Parens for params

src/compiler/scala/tools/nsc/ast/parser/Parsers.scala

@@ -777,8 +777,16 @@ self =>
 
     /** Convert tree to formal parameter list. */
     def convertToParams(tree: Tree): List[ValDef] = tree match {
-      case Parens(ts) => ts map convertToParam
-      case _          => List(convertToParam(tree))
+      case Parens(ts) => ts.map(convertToParam)
+      case Typed(Ident(_), _) =>
+        val msg = "parentheses are required around the parameter of a lambda"
+        val silencing = s"$msg (Or use '-Wconf:msg=$msg:s' to silence this warning)"
+        if (currentRun.isScala3 && settings.Wsource.value)
+          syntaxError(tree.pos.point, silencing)
+        else if (currentRun.isScala3 || settings.Wsource.value)
+          deprecationWarning(tree.pos.point, silencing, "2.13.11")
+        List(convertToParam(tree))
+      case _ => List(convertToParam(tree))
     }
 
     /** Convert tree to formal parameter. */

src/compiler/scala/tools/nsc/settings/Warnings.scala

@@ -27,6 +27,8 @@ trait Warnings {
   // Warning semantics.
   val fatalWarnings = BooleanSetting("-Werror", "Fail the compilation if there are any warnings.") withAbbreviation "-Xfatal-warnings"
 
+  val Wsource       = BooleanSetting("-Wsource", "Increase warnings/errors due to -Xsource.")
+
   private val WconfDefault = List("cat=deprecation:ws", "cat=feature:ws", "cat=optimizer:ws")
   // Note: user-defined settings are added on the right, but the value is reversed before
   // it's parsed, so that later defined settings take precedence.

test/files/neg/parens-for-params-3.check

@@ -0,0 +1,4 @@
+parens-for-params-3.scala:5: error: parentheses are required around the parameter of a lambda (Or use '-Wconf:msg=parentheses are required around the parameter of a lambda:s' to silence this warning)
+    x: Int => x * 2
+     ^
+1 error

test/files/neg/parens-for-params-3.scala

@@ -0,0 +1,10 @@
+// scalac: -Werror -Wsource -Xsource:3
+
+class C {
+  def f = {
+    x: Int => x * 2
+  }
+  def g = {
+    (x: Int) => x * 2
+  }
+}

test/files/neg/parens-for-params.check

@@ -0,0 +1,6 @@
+parens-for-params.scala:5: warning: parentheses are required around the parameter of a lambda (Or use '-Wconf:msg=parentheses are required around the parameter of a lambda:s' to silence this warning)
+    x: Int => x * 2
+     ^
+error: No warnings can be incurred under -Werror.
+1 warning
+1 error

test/files/neg/parens-for-params.scala

@@ -0,0 +1,10 @@
+// scalac: -Werror -Wsource -Xlint
+
+class C {
+  def f = {
+    x: Int => x * 2
+  }
+  def g = {
+    (x: Int) => x * 2
+  }
+}