Fixes scala#18123

Author: julienrf

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

@@ -106,16 +106,18 @@ object ProtoTypes {
       if !res then ctx.typerState.constraint = savedConstraint
       res
 
-    /** Constrain result with special case if `meth` is a transparent inlineable method in an inlineable context.
-     *  In that case, we should always succeed and not constrain type parameters in the expected type,
-     *  because the actual return type can be a subtype of the currently known return type.
+    /** Constrain result with special case if `meth` is an inlineable method in an inlineable context.
+     *  In that case, if the method is transparent we should always succeed and not constrain type
+     *  parameters in the expected type, because the actual return type can be a subtype of the currently
+     *  known return type.
      *  However, we should constrain parameters of the declared return type. This distinction is
      *  achieved by replacing expected type parameters with wildcards.
      */
     def constrainResult(meth: Symbol, mt: Type, pt: Type)(using Context): Boolean =
-      if (Inlines.isInlineable(meth) && meth.is(Transparent)) {
-        constrainResult(mt, wildApprox(pt))
-        true
+      if (Inlines.isInlineable(meth)) {
+        val methodMatchedType = constrainResult(mt, wildApprox(pt))
+        // do not constrain the result type of transparent inline methods
+        meth.is(Transparent) || methodMatchedType
       }
       else constrainResult(mt, pt)
   }

tests/pos-macros/i18123.scala

@@ -0,0 +1,24 @@
+package pkg
+
+trait P[+T]
+
+extension [T](inline parse0: P[T])
+  inline def | [V >: T](inline other: P[V]): P[V] = ???
+
+extension [T](inline parse0: => P[T])
+  inline def rep[V](inline min: Int = 0)(using repeater: Implicits.Repeater[T, V]): P[V] = ???
+
+object Implicits:
+  trait Repeater[-T, R]
+  object Repeater:
+    implicit def GenericRepeaterImplicit[T]: Repeater[T, Seq[T]] = ???
+
+sealed trait RegexTree
+abstract class Node extends RegexTree
+class CharClassIntersection() extends Node
+
+def classItem: P[RegexTree] = ???
+def charClassIntersection: P[CharClassIntersection] = ???
+
+def x =
+  (charClassIntersection.rep() | classItem.rep())