More careful type variable instance improvements (scala#19659)

The previous code tried to recursively apply the current instance of
FullyDefinedAccumulator to the prospective instance type. This can have
unforeseen side-effects, as i19637 shows. We now are more conservative:
We check that the prospective instance type is already fully defined
without the possibility to instantiate more type variables. This still
passes the test cases that type variable improvement solves and avoids
the problem with scala#19637.

Fixes scala#19637

compiler/src/dotty/tools/dotc/typer/Inferencing.scala

@@ -205,7 +205,7 @@ object Inferencing {
     private def instantiate(tvar: TypeVar, fromBelow: Boolean): Boolean =
       if fromBelow && force.canImprove(tvar) then
         val inst = tvar.typeToInstantiateWith(fromBelow = true)
-        if apply(true, inst) then
+        if isFullyDefined(inst, ForceDegree.none) then
           // need to recursively check before improving, since improving adds type vars
           // which should not be instantiated at this point
           val better = improve(tvar)(inst)

tests/pos/i19637.scala

@@ -0,0 +1,11 @@
+import java.util.stream.*
+import java.util.function.*
+
+val map: java.util.Map[String, String] = Stream.of("1", "2", "3").collect(Collectors.toMap(
+  (s: String) => s,
+  Function.identity(),
+  {
+    case ("1", "1") => "1"
+    case (_, l) => l
+  }
+))