NI: Prefer nullable lower bound to flexible one when substitution of type variable is performed

^KT-32435 Fixed

compiler/testData/diagnostics/tests/nullableTypes/inferenceFlexibleTToNullable.kt

@@ -0,0 +1,63 @@
+// !DIAGNOSTICS: -UNUSED_VARIABLE -UNCHECKED_CAST -UNUSED_VALUE -ASSIGNED_BUT_NEVER_ACCESSED_VARIABLE
+// !LANGUAGE: +NewInference
+// SKIP_TXT
+
+// FILE: Foo.java
+import org.jetbrains.annotations.NotNull;
+
+public class Foo<T>  {
+    T x;
+
+    public Foo(T x) {
+        this.x = x;
+    }
+
+    public static Number bar() { return null; }
+
+    public static <K> K simpleId(K k) {
+        return k;
+    }
+
+    public T produceT() {
+        return x;
+    }
+
+    @NotNull
+    public T produceNotNullT() {
+        return x;
+    }
+
+    public void consumeT(T x) {}
+}
+
+// FILE: main.kt
+fun <T> bar(n: Number?, d: T, e: T) {
+    val a: Number = <!TYPE_MISMATCH!>Foo.<!TYPE_MISMATCH!>simpleId(n)<!><!>
+    val b: Number? = Foo.simpleId(n)
+    val c = <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Number?")!>Foo.simpleId(n)<!>
+
+    val x4 = Foo(if (true) 10 else null)
+    val x5: Number = <!TYPE_MISMATCH!>x4.produceT()<!>
+    val x6: Number? = x4.produceT()
+    val x7 = <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int?")!>x4.produceT()<!>
+    val x8 = <!DEBUG_INFO_EXPRESSION_TYPE("kotlin.Int")!>x4.produceNotNullT()<!>
+
+    x4.consumeT(x7)
+
+    val x9: T = Foo.simpleId(d)
+    val x10: T? = Foo.simpleId(d)
+
+    if (e != null) {
+        var x11 = e
+        x11 = Foo.simpleId(d) // assign to definitely not-null T, the lack an error is consistent with old inference
+    }
+
+    var x11 = Foo<T>(null as T).x
+    x11 = Foo.simpleId(d) // assign to flexible T
+
+    var x12 = Foo.bar()
+    x12 = Foo.simpleId(n) // assign to flexible Number
+
+    var x13 = e
+    x13 = Foo.simpleId(d)
+}