Merge pull request #3402 from dotty-staging/fix-#3246

Fix #3246: Refine handling of postfix _ for non-functions

Author: allanrenucci

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -527,6 +527,7 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
     def unapply(tree: Tree): Option[(List[Tree], Tree, Tree)] = tree match {
       case Block(_, expr) => unapply(expr)
       case Closure(env, meth, tpt) => Some(env, meth, tpt)
+      case Typed(expr, _)  => unapply(expr)
       case _ => None
     }
   }

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

@@ -1548,17 +1548,21 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
   def typedAsFunction(tree: untpd.PostfixOp, pt: Type)(implicit ctx: Context): Tree = {
     val untpd.PostfixOp(qual, Ident(nme.WILDCARD)) = tree
     val pt1 = if (defn.isFunctionType(pt)) pt else AnyFunctionProto
-    var res = typed(qual, pt1)
-    if (pt1.eq(AnyFunctionProto) && !defn.isFunctionClass(res.tpe.classSymbol)) {
-      ctx.errorOrMigrationWarning(OnlyFunctionsCanBeFollowedByUnderscore(res.tpe), tree.pos)
-      if (ctx.scala2Mode) {
-        // Under -rewrite, patch `x _` to `(() => x)`
-        patch(Position(tree.pos.start), "(() => ")
-        patch(Position(qual.pos.end, tree.pos.end), ")")
-        res = typed(untpd.Function(Nil, untpd.TypedSplice(res)))
-      }
+    val nestedCtx = ctx.fresh.setNewTyperState()
+    val res = typed(qual, pt1)(nestedCtx)
+    res match {
+      case res @ closure(_, _, _) =>
+      case _ =>
+        ctx.errorOrMigrationWarning(OnlyFunctionsCanBeFollowedByUnderscore(res.tpe), tree.pos)
+        if (ctx.scala2Mode) {
+          // Under -rewrite, patch `x _` to `(() => x)`
+          patch(Position(tree.pos.start), "(() => ")
+          patch(Position(qual.pos.end, tree.pos.end), ")")
+          return typed(untpd.Function(Nil, qual), pt)
+        }
     }
-    else if (ctx.settings.strict.value) {
+    nestedCtx.typerState.commit()
+    if (ctx.settings.strict.value) {
       lazy val (prefix, suffix) = res match {
         case Block(mdef @ DefDef(_, _, vparams :: Nil, _, _) :: Nil, _: Closure) =>
           val arity = vparams.length

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

@@ -182,6 +182,7 @@ class CompilationTests extends ParallelTesting {
     compileFile("../tests/neg/tailcall/tailrec.scala", defaultOptions) +
     compileFile("../tests/neg/tailcall/tailrec-2.scala", defaultOptions) +
     compileFile("../tests/neg/tailcall/tailrec-3.scala", defaultOptions) +
+    compileFile("../tests/neg/i3246.scala", scala2Mode) +
     compileDir("../tests/neg/typedIdents", defaultOptions)
   }.checkExpectedErrors()
 

tests/neg/i3246.scala

@@ -0,0 +1,4 @@
+class Test {
+  def foo(x: Int) = 1
+  val bar: () => Int = foo _ // error: type mismatch
+}

tests/pos-scala2/i3246.scala

@@ -0,0 +1,11 @@
+class Test {
+  def foo(x: => Int) = bar(x _)
+  def bar(x: () => Int) = ???
+  def baz = 1
+  def bam: () => Int = baz _
+  def ban: () => Int = 1 _
+
+  def titi(fun: () => Unit) = ???
+  def toto(fun: => Int) = titi(fun _)
+  titi(1 _) // rejected by scalac
+}

tests/pos/i3246.scala

@@ -0,0 +1,8 @@
+class Test {
+  class A {
+    type T = Int
+    def f(x: T): T = ???
+  }
+  def a: A = new A
+  val f = a.f _
+}