Drop lazy recursive application in approximateParent

This plays badly with any code that inspects bounds deeply when creating constraints
since the LazyRefs create new type variables at unexpected times. Instead, create
the type variables first with an empty constraint of the right kind and then add
the bounds to the constraint using subtype tests.

Author: odersky

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

@@ -770,20 +770,15 @@ object TypeOps:
           tref
 
         case tp: TypeRef if !tp.symbol.isClass =>
-          def lo = LazyRef.of(apply(tp.underlying.loBound))
-          def hi = LazyRef.of(apply(tp.underlying.hiBound))
           val lookup = boundTypeParams.lookup(tp)
           if lookup != null then lookup
           else
-            val tv = newTypeVar(TypeBounds(lo, hi))
+            val TypeBounds(lo, hi) = tp.underlying.bounds
+            val tv = newTypeVar(TypeBounds(defn.NothingType, hi.topType))
             boundTypeParams(tp) = tv
-            // Force lazy ref eagerly using current context
-            // Otherwise, the lazy ref will be forced with a unknown context,
-            // which causes a problem in tests/patmat/i3645e.scala
-            lo.ref
-            hi.ref
+            assert(tv <:< apply(hi))
+            apply(lo) <:< tv //  no assert, since bounds might conflict
             tv
-          end if
 
         case tp @ AppliedType(tycon: TypeRef, _) if !tycon.dealias.typeSymbol.isClass && !tp.isMatchAlias =>