Lint function arrow intended context function (scala#23847)

Fixes scala#21187

If a function literal `x => body` has an expected type `X ?=> ?` then
maybe they intended to write `x ?=> body`.

As shown in the test, maybe types will misalign in other ways to emit
warnings.

Author: som-snytt

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

@@ -176,6 +176,7 @@ private sealed trait WarningSettings:
   private val WenumCommentDiscard = BooleanSetting("-Wenum-comment-discard", "Warn when a comment ambiguously assigned to multiple enum cases is discarded.")
   private val WtoStringInterpolated = BooleanSetting("-Wtostring-interpolated", "Warn a standard interpolator used toString on a reference type.")
   private val WrecurseWithDefault = BooleanSetting("-Wrecurse-with-default", "Warn when a method calls itself with a default argument.")
+  private val WwrongArrow = BooleanSetting("-Wwrong-arrow", "Warn if function arrow was used instead of context literal ?=>.")
   private val Wunused: Setting[List[ChoiceWithHelp[String]]] = MultiChoiceHelpSetting(
     name = "-Wunused",
     helpArg = "warning",
@@ -295,6 +296,7 @@ private sealed trait WarningSettings:
     def toStringInterpolated(using Context): Boolean = allOr(WtoStringInterpolated)
     def recurseWithDefault(using Context): Boolean = allOr(WrecurseWithDefault)
     def checkInit(using Context): Boolean = allOr(YcheckInit)
+    def wrongArrow(using Context): Boolean = allOr(WwrongArrow)
 
 /** -X "Extended" or "Advanced" settings */
 private sealed trait XSettings:

compiler/src/dotty/tools/dotc/transform/init/Checker.scala

@@ -46,7 +46,7 @@ class Checker extends Phase:
     cancellable {
       val classes = traverser.getClasses()
 
-      if ctx.settings.Whas.checkInit then
+      if ctx.settings.Whas.safeInit then
         Semantic.checkClasses(classes)(using checkCtx)
     }
 

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

@@ -3283,6 +3283,16 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     val ifun = desugar.makeContextualFunction(paramTypes, 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.wrongArrow =>
+          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` */

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

@@ -132,7 +132,7 @@ class CompilationTests {
       compileFilesInDir("tests/neg-deep-subtype", allowDeepSubtypes),
       compileFilesInDir("tests/neg-custom-args/captures", defaultOptions.and("-language:experimental.captureChecking")),
       compileFile("tests/neg-custom-args/sourcepath/outer/nested/Test1.scala", defaultOptions.and("-sourcepath", "tests/neg-custom-args/sourcepath")),
-      compileDir("tests/neg-custom-args/sourcepath2/hi", defaultOptions.and("-sourcepath", "tests/neg-custom-args/sourcepath2", "-Xfatal-warnings")),
+      compileDir("tests/neg-custom-args/sourcepath2/hi", defaultOptions.and("-sourcepath", "tests/neg-custom-args/sourcepath2", "-Werror")),
       compileList("duplicate source", List(
         "tests/neg-custom-args/toplevel-samesource/S.scala",
         "tests/neg-custom-args/toplevel-samesource/nested/S.scala"),
@@ -214,7 +214,7 @@ class CompilationTests {
     compileFilesInDir("tests/init/neg", options).checkExpectedErrors()
     compileFilesInDir("tests/init/warn", defaultOptions.and("-Ysafe-init")).checkWarnings()
     compileFilesInDir("tests/init/pos", options).checkCompile()
-    compileFilesInDir("tests/init/crash", options.without("-Xfatal-warnings")).checkCompile()
+    compileFilesInDir("tests/init/crash", options.without("-Werror")).checkCompile()
     // The regression test for i12128 has some atypical classpath requirements.
     // The test consists of three files: (a) Reflect_1  (b) Macro_2  (c) Test_3
     // which must be compiled separately. In addition:
@@ -223,7 +223,7 @@ class CompilationTests {
     //   - the output from (a) _must not_ be on the classpath while compiling (c)
     locally {
       val i12128Group = TestGroup("checkInit/i12128")
-      val i12128Options = options.without("-Xfatal-warnings")
+      val i12128Options = options.without("-Werror")
       val outDir1 = defaultOutputDir + i12128Group + "/Reflect_1/i12128/Reflect_1"
       val outDir2 = defaultOutputDir + i12128Group + "/Macro_2/i12128/Macro_2"
 
@@ -242,7 +242,7 @@ class CompilationTests {
      * an error when reading the files' TASTy trees. */
     locally {
       val tastyErrorGroup = TestGroup("checkInit/tasty-error/val-or-defdef")
-      val tastyErrorOptions = options.without("-Xfatal-warnings")
+      val tastyErrorOptions = options.without("-Werror")
 
       val classA0 = defaultOutputDir + tastyErrorGroup + "/A/v0/A"
       val classA1 = defaultOutputDir + tastyErrorGroup + "/A/v1/A"
@@ -265,7 +265,7 @@ class CompilationTests {
      * an error when reading the files' TASTy trees. This fact is demonstrated by the compilation of Main. */
     locally {
       val tastyErrorGroup = TestGroup("checkInit/tasty-error/typedef")
-      val tastyErrorOptions = options.without("-Xfatal-warnings").without("-Ycheck:all")
+      val tastyErrorOptions = options.without("-Werror").without("-Ycheck:all")
 
       val classC = defaultOutputDir + tastyErrorGroup + "/C/typedef/C"
       val classA0 = defaultOutputDir + tastyErrorGroup + "/A/v0/A"

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"))