Fix insertion of using in applications with trailing lambda syntax

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

@@ -130,14 +130,24 @@ trait Migrations:
   def implicitParams(tree: Tree, tp: MethodOrPoly, pt: FunProto)(using Context): Unit =
     val mversion = mv.ImplicitParamsWithoutUsing
     if tp.companion == ImplicitMethodType && pt.applyKind != ApplyKind.Using && pt.args.nonEmpty then
-      val rewriteMsg = Message.rewriteNotice("This code", mversion.patchFrom)
+      // The application can only be rewritten if it uses parentheses syntax.
+      // See issue #22927 and related tests.
+      val hasParentheses =
+        ctx.source.content
+          .slice(tree.span.end, pt.args.head.span.start)
+          .exists(_ == '(')
+      val rewriteMsg =
+        if hasParentheses then
+          Message.rewriteNotice("This code", mversion.patchFrom)
+        else
+          ""
       report.errorOrMigrationWarning(
         em"""Implicit parameters should be provided with a `using` clause.$rewriteMsg
             |To disable the warning, please use the following option: 
             |  "-Wconf:msg=Implicit parameters should be provided with a `using` clause:s"
             |""", 
         pt.args.head.srcPos, mversion)
-      if mversion.needsPatch then
+      if hasParentheses && mversion.needsPatch then
         patch(Span(pt.args.head.span.start), "using ")
   end implicitParams
 

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -88,7 +88,9 @@ class CompilationTests {
       compileFile("tests/rewrites/ambiguous-named-tuple-assignment.scala", defaultOptions.and("-rewrite", "-source:3.6-migration")),
       compileFile("tests/rewrites/i21382.scala", defaultOptions.and("-indent", "-rewrite")),
       compileFile("tests/rewrites/unused.scala", defaultOptions.and("-rewrite", "-Wunused:all")),
-      compileFile("tests/rewrites/i22440.scala", defaultOptions.and("-rewrite"))
+      compileFile("tests/rewrites/i22440.scala", defaultOptions.and("-rewrite")),
+      compileFile("tests/rewrites/i22731.scala", defaultOptions.and("-rewrite", "-source:3.7-migration")),
+      compileFile("tests/rewrites/i22731b.scala", defaultOptions.and("-rewrite", "-source:3.7-migration")),
     ).checkRewrites()
   }
 

tests/rewrites/i22731.check

@@ -0,0 +1,11 @@
+class Input(x: String)
+
+trait Decoder[T]
+
+object Decoder:
+  inline def apply[T](implicit f: () => Unit): Decoder[T] = ???
+
+object Input:
+  given Decoder[Input] = Decoder { () =>
+    Input("")
+  }

tests/rewrites/i22731.scala

@@ -0,0 +1,13 @@
+class Input(x: String)
+
+trait Decoder[T]
+
+object Decoder {
+  inline def apply[T](implicit f: () => Unit): Decoder[T] = ???
+}
+
+object Input {
+  given Decoder[Input] = Decoder { () =>
+    Input("")
+  }
+}

tests/rewrites/i22731b.check

@@ -0,0 +1,50 @@
+def foo(implicit f: () => Unit): Unit = ???
+def bar(a: Int)(implicit f: () => Unit): Unit = ???
+
+@main def main =
+  // `using` can automatically be added when the application is done with parentheses
+  foo ( using () => 43 )
+  foo ( using () =>
+    val x = 42
+    43
+  )
+  foo( using () =>
+    val x = 42
+    43
+  )
+  foo (using () =>
+    val x = 42
+    43
+  )
+  bar(1) ( using () =>
+    val x = 42
+    43 )
+
+  // `using` cannot automatically be added when the application is done with trailing lambda syntax
+  foo { () => 43 }
+  foo { () => val x = 42; 43 }
+  foo{ () => val x = 42; 43 }
+  foo {() => val x = 42; 43}
+  bar(1) { () =>
+    val x = 42
+    43 }
+  foo: () =>
+    43
+  foo : () =>
+    val x = 42
+    43
+  foo :() =>
+    val x = 42
+    43
+  foo
+  : () =>
+    val x = 42
+    43
+  foo
+  :
+    () =>
+      val x = 42
+      43
+  bar(1) : () =>
+    val x = 42
+    43

tests/rewrites/i22731b.scala

@@ -0,0 +1,50 @@
+def foo(implicit f: () => Unit): Unit = ???
+def bar(a: Int)(implicit f: () => Unit): Unit = ???
+
+@main def main =
+  // `using` can automatically be added when the application is done with parentheses
+  foo ( () => 43 )
+  foo ( () =>
+    val x = 42
+    43
+  )
+  foo( () =>
+    val x = 42
+    43
+  )
+  foo (() =>
+    val x = 42
+    43
+  )
+  bar(1) ( () =>
+    val x = 42
+    43 )
+
+  // `using` cannot automatically be added when the application is done with trailing lambda syntax
+  foo { () => 43 }
+  foo { () => val x = 42; 43 }
+  foo{ () => val x = 42; 43 }
+  foo {() => val x = 42; 43}
+  bar(1) { () =>
+    val x = 42
+    43 }
+  foo: () =>
+    43
+  foo : () =>
+    val x = 42
+    43
+  foo :() =>
+    val x = 42
+    43
+  foo
+  : () =>
+    val x = 42
+    43
+  foo
+  :
+    () =>
+      val x = 42
+      43
+  bar(1) : () =>
+    val x = 42
+    43

tests/warn/i22731.check

@@ -0,0 +1,13 @@
+-- Warning: tests/warn/i22731.scala:5:11 -------------------------------------------------------------------------------
+5 |  foo ( () => 43 ) // warn
+  |        ^^^^^^^^
+  |        Implicit parameters should be provided with a `using` clause.
+  |        This code can be rewritten automatically under -rewrite -source 3.7-migration.
+  |        To disable the warning, please use the following option: 
+  |          "-Wconf:msg=Implicit parameters should be provided with a `using` clause:s"
+-- Warning: tests/warn/i22731.scala:7:6 --------------------------------------------------------------------------------
+7 |  foo { () => 43 } // warn
+  |      ^^^^^^^^^^^^
+  |      Implicit parameters should be provided with a `using` clause.
+  |      To disable the warning, please use the following option: 
+  |        "-Wconf:msg=Implicit parameters should be provided with a `using` clause:s"

tests/warn/i22731.scala

@@ -0,0 +1,7 @@
+def foo(implicit f: () => Unit): Unit = ???
+
+@main def main =
+  // `using` can automatically be added when the application is done with parentheses
+  foo ( () => 43 ) // warn
+  // `using` cannot automatically be added when the application is done with trailing lambda syntax
+  foo { () => 43 } // warn