Tolerate some faults when copying trees in InlineTreeMap

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

@@ -37,7 +37,8 @@ class TreeTypeMap(
   val oldOwners: List[Symbol] = Nil,
   val newOwners: List[Symbol] = Nil,
   val substFrom: List[Symbol] = Nil,
-  val substTo: List[Symbol] = Nil)(using Context) extends tpd.TreeMap {
+  val substTo: List[Symbol] = Nil,
+  cpy: tpd.TreeCopier = tpd.cpy)(using Context) extends tpd.TreeMap(cpy) {
   import tpd._
 
   def copy(

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

@@ -903,6 +903,15 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(using Context) {
     def inlinedFromOutside(tree: Tree)(span: Span): Tree =
       Inlined(EmptyTree, Nil, tree)(using ctx.withSource(inlinedMethod.topLevelClass.source)).withSpan(span)
 
+    // InlineCopier is a more fault-tolerant copier that does not cause errors when
+    // function types in applications are undefined. This is necessary since we copy at
+    // the same time as establishing the proper context in which the copied tree should
+    // be evaluated. This matters for opaque types, see neg/i14653.scala.
+    class InlineCopier() extends TypedTreeCopier:
+      override def Apply(tree: Tree)(fun: Tree, args: List[Tree])(using Context): Apply =
+        if fun.tpe.widen.exists then super.Apply(tree)(fun, args)
+        else untpd.cpy.Apply(tree)(fun, args).withTypeUnchecked(tree.tpe)
+
     // InlinerMap is a TreeTypeMap with special treatment for inlined arguments:
     // They are generally left alone (not mapped further, and if they wrap a type
     // the type Inlined wrapper gets dropped
@@ -913,7 +922,8 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(using Context) {
         newOwners: List[Symbol],
         substFrom: List[Symbol],
         substTo: List[Symbol])(using Context)
-      extends TreeTypeMap(typeMap, treeMap, oldOwners, newOwners, substFrom, substTo):
+      extends TreeTypeMap(
+        typeMap, treeMap, oldOwners, newOwners, substFrom, substTo, InlineCopier()):
 
       override def copy(
           typeMap: Type => Type,

tests/neg/i14653.scala

@@ -0,0 +1,18 @@
+type Amount = Amount.Type
+object Amount:
+  opaque type Type = BigDecimal
+  inline def apply(inline dec: BigDecimal): Type = dec
+
+  extension (self: Type)
+    inline def value: BigDecimal = self
+    inline def +(y: Type): Type = self + y
+
+@main def r(): Unit =
+  val aa: Amount = Amount(1)
+  val ab: Amount = Amount(2)
+  val ac: Amount = Amount(2)
+  val as1: Amount = aa + ab  // error
+  val as2: Amount = aa + ab + ac // error
+
+  println(s"aa + ab = ${as1}")
+  println(s"aa + ab = ${as2}")