Only strip ascriptions that do not break subtyping

Author: mbovel

community-build/community-projects/requests-scala

@@ -0,0 +1 @@
+Subproject commit 8e4a40588491608aa40099f79c881d54a5094e75

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

@@ -1047,11 +1047,18 @@ class Inliner(val call: tpd.Tree)(using Context):
         reduceInlineMatchExpr(sel)
       }
 
+    private def shouldStripAscription(tree: Typed)(using Context): Boolean =
+      val exprTp = tree.expr.tpe
+      tree.hasAttachment(PrepareInlineable.InlineResultAscription)
+      && exprTp.exists
+      && !exprTp.widen.isRef(defn.NothingClass)
+      && !exprTp.widen.isRef(defn.NullClass)
+      && (exprTp frozen_<:< tree.tpe)
+
     override def typedTyped(tree: untpd.Typed, pt: Type)(using Context): Tree =
-      if tree.hasAttachment(PrepareInlineable.InlineResultAscription) then
-        typedExpr(tree.expr, pt)
-      else
-        super.typedTyped(tree, pt)
+      super.typedTyped(tree, pt) match
+        case typedTree: Typed if shouldStripAscription(typedTree) => typedTree.expr
+        case typedTree => typedTree
 
     override def newLikeThis(nestingLevel: Int): Typer = new InlineTyper(initialErrorCount, nestingLevel)