From 0a416d88b5810b1fc86877c38b17e6c833a4c5e9 Mon Sep 17 00:00:00 2001
From: odersky <odersky@gmail.com>
Date: Thu, 3 Oct 2024 18:31:35 +0200
Subject: [PATCH] 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 |  6 ++++++
 compiler/src/dotty/tools/dotc/typer/Typer.scala     | 10 +++++++++-
 tests/pos/i21413.scala                              |  2 ++
 3 files changed, 17 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos/i21413.scala

diff --git a/compiler/src/dotty/tools/dotc/inlines/Inliner.scala b/compiler/src/dotty/tools/dotc/inlines/Inliner.scala
index 7c79e972c126..103f3aac7630 100644
--- a/compiler/src/dotty/tools/dotc/inlines/Inliner.scala
+++ b/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.
diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index 93ea3f3c3ae0..57bbc3ee98e8 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/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
 
diff --git a/tests/pos/i21413.scala b/tests/pos/i21413.scala
new file mode 100644
index 000000000000..d2dc52e34630
--- /dev/null
+++ b/tests/pos/i21413.scala
@@ -0,0 +1,2 @@
+val x = (aaa = 1).aaa
+//val y = x.aaa
\ No newline at end of file