Be still more careful when computing denotations of class parameters (#16112)

Be still more careful when intersecting info and arguments of a class
type parameter. This is the latest installment of a never-ending story.

The problem is this: Given
```scala
class C[T >: L1 <: H1] { val x: T }
def f(): C[? >: L2 <: H2]
```
what is the type of `f().x`?

With capture conversion (an extremely tricky aspect of the type system
forced on us since Java does it), the type is something like `?1.T`
where `?1` is a skolem variable of type `C[? >: L2 <: H2]`. OK, but what
is the underlying (widened) type of `?1.T`?

We used to say it's `C[T >: L1 <: H1]`. I.e. we forgot about the actual
arguments. But then we had to change untupling desugarings from defs to
vals in #14816 to fix #14783 and it turned out that was not good enough,
we needed the information of the actual arguments, i.e. the type should
be `C[T >: L2 <: H2]`. Then something else started failing which relied
on the formal arguiment bounds being preserved. So the new resolution
was that the type would be the intersection of the formal parameter
bounds and the actual bounds, i.e. `C[T >: L1 | L2 <: H1 & H2]`. Then
there was a series of problems where _that_ failed, either because the
parameter bound was F-bounded or because the intersection was an empty
type. The latest installment is that the parameter bounds refer to
another parameter in the same class, which requires a simultaneous
substitution of all skolemized bounds for all dependent parameter
references in the parameter bounds. But that's impossible or at least
very hard to achieve since we are looking at the type of a single
parameter after capture conversion here. So the current solution is to
also back out of the intersection if there are cross-parameter
references and to use just the argument bounds instead.

Author: odersky

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

@@ -1076,6 +1076,7 @@ object Denotations {
     def aggregate[T](f: SingleDenotation => T, g: (T, T) => T): T = f(this)
 
     type AsSeenFromResult = SingleDenotation
+
     protected def computeAsSeenFrom(pre: Type)(using Context): SingleDenotation = {
       val symbol = this.symbol
       val owner = this match {
@@ -1120,19 +1121,31 @@ object Denotations {
       then this
       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
-              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)
+        if arg.exists
+        then derivedSingleDenotation(symbol, normalizedArgBounds(arg.bounds), pre)
         else derived(symbol.info)
       else derived(symbol.info)
     }
+
+    /** The argument bounds, possibly intersected with the parameter's info TypeBounds,
+     *  if the latter is not F-bounded and does not refer to other type parameters
+     *  of the same class, and the intersection is provably nonempty.
+     */
+    private def normalizedArgBounds(argBounds: TypeBounds)(using Context): TypeBounds =
+      if symbol.isCompleted && !hasBoundsDependingOnParamsOf(symbol.owner) then
+        val combined @ TypeBounds(lo, hi) = symbol.info.bounds & argBounds
+        if (lo frozen_<:< hi) then combined
+        else argBounds
+      else argBounds
+
+    private def hasBoundsDependingOnParamsOf(cls: Symbol)(using Context): Boolean =
+      val acc = new TypeAccumulator[Boolean]:
+        def apply(x: Boolean, tp: Type): Boolean = tp match
+          case _: LazyRef => true
+          case tp: TypeRef
+          if tp.symbol.isAllOf(ClassTypeParam) && tp.symbol.owner == cls => true
+          case _ => foldOver(x, tp)
+      acc(false, symbol.info)
   }
 
   abstract class NonSymSingleDenotation(symbol: Symbol, initInfo: Type, override val prefix: Type) extends SingleDenotation(symbol, initInfo) {

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

@@ -442,14 +442,6 @@ object Types {
     final def containsWildcardTypes(using Context) =
       existsPart(_.isInstanceOf[WildcardType], StopAt.Static, forceLazy = false)
 
-    /** Does this type contain LazyRef types? */
-    final def containsLazyRefs(using Context) =
-      val acc = new TypeAccumulator[Boolean]:
-        def apply(x: Boolean, tp: Type): Boolean = tp match
-          case _: LazyRef => true
-          case _ => x || foldOver(x, tp)
-      acc(false, this)
-
 // ----- Higher-order combinators -----------------------------------
 
     /** Returns true if there is a part of this type that satisfies predicate `p`.

tests/pos/i16105.scala

@@ -0,0 +1,12 @@
+trait SQLSyntaxSupport[A]
+
+trait ResultNameSQLSyntaxProvider[S <: SQLSyntaxSupport[A], A]
+trait QuerySQLSyntaxProvider[S <: SQLSyntaxSupport[A], A]{
+  def resultName: ResultNameSQLSyntaxProvider[S, A] = ???
+}
+
+def include(syntaxProviders: QuerySQLSyntaxProvider[_, _]*) = {
+  syntax(syntaxProviders.map(_.resultName): _*)
+}
+
+def syntax(resultNames: ResultNameSQLSyntaxProvider[_, _]*) = ???