Change result type of by-name closures

Make it the formal type rather than the actual one. This avoids messing up
capture annotations.

Author: odersky

compiler/src/dotty/tools/dotc/transform/ElimByName.scala

@@ -92,6 +92,7 @@ class ElimByName extends MiniPhase, InfoTransformer:
     sym.is(Method) || exprBecomesFunction(sym)
 
   def byNameClosure(arg: Tree, argType: Type)(using Context): Tree =
+    log(i"creating by name closure for $argType")
     val meth = newAnonFun(ctx.owner, MethodType(Nil, argType), coord = arg.span)
     Closure(meth,
         _ => arg.changeOwnerAfter(ctx.owner, meth, thisPhase),
@@ -135,12 +136,8 @@ class ElimByName extends MiniPhase, InfoTransformer:
             if isByNameRef(qual) && (isPureExpr(qual) || qual.symbol.isAllOf(InlineParam)) =>
               qual
             case _ =>
-              if isByNameRef(arg) || arg.symbol.name.is(SuperArgName)
-              then arg
-              else
-                var argType = arg.tpe.widenIfUnstable
-                if argType.isBottomType then argType = formalResult
-                byNameClosure(arg, argType)
+              if isByNameRef(arg) || arg.symbol.name.is(SuperArgName) then arg
+              else byNameClosure(arg, formalResult)
         case _ =>
           arg
 

tests/neg-custom-args/captures/byname.check

@@ -1,18 +1,18 @@
--- Warning: tests/neg-custom-args/captures/byname.scala:14:18 ----------------------------------------------------------
-14 |  def h(x: {cap1} -> I) = x // warning
+-- Warning: tests/neg-custom-args/captures/byname.scala:17:18 ----------------------------------------------------------
+17 |  def h(x: {cap1} -> I) = x // warning
    |                  ^
    |                  Style: by-name `->` should immediately follow closing `}` of capture set
    |                  to avoid confusion with function type.
    |                  That is, `{c}-> T` instead of `{c} -> T`.
--- [E007] Type Mismatch Error: tests/neg-custom-args/captures/byname.scala:7:5 -----------------------------------------
-7 |  h(f())  // error
-  |    ^^^
-  |    Found:    {cap2} (x$0: Int) -> Int
-  |    Required: Int -> Int
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/byname.scala:10:6 ----------------------------------------
+10 |  h(f2()) // error
+   |    ^^^^
+   |    Found:    {cap1} (x$0: Int) -> Int
+   |    Required: {cap2} Int -> Int
 
 longer explanation available when compiling with `-explain`
--- [E007] Type Mismatch Error: tests/neg-custom-args/captures/byname.scala:16:5 ----------------------------------------
-16 |  h(g()) // error
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/byname.scala:19:5 ----------------------------------------
+19 |  h(g()) // error
    |    ^^^
    |    Found:    {cap2} () ?-> I
    |    Required: {cap1} () ?-> I

tests/neg-custom-args/captures/byname.scala

@@ -3,8 +3,11 @@
 def test(cap1: Cap, cap2: Cap) =
   def f() = if cap1 == cap1 then g else g
   def g(x: Int) = if cap2 == cap2 then 1 else x
+  def g2(x: Int) = if cap1 == cap1 then 1 else x
+  def f2() = if cap1 == cap1 then g2 else g2
   def h(ff: => {cap2} Int -> Int) = ff
-  h(f())  // error
+  h(f())  // ok
+  h(f2()) // error
 
 class I
 

tests/pos-custom-args/captures/lazylists-exceptions.scala

@@ -42,14 +42,17 @@ extension [A](xs: {*} LazyList[A])
 
   def concat(ys: {*} LazyList[A]): {xs, ys} LazyList[A] =
     if xs.isEmpty then ys
-    else LazyCons(xs.head, () => xs.tail.concat(ys))
+    else lazyCons(xs.head, xs.tail.concat(ys))
 end extension
 
+def lazyCons[A](x: A, xs1: => {*} LazyList[A]): {xs1} LazyList[A] =
+  LazyCons(x, () => xs1)
+
 class Ex1 extends Exception
 class Ex2 extends Exception
 
 def test(using cap1: CanThrow[Ex1], cap2: CanThrow[Ex2]) =
-  val xs = LazyCons(1, () => LazyNil)
+  val xs = lazyCons(1, LazyNil)
 
   def f(x: Int): Int throws Ex1 =
     if x < 0 then throw Ex1()