Backport "Patmat: Use less type variables in prefix inference" (#17440)

Backports #16827

Solves #17368

Author: Kordyjan

compiler/src/dotty/tools/dotc/core/TypeOps.scala

@@ -2,7 +2,7 @@ package dotty.tools
 package dotc
 package core
 
-import Contexts._, Types._, Symbols._, Names._, Flags._
+import Contexts._, Types._, Symbols._, Names._, NameKinds.*, Flags._
 import SymDenotations._
 import util.Spans._
 import util.Stats
@@ -839,40 +839,63 @@ object TypeOps:
       }
     }
 
-    // Prefix inference, replace `p.C.this.Child` with `X.Child` where `X <: p.C`
-    // Note: we need to strip ThisType in `p` recursively.
+    /** Gather GADT symbols and `ThisType`s found in `tp2`, ie. the scrutinee. */
+    object TraverseTp2 extends TypeTraverser:
+      val thisTypes = util.HashSet[ThisType]()
+      val gadtSyms  = new mutable.ListBuffer[Symbol]
+
+      def traverse(tp: Type) = {
+        val tpd = tp.dealias
+        if tpd ne tp then traverse(tpd)
+        else tp match
+          case tp: ThisType if !tp.tref.symbol.isStaticOwner && !thisTypes.contains(tp) =>
+            thisTypes += tp
+            traverseChildren(tp.tref)
+          case tp: TypeRef if tp.symbol.isAbstractOrParamType =>
+            gadtSyms += tp.symbol
+            traverseChildren(tp)
+          case _ =>
+            traverseChildren(tp)
+      }
+    TraverseTp2.traverse(tp2)
+    val thisTypes = TraverseTp2.thisTypes
+    val gadtSyms  = TraverseTp2.gadtSyms.toList
+
+    // Prefix inference, given `p.C.this.Child`:
+    //   1. return it as is, if `C.this` is found in `tp`, i.e. the scrutinee; or
+    //   2. replace it with `X.Child` where `X <: p.C`, stripping ThisType in `p` recursively.
     //
-    // See tests/patmat/i3938.scala
+    // See tests/patmat/i3938.scala, tests/pos/i15029.more.scala, tests/pos/i16785.scala
     class InferPrefixMap extends TypeMap {
       var prefixTVar: Type | Null = null
       def apply(tp: Type): Type = tp match {
-        case ThisType(tref: TypeRef) if !tref.symbol.isStaticOwner =>
+        case tp @ ThisType(tref) if !tref.symbol.isStaticOwner =>
           val symbol = tref.symbol
-          if (symbol.is(Module))
+          if thisTypes.contains(tp) then
+            prefixTVar = tp // e.g. tests/pos/i16785.scala, keep Outer.this
+            prefixTVar.uncheckedNN
+          else if symbol.is(Module) then
             TermRef(this(tref.prefix), symbol.sourceModule)
           else if (prefixTVar != null)
             this(tref)
           else {
             prefixTVar = WildcardType  // prevent recursive call from assigning it
-            val tvars = tref.typeParams.map { tparam => newTypeVar(tparam.paramInfo.bounds) }
+            // e.g. tests/pos/i15029.more.scala, create a TypeVar for `Instances`' B, so we can disregard `Ints`
+            val tvars = tref.typeParams.map { tparam => newTypeVar(tparam.paramInfo.bounds, DepParamName.fresh(tparam.paramName)) }
             val tref2 = this(tref.applyIfParameterized(tvars))
-            prefixTVar = newTypeVar(TypeBounds.upper(tref2))
+            prefixTVar = newTypeVar(TypeBounds.upper(tref2), DepParamName.fresh(tref.name))
             prefixTVar.uncheckedNN
           }
         case tp => mapOver(tp)
       }
     }
 
     val inferThisMap = new InferPrefixMap
-    val tvars = tp1.typeParams.map { tparam => newTypeVar(tparam.paramInfo.bounds) }
+    val tvars = tp1.typeParams.map { tparam => newTypeVar(tparam.paramInfo.bounds, DepParamName.fresh(tparam.paramName)) }
     val protoTp1 = inferThisMap.apply(tp1).appliedTo(tvars)
 
-    val getAbstractSymbols = new TypeAccumulator[List[Symbol]]:
-      def apply(xs: List[Symbol], tp: Type) = tp.dealias match
-        case tp: TypeRef if tp.symbol.exists && !tp.symbol.isClass => foldOver(tp.symbol :: xs, tp)
-        case tp                                => foldOver(xs, tp)
-    val syms2 = getAbstractSymbols(Nil, tp2).reverse
-    if syms2.nonEmpty then ctx.gadtState.addToConstraint(syms2)
+    if gadtSyms.nonEmpty then
+      ctx.gadtState.addToConstraint(gadtSyms)
 
     // If parent contains a reference to an abstract type, then we should
     // refine subtype checking to eliminate abstract types according to

tests/patmat/i12408.check

@@ -1,2 +1,2 @@
-13: Pattern Match Exhaustivity: X[<?>] & (X.this : X[T]).A(_), X[<?>] & (X.this : X[T]).C(_)
+13: Pattern Match Exhaustivity: A(_), C(_)
 21: Pattern Match

tests/pos/i16785.scala

@@ -0,0 +1,11 @@
+class VarImpl[Lbl, A]
+
+class Outer[|*|[_, _], Lbl1]:
+  type Var[A1] = VarImpl[Lbl1, A1]
+
+  sealed trait Foo[G]
+  case class Bar[T, U]()
+    extends Foo[Var[T] |*| Var[U]]
+
+  def go[X](scr: Foo[Var[X]]): Unit = scr match // was: compile hang
+    case Bar() => ()