draft: better error for impure lambda in stmt position

Author: i10416

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

@@ -4412,7 +4412,6 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     if !tree.tpe.isErroneous
       && !ctx.isAfterTyper
       && !tree.isInstanceOf[Inlined]
-      && isPureExpr(tree)
       && !isSelfOrSuperConstrCall(tree)
     then tree match
       case closureDef(meth)
@@ -4425,7 +4424,10 @@ 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 isPureExpr(tree) then
+          missingArgs(tree, tree.tpe.widen)
+        else
+          report.warning(UnusedNonUnitValue(tree.tpe))
       case _ if isUnitExpr =>
         report.warning(PureUnitExpression(original, tree.tpe), original.srcPos)
       case _ =>

tests/neg/i19266.check

@@ -0,0 +1,18 @@
+-- [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`
+-- [E129] Potential Issue Warning: tests/neg/i19266.scala:9:4 ----------------------------------------------------------
+9 |    fn1(1) // warn
+  |    ^^^
+  |    A pure expression does nothing in statement position
+  |
+  | longer explanation available when compiling with `-explain`

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