Fix #7249: Search for hidden givens only at whole type

[odersky]

Don't try to find a hidden implicit for a part of type type that
was searched for, unless that part is fully defined.

The reason is that any type variables in a subtype might be associated
with constraints that are lost at the point of issuing a "hidden
givens" addendum.

The commit also contains a tweak to "isFullyDefined" which makes
an uninstantiated type variable that is not bound in the current
constraint count as "not fully defined", where previously it was
"fully defined".

[odersky]

This fixes also #7248

[anatoliykmetyuk]

This summon shouldn't succeed though on compile time. Starting from println:

// 1) initial call
f[B[Int]]

// 2) inlining `f`
inline erasedValue[B[Int]] match
  case _: A => new A{}.asInstanceOf[B[Int]]
  case _: B[a] => summon[a].asInstanceOf[B[Int]]

// 3) second case matched and inlined with a = Int
summon[Int].asInstanceOf[B[Int]]

summon[Int] should fail since there's no Int given in scope, shouldn't it?

scala> summon[Int]
1 |summon[Int]
  |           ^
  |no implicit argument of type Int was found for parameter x of method summon in object DottyPredef

[anatoliykmetyuk]

Even if it tries Int & Foo it still won't find anything suitable, since f returns T <: Foo and nothing about it says that it is an intersection with Int.

[odersky]

It actually tries f[Nothing] and succeeds with the A case.

[anatoliykmetyuk]

Oh right. So the behaviour here stays the same as in the #7250 second example: if we run this test, it will fail at runtime with ClassCastException. It looks like it is the same problem as #7293, where Nothing appears in unexpected places.

[anatoliykmetyuk]

Actually I remember testing it with inline given f[T >: Nothing <: Foo]: T, and it still finds f when looking for Nothing (as understood from the unchanged behaviour in that case).