Deprecation waring for trailing _ to force eta expansion

This is part of #18867

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -2993,26 +2993,28 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         }
     }
     nestedCtx.typerState.commit()
-    if sourceVersion.isAtLeast(future) then
-      lazy val (prefix, suffix) = res match {
-        case Block(mdef @ DefDef(_, vparams :: Nil, _, _) :: Nil, _: Closure) =>
-          val arity = vparams.length
-          if (arity > 0) ("", "") else ("(() => ", "())")
-        case _ =>
-          ("(() => ", ")")
-      }
-      def remedy =
-        if ((prefix ++ suffix).isEmpty) "simply leave out the trailing ` _`"
-        else s"use `$prefix<function>$suffix` instead"
-      report.errorOrMigrationWarning(
-        em"""The syntax `<function> _` is no longer supported;
-            |you can $remedy""",
-        tree.srcPos,
-        from = future)
-      if sourceVersion.isMigrating then
-        patch(Span(tree.span.start), prefix)
-        patch(Span(qual.span.end, tree.span.end), suffix)
-    end if
+
+    lazy val (prefix, suffix) = res match {
+      case Block(mdef @ DefDef(_, vparams :: Nil, _, _) :: Nil, _: Closure) =>
+        val arity = vparams.length
+        if (arity > 0) ("", "") else ("(() => ", "())")
+      case _ =>
+        ("(() => ", ")")
+    }
+    def remedy =
+      if ((prefix ++ suffix).isEmpty) "simply leave out the trailing ` _`"
+      else s"use `$prefix<function>$suffix` instead"
+    def rewrite = Message.rewriteNotice("This construct", `3.4-migration`)
+    report.gradualErrorOrMigrationWarning(
+      em"""The syntax `<function> _` is no longer supported;
+          |you can $remedy$rewrite""",
+      tree.srcPos,
+      warnFrom = `3.4`,
+      errorFrom = future)
+    if sourceVersion.isMigrating && sourceVersion.isAtLeast(`3.4-migration`) then
+      patch(Span(tree.span.start), prefix)
+      patch(Span(qual.span.end, tree.span.end), suffix)
+
     res
   }
 

tests/neg/i18867-3.4.scala

@@ -0,0 +1,7 @@
+//> using options -Werror
+
+import language.`3.4`
+
+def foo(x: Int) = x
+
+def test = foo _ // error

tests/neg/i18867.check

@@ -0,0 +1,6 @@
+-- Error: tests/neg/i18867.scala:5:15 ----------------------------------------------------------------------------------
+5 |def test = foo _ // error
+  |           ^^^^^
+  |           The syntax `<function> _` is no longer supported;
+  |           you can simply leave out the trailing ` _`
+  |           This construct can be rewritten automatically under -rewrite -source 3.4-migration.

tests/neg/i18867.scala

@@ -0,0 +1,5 @@
+//> using options -Werror
+
+def foo(x: Int) = x
+
+def test = foo _ // error

tests/pos/i18867-3.3.scala

@@ -0,0 +1,7 @@
+//> using options -Werror
+
+import language.`3.3`
+
+def foo(x: Int) = x
+
+def test = foo _

tests/pos/i18867-3.4.scala

@@ -0,0 +1,5 @@
+import language.`3.4`
+
+def foo(x: Int) = x
+
+def test = foo _ // warn

tests/rewrites/rewrites3x.check

@@ -8,3 +8,6 @@ def test =
 def g = { implicit (x: Int) =>
   x + 1
 }
+
+def foo(x: Int) = x
+def testTrailingUnderscoreEtaExpansion = foo

tests/rewrites/rewrites3x.scala

@@ -7,4 +7,7 @@ def test =
 
 def g = { implicit x: Int =>
   x + 1
-}
+}
+
+def foo(x: Int) = x
+def testTrailingUnderscoreEtaExpansion = foo _