Backport "Be even more careful when combining argument and info in computeAsSeenFrom" (#16101)

Backports #16070

Author: Kordyjan

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

@@ -1121,9 +1121,13 @@ object Denotations {
       else if symbol.isAllOf(ClassTypeParam) then
         val arg = symbol.typeRef.argForParam(pre, widenAbstract = true)
         if arg.exists then
+          // take the argument bounds, but intersect with the symbols bounds if
+          // this forces nothing and gives a non-empty type.
           val newBounds =
-            if symbol.isCompleted && !symbol.info.containsLazyRefs
-            then symbol.info.bounds & arg.bounds
+            if symbol.isCompleted && !symbol.info.containsLazyRefs then
+              val combined @ TypeBounds(lo, hi) = symbol.info.bounds & arg.bounds
+              if lo frozen_<:< hi then combined
+              else arg.bounds
             else arg.bounds
           derivedSingleDenotation(symbol, newBounds, pre)
         else derived(symbol.info)

tests/pos/i16049.scala

@@ -0,0 +1,10 @@
+trait BatchDiffFunction[T]
+
+abstract class FirstOrderMinimizer[T, DF <: BatchDiffFunction[T]]:
+  type State = FirstOrderMinimizer.State[T]
+
+object FirstOrderMinimizer:
+  case class State[+T](x: T)
+
+  class OptParams:
+    def iterations[T](init: T): Iterator[FirstOrderMinimizer[T, BatchDiffFunction[T]]#State] = ???

tests/pos/i16076.scala

@@ -0,0 +1,9 @@
+trait Column[V]
+trait ColumnPath
+
+trait ColumnFactory[V, C <: Column[V]]:
+  def apply(columnPath: ColumnPath): C
+
+object ColumnFactory:
+  private def apply[V, C <: Column[V]](f: String => C): ColumnFactory[V, C] =
+    columnPath => f(columnPath.toString())