Fix bounds of encoded type variables in quote patterns

When we encode quote patterns into `unapply` methods, we need to create
a copy of each type variable. One copy is kept within the quote(in the
next stage) and the other is used in the `unapply` method to define the
usual pattern type variables. When creating the latter we copied the
symbols but did not update the infos. This implies that if type variables
would be bounded by each other, the bounds of the copies would be the
original types instead of the copies. We need to update those references.

To update the info we now create all the symbols in one pass and the
update all their infos in a second pass. This also implies that we cannot
use the `newPatternBoundSymbol` to create the symbol as this constructor
will register the info into GADT bounds. Instead we use the plain
`newSymbol`. Then in the second pass, when we have updated the infos, we
register the symbol into GADT bounds.

Note that the code in the added test does compiles correctly, but it had
the inconsistent bounds. This test is added in case we need to manually
inspect the bounds latter. This test does fail to compile in
#17935 if this fix is not applied.

Author: nicolasstucki

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

@@ -202,13 +202,7 @@ trait QuotesAndSplices {
   private def splitQuotePattern(quoted: Tree)(using Context): (collection.Map[Symbol, Bind], Tree, List[Tree]) = {
     val ctx0 = ctx
 
-    val typeBindings: mutable.Map[Symbol, Bind] = mutable.LinkedHashMap.empty
-    def getBinding(sym: Symbol): Bind =
-      typeBindings.getOrElseUpdate(sym, {
-        val bindingBounds = sym.info
-        val bsym = newPatternBoundSymbol(sym.name.toString.stripPrefix("$").toTypeName, bindingBounds, quoted.span)
-        Bind(bsym, untpd.Ident(nme.WILDCARD).withType(bindingBounds)).withSpan(quoted.span)
-      })
+    val bindSymMapping: collection.Map[Symbol, Bind] = unapplyBindingsMapping(quoted)
 
     object splitter extends tpd.TreeMap {
       private var variance: Int = 1
@@ -288,7 +282,7 @@ trait QuotesAndSplices {
           report.error(IllegalVariableInPatternAlternative(tdef.symbol.name), tdef.srcPos)
         if variance == -1 then
           tdef.symbol.addAnnotation(Annotation(New(ref(defn.QuotedRuntimePatterns_fromAboveAnnot.typeRef)).withSpan(tdef.span)))
-        val bindingType = getBinding(tdef.symbol).symbol.typeRef
+        val bindingType = bindSymMapping(tdef.symbol).symbol.typeRef
         val bindingTypeTpe = AppliedType(defn.QuotedTypeClass.typeRef, bindingType :: Nil)
         val sym = newPatternBoundSymbol(nameOfSyntheticGiven, bindingTypeTpe, tdef.span, flags = ImplicitVal)(using ctx0)
         buff += Bind(sym, untpd.Ident(nme.WILDCARD).withType(bindingTypeTpe)).withSpan(tdef.span)
@@ -325,7 +319,56 @@ trait QuotesAndSplices {
         new TreeTypeMap(typeMap = typeMap).transform(shape1)
       }
 
-    (typeBindings, shape2, patterns)
+    (bindSymMapping, shape2, patterns)
+  }
+
+  /** For each type variable defined in the quote pattern we generate an equivalent
+   *  binding that will be as type variable in the encoded `unapply` of the quote pattern.
+   *
+   *  @return Mapping from type variable symbols defined in the quote pattern into
+   *          type variable `Bind` definitions for the `unapply` of the quote pattern.
+   *          This mapping retains the original type variable definition order.
+   */
+  private def unapplyBindingsMapping(quoted: Tree)(using Context): collection.Map[Symbol, Bind] = {
+    // Collect all existing type variable bindings and create new symbols for them.
+    // The old info is used, it may contain references to the old symbols.
+    val (oldBindings, newBindings) = {
+      val seen = mutable.Set.empty[Symbol]
+      val oldBindingsBuffer = mutable.LinkedHashSet.empty[Symbol]
+      val newBindingsBuffer = mutable.ListBuffer.empty[Symbol]
+
+      new tpd.TreeTraverser {
+        def traverse(tree: Tree)(using Context): Unit = tree match {
+          case _: SplicePattern =>
+          case Select(pat: Bind, _) if tree.symbol.isTypeSplice =>
+            val sym = tree.tpe.dealias.typeSymbol
+            if sym.exists then registerNewBindSym(sym)
+          case tdef: TypeDef  =>
+            if tdef.symbol.hasAnnotation(defn.QuotedRuntimePatterns_patternTypeAnnot) then
+              registerNewBindSym(tdef.symbol)
+            traverseChildren(tdef)
+          case _ =>
+            traverseChildren(tree)
+        }
+        private def registerNewBindSym(sym: Symbol): Unit =
+          if !seen(sym) then
+            seen += sym
+            oldBindingsBuffer += sym
+            newBindingsBuffer += newSymbol(ctx.owner, sym.name.toString.stripPrefix("$").toTypeName, Case | sym.flags, sym.info, coord = quoted.span)
+      }.traverse(quoted)
+      (oldBindingsBuffer.toList, newBindingsBuffer.toList)
+    }
+
+    // Replace symbols in `mapping` in the infos of the new symbol and register GADT bounds.
+    // GADT bounds need to be added after the info is updated to avoid references to the old symbols.
+    val newBindingsRefs = newBindings.map(_.typeRef)
+    for newBindings <- newBindings do
+      newBindings.info = newBindings.info.subst(oldBindings.toList, newBindingsRefs)
+      ctx.gadtState.addToConstraint(newBindings) // This must be performed after the info has been updated
+
+    // Map into Bind nodes retaining the original order
+    val newBindingBinds = newBindings.map(newSym => Bind(newSym, untpd.Ident(nme.WILDCARD).withType(newSym.info)).withSpan(quoted.span))
+    mutable.LinkedHashMap.from(oldBindings.lazyZip(newBindingBinds))
   }
 
   /** Type a quote pattern `case '{ <quoted> } =>` qiven the a current prototype. Typing the pattern

tests/pos-macros/quote-pattern-type-variable-bounds.scala

@@ -0,0 +1,12 @@
+import quoted.*
+
+def foo(using Quotes)(x: Expr[Int]) =
+  x match
+    case '{ type t; type u <: `t`; f[`t`, `u`] } =>
+    case '{ type u <: `t`; type t; f[`t`, `u`] } =>
+    case '{ type t; type u <: `t`; g[F[`t`, `u`]] } =>
+    case '{ type u <: `t`; type t; g[F[`t`, `u`]] } =>
+
+def f[T, U <: T] = ???
+def g[T] = ???
+type F[T, U <: T]