Lint function arrow intended context function

Author: som-snytt

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -168,6 +168,7 @@ private sealed trait WarningSettings:
   private val WunstableInlineAccessors = BooleanSetting(WarningSetting, "WunstableInlineAccessors", "Warn an inline methods has references to non-stable binary APIs.")
   private val WtoStringInterpolated = BooleanSetting(WarningSetting, "Wtostring-interpolated", "Warn a standard interpolator used toString on a reference type.")
   private val WrecurseWithDefault = BooleanSetting(WarningSetting, "Wrecurse-with-default", "Warn when a method calls itself with a default argument.")
+  private val WdubiousContextual = BooleanSetting(WarningSetting, "Wwrong-arrow", "Warn if function arrow was used instead of context literal ?=>.")
   private val Wunused: Setting[List[ChoiceWithHelp[String]]] = MultiChoiceHelpSetting(
     WarningSetting,
     name = "Wunused",
@@ -311,6 +312,7 @@ private sealed trait WarningSettings:
     def unstableInlineAccessors(using Context): Boolean = allOr(WunstableInlineAccessors)
     def toStringInterpolated(using Context): Boolean = allOr(WtoStringInterpolated)
     def recurseWithDefault(using Context): Boolean = allOr(WrecurseWithDefault)
+    def dubiousContextual(using Context): Boolean = allOr(WdubiousContextual)
     def checkInit(using Context): Boolean = allOr(WcheckInit)
 
 /** -X "Extended" or "Advanced" settings */

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

@@ -3824,6 +3824,16 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     val ifun = desugar.makeContextualFunction(paramTypes, paramNamesOrNil, tree, erasedParams)
     typr.println(i"make contextual function $tree / $pt ---> $ifun")
     typedFunctionValue(ifun, pt)
+      .tap:
+        case tree @ Block((m1: DefDef) :: _, _: Closure) if ctx.settings.Whas.dubiousContextual =>
+          m1.rhs match
+          case Block((m2: DefDef) :: _, _: Closure) if m1.paramss.lengthCompare(m2.paramss) == 0 =>
+            val p1s = m1.symbol.info.asInstanceOf[MethodType].paramInfos
+            val p2s = m2.symbol.info.asInstanceOf[MethodType].paramInfos
+            if p1s.corresponds(p2s)(_ =:= _) then
+              report.warning(em"Context function adapts a lambda with the same parameter types, possibly ?=> was intended.", tree.srcPos)
+          case _ =>
+        case _ =>
   }
 
   /** Typecheck and adapt tree, returning a typed tree. Parameters as for `typedUnadapted` */

tests/warn/i21187.scala

@@ -0,0 +1,22 @@
+//> using options -Wall
+
+def oops(msg: String) = sys.error(msg)
+
+class Zone
+object Zone:
+  inline def apply[T](inline f: Zone ?=> T): T = f(using new Zone)
+
+inline def zone[A](inline f: Zone ?=> A) = Zone.apply(z => f(using z)) // warn suspicious contextualizing
+
+def zone_?[A](f: Zone ?=> A) = Zone.apply(z => f(using z)) // warn
+
+// intended
+//inline def zone[A](inline f: Zone ?=> A): A = Zone.apply(z ?=> f(using z))
+
+@main def hello = 
+  // this swallows exceptions!
+  zone(oops("here")) // warn function value is not used
+  zone_?(oops("here")) // warn
+  
+  // this doesn't
+  Zone(oops("not here"))