Tighten closure extractor in TreeInfo (#21621)

The previous extractor for closures matches also arbitrary blocks that
ended in a (possible deeply nested) closure. This caused wrong use sets
in #21620. The new definition is stricter. There is also a new
blockEndingInclosure extractor that keeps the old behavior.

Fixes #21620

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

@@ -814,17 +814,31 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
       case _ => false
     }
 
-  /** An extractor for closures, either contained in a block or standalone.
+  /** An extractor for closures, possibly typed, and possibly including the
+   *  definition of the anonymous def.
    */
   object closure {
-    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)
+    def unapply(tree: Tree)(using Context): Option[(List[Tree], Tree, Tree)] = tree match {
+      case Block((meth : DefDef) :: Nil, closure: Closure) if meth.symbol == closure.meth.symbol =>
+        unapply(closure)
+      case Block(Nil, expr) =>
+        unapply(expr)
+      case Closure(env, meth, tpt) =>
+        Some(env, meth, tpt)
+      case Typed(expr, _)  =>
+        unapply(expr)
       case _ => None
     }
   }
 
+  /** An extractor for a closure or a block ending in one. This was
+   *  previously `closure` before that one was tightened.
+   */
+  object blockEndingInClosure:
+    def unapply(tree: Tree)(using Context): Option[(List[Tree], Tree, Tree)] = tree match
+      case Block(_, expr) => unapply(expr)
+      case _ => closure.unapply(tree)
+
   /** An extractor for def of a closure contained the block of the closure. */
   object closureDef {
     def unapply(tree: Tree)(using Context): Option[DefDef] = tree match {

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

@@ -57,7 +57,7 @@ trait Migrations:
     val nestedCtx = ctx.fresh.setNewTyperState()
     val res = typed(qual, pt1)(using nestedCtx)
     res match {
-      case closure(_, _, _) =>
+      case blockEndingInClosure(_, _, _) =>
       case _ =>
         val recovered = typed(qual)(using ctx.fresh.setExploreTyperState())
         val msg = OnlyFunctionsCanBeFollowedByUnderscore(recovered.tpe.widen, tree)

tests/neg-custom-args/captures/i21620.check

@@ -0,0 +1,13 @@
+-- [E129] Potential Issue Warning: tests/neg-custom-args/captures/i21620.scala:5:6 -------------------------------------
+5 |      x
+  |      ^
+  |      A pure expression does nothing in statement position
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E007] Type Mismatch Error: tests/neg-custom-args/captures/i21620.scala:9:31 ----------------------------------------
+9 |  val _: () -> () ->{x} Unit = f // error
+  |                               ^
+  |                               Found:    () ->{f} () ->{x} Unit
+  |                               Required: () -> () ->{x} Unit
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg-custom-args/captures/i21620.scala

@@ -0,0 +1,10 @@
+class C
+def test(x: C^) =
+  val f = () =>
+    def foo() =
+      x
+      ()
+    println(s"hey: $x")
+    () => foo()
+  val _: () -> () ->{x} Unit = f // error
+  ()

tests/pos-custom-args/captures/i21620.scala

@@ -0,0 +1,11 @@
+class C
+def test(x: C^) =
+  def foo() =
+    x
+    ()
+  val f = () =>
+    // println() //  uncomenting would give an error, but with
+                 //  a different way of handling curried functions should be OK
+    () => foo()
+  val _: () -> () ->{x} Unit = f
+  ()