From 2064c89591ad8d1c82cd4669d2e9192a81046a06 Mon Sep 17 00:00:00 2001
From: Som Snytt <som.snytt@gmail.com>
Date: Mon, 20 Feb 2023 23:39:00 -0800
Subject: [PATCH] Parens for params

---
 .../scala/tools/nsc/ast/parser/Parsers.scala         | 12 ++++++++++--
 src/compiler/scala/tools/nsc/settings/Warnings.scala |  2 ++
 test/files/neg/parens-for-params-3.check             |  4 ++++
 test/files/neg/parens-for-params-3.scala             | 10 ++++++++++
 test/files/neg/parens-for-params.check               |  6 ++++++
 test/files/neg/parens-for-params.scala               | 10 ++++++++++
 6 files changed, 42 insertions(+), 2 deletions(-)
 create mode 100644 test/files/neg/parens-for-params-3.check
 create mode 100644 test/files/neg/parens-for-params-3.scala
 create mode 100644 test/files/neg/parens-for-params.check
 create mode 100644 test/files/neg/parens-for-params.scala

diff --git a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala b/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
index ab125e793582..af6443d3afca 100644
--- a/src/compiler/scala/tools/nsc/ast/parser/Parsers.scala
+++ b/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. */
diff --git a/src/compiler/scala/tools/nsc/settings/Warnings.scala b/src/compiler/scala/tools/nsc/settings/Warnings.scala
index 94f9a49c9604..d9497a0d1368 100644
--- a/src/compiler/scala/tools/nsc/settings/Warnings.scala
+++ b/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.
diff --git a/test/files/neg/parens-for-params-3.check b/test/files/neg/parens-for-params-3.check
new file mode 100644
index 000000000000..c4d1d99d4af9
--- /dev/null
+++ b/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
diff --git a/test/files/neg/parens-for-params-3.scala b/test/files/neg/parens-for-params-3.scala
new file mode 100644
index 000000000000..93ac43e199cf
--- /dev/null
+++ b/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
+  }
+}
diff --git a/test/files/neg/parens-for-params.check b/test/files/neg/parens-for-params.check
new file mode 100644
index 000000000000..e6477fe1c727
--- /dev/null
+++ b/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
diff --git a/test/files/neg/parens-for-params.scala b/test/files/neg/parens-for-params.scala
new file mode 100644
index 000000000000..435cb967269a
--- /dev/null
+++ b/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
+  }
+}