Int)(implicit j: Int)(implicit k: Int)(n: Int) {
   def f = n
 }
+
+class D(private implicit val i: Int)(implicit s: String)