draft: better error for impure lambda in stmt position

i10416 · i10416 · commit a953135b25ba · 2024-01-27T05:17:32.000+09:00
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -4409,10 +4409,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       case _ =>
 
   private def checkStatementPurity(tree: tpd.Tree)(original: untpd.Tree, exprOwner: Symbol, isUnitExpr: Boolean = false)(using Context): Unit =
+    val isPure = isPureExpr(tree)
     if !tree.tpe.isErroneous
       && !ctx.isAfterTyper
       && !tree.isInstanceOf[Inlined]
-      && isPureExpr(tree)
       && !isSelfOrSuperConstrCall(tree)
     then tree match
       case closureDef(meth)
@@ -4425,11 +4425,16 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         // if the original tree was a lambda. This does not work always either since
         // sometimes we do not have the original anymore and use the transformed tree instead.
         // But taken together, the two criteria are quite accurate.
-        missingArgs(tree, tree.tpe.widen)
+        if isPure then
+          missingArgs(tree, tree.tpe.widen)
+        else
+          report.warning(UnusedNonUnitValue(tree.tpe))
       case _ if isUnitExpr =>
-        report.warning(PureUnitExpression(original, tree.tpe), original.srcPos)
-      case _ =>
-        report.warning(PureExpressionInStatementPosition(original, exprOwner), original.srcPos)
+        if isPure then
+          report.warning(PureUnitExpression(original, tree.tpe), original.srcPos)
+      case _  =>
+        if isPure then
+          report.warning(PureExpressionInStatementPosition(original, exprOwner), original.srcPos)
 
   /** Types the body Scala 2 macro declaration `def f = macro <body>` */
   protected def typedScala2MacroBody(call: untpd.Tree)(using Context): Tree =
diff --git a/tests/neg/i19266.check b/tests/neg/i19266.check
@@ -0,0 +1,12 @@
+-- [E178] Type Error: tests/neg/i19266.scala:13:4 ----------------------------------------------------------------------
+13 |    fn2(2) // error
+   |    ^^^^^^
+   |    missing argument list for value of type String => Some[String]
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E178] Type Error: tests/neg/i19266.scala:19:4 ----------------------------------------------------------------------
+19 |    fn3(4) // error
+   |    ^^^^^^
+   |    missing argument list for value of type Double => Some[String]
+   |
+   | longer explanation available when compiling with `-explain`
diff --git a/tests/neg/i19266.scala b/tests/neg/i19266.scala
@@ -0,0 +1,20 @@
+object i19266:
+  def fn1(x: Int, y: String = "y")(z: Double) = Some(s"$x$y$z")
+  def fn2(p: Int)(q: String) = Some(s"$p$q")
+  def fn3(x: Int, y: => String = "y")(z: Double) = Some(s"$x$y$z")
+
+  def checkCompile =
+    // It compiles because default value for by-value
+    // parameter is impure and may perform side effect.
+    fn1(1) // warn
+    // This does not compile because (pure) synthesized lambda from
+    // eta-expansion in statement position is prohibited.
+    // See https://github.com/lampepfl/dotty/pull/11769
+    fn2(2) // error
+    // This compiles.
+    val a = fn2(3)
+    // This does not compile because default value for by-name parameter
+    // is still pure. Thus, it also violates the rule for lambda in 
+    // statement position.
+    fn3(4) // error
+    1
diff --git a/tests/pos/i19266.scala b/tests/pos/i19266.scala
