Add an adaptation step in Inliner

We sometimes face a problem that we inline a reference `x: T` which upon further inlining
is adapted to an expected type `x`. It only seems to occur in complicated scenarios. I could
not completely narrow it down. But in any case it's safe to drop the widening cast in order
to avoid a type error here. We do that in a last-effort adaptation step that's only enabled
in the Inliner: Faced with an expression `x: T` and a singleton expected type `y.type` where
`x.type <: y.type`, rewrite to `x`.

compiler/src/dotty/tools/dotc/inlines/Inliner.scala

@@ -957,6 +957,12 @@ class Inliner(val call: tpd.Tree)(using Context):
             case None => tree
         case _ =>
           tree
+
+    /** For inlining only: Given `(x: T)` with expected type `x.type`, replace the tree with `x`.
+     */
+    override def healAdapt(tree: Tree, pt: Type)(using Context): Tree = (tree, pt) match
+      case (Typed(tree1, _), pt: SingletonType) if tree1.tpe <:< pt => tree1
+      case _ => tree
   end InlineTyper
 
   /** Drop any side-effect-free bindings that are unused in expansion or other reachable bindings.

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

@@ -4602,7 +4602,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
 
       def recover(failure: SearchFailureType) =
         if canDefineFurther(wtp) || canDefineFurther(pt) then readapt(tree)
-        else err.typeMismatch(tree, pt, failure)
+        else
+          val tree1 = healAdapt(tree, pt)
+          if tree1 ne tree then readapt(tree1)
+          else err.typeMismatch(tree, pt, failure)
 
       pt match
         case _: SelectionProto =>
@@ -4751,6 +4754,11 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     }
   }
 
+  /** Hook for inheriting Typers to do a last-effort adaptation. If a different
+   *  tree is returned, we will readpat that one, ptherwise we issue a type error afterwards.
+   */
+  protected def healAdapt(tree: Tree, pt: Type)(using Context): Tree = tree
+
   /** True if this inline typer has already issued errors */
   def hasInliningErrors(using Context): Boolean = false
 

tests/pos/i21413.scala

@@ -0,0 +1,2 @@
+val x = (aaa = 1).aaa
+//val y = x.aaa