Fix #9067: avoid instantiate child parameters to Wildcard if possible

In tests/patmat/i9067.scala, for Foo[X1, X2] <: Base[Any, Any],
X1 and X2 can both be `Any` instead of `Wildcard`.
Inferring `Wildcard` will lead to the signature of `Foo.unapply`
be inferred as `Nothing => Nothing`, which causes the checker
to issue a false unexhaustive warning.

Author: liufengyun

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

@@ -18,7 +18,7 @@ import config.Feature
 import typer.Applications._
 import typer.ProtoTypes._
 import typer.ForceDegree
-import typer.Inferencing.isFullyDefined
+import typer.Inferencing._
 import typer.IfBottom
 
 import scala.annotation.internal.sharable
@@ -618,8 +618,9 @@ object TypeOps:
 
     val childTp = if (child.isTerm) child.termRef else child.typeRef
 
-    instantiateToSubType(childTp, parent)(using ctx.fresh.setNewTyperState())
-      .dealias
+    inContext(ctx.fresh.setExploreTyperState().setFreshGADTBounds) {
+      instantiateToSubType(childTp, parent).dealias
+    }
   }
 
   /** Instantiate type `tp1` to be a subtype of `tp2`
@@ -685,10 +686,15 @@ object TypeOps:
       }
     }
 
+    def isPatternTypeSymbol(sym: Symbol) = !sym.isClass && sym.is(Case)
+
     // replace uninstantiated type vars with WildcardType, check tests/patmat/3333.scala
     def instUndetMap(implicit ctx: Context) = new TypeMap {
       def apply(t: Type): Type = t match {
-        case tvar: TypeVar if !tvar.isInstantiated => WildcardType(tvar.origin.underlying.bounds)
+        case tvar: TypeVar if !tvar.isInstantiated =>
+          WildcardType(tvar.origin.underlying.bounds)
+        case tref: TypeRef if isPatternTypeSymbol(tref.typeSymbol) =>
+          WildcardType(tref.underlying.bounds)
         case _ => mapOver(t)
       }
     }
@@ -715,9 +721,10 @@ object TypeOps:
       }
     }
 
-    if (protoTp1 <:< tp2)
-      if (isFullyDefined(protoTp1, force)) protoTp1
-      else instUndetMap.apply(protoTp1)
+    if (protoTp1 <:< tp2) {
+      maximizeType(protoTp1, NoSpan, fromScala2x = false)
+      instUndetMap.apply(protoTp1)
+    }
     else {
       val protoTp2 = maxTypeMap.apply(tp2)
       if (protoTp1 <:< protoTp2 || parentQualify)

tests/patmat/i9067.scala

@@ -0,0 +1,4 @@
+sealed trait Base[A, +B]
+case class Foo[A, +B](f: A => B) extends Base[A, B]
+
+def bar(x: Base[Any, Any]): Unit = x match { case Foo(_) => }