fixed bug in constraint system

For parameter type T constraint T? <: Int? should NOT transform to T <: Int, it should be T <: Int?
equality constraint T? = Int? should transform to T <: Int? && T >: Int

compiler/testData/diagnostics/tests/inference/constraints/equalityConstraintOnNullableType.kt

@@ -0,0 +1,21 @@
+// !DIAGNOSTICS: -BASE_WITH_NULLABLE_UPPER_BOUND
+class TypeOf<T>(t: T)
+
+trait A<T>
+
+fun <T> foo(a: A<T>, aN: A<T?>): T = throw Exception("$a $aN")
+
+fun <T> doA(a: A<T>): T = throw Exception("$a")
+
+fun test(a: A<Int>, aN: A<Int?>) {
+    val aa = doA(aN)
+    TypeOf(aa): TypeOf<Int?>
+
+    val nullable = foo(aN, aN)
+    //T = Int?, T? = Int? => T = Int?
+    TypeOf(nullable): TypeOf<Int?>
+
+    val notNullable = foo(a, aN)
+    //T = Int, T? = Int? => T = Int
+    TypeOf(notNullable): TypeOf<Int>
+}

compiler/testData/diagnostics/tests/inference/constraints/notNullConstraintOnNullableType.kt

@@ -0,0 +1,25 @@
+class TypeOf<T>(t: T)
+
+trait A<T>
+
+trait In<in T>
+
+trait Out<out T>
+
+fun <T: Any> doT(t: T?): T = throw Exception("$t")
+fun <T: Any> doOut(o: Out<T?>): T { throw Exception("$o") }
+fun <T: Any> doIn(i: In<T?>) { throw Exception("$i") }
+fun <T: Any> doA(i: A<T?>) { throw Exception("$i") }
+
+fun test(out: Out<Int>, i: In<Int>, inv: A<Int>) {
+    // T? >: Int => T = Int
+    doT(1)
+    val r = doOut(out)
+    TypeOf(r): TypeOf<Int>
+
+    // T? <: Int => error
+    doIn(<!TYPE_MISMATCH!>i<!>)
+
+    // T? >: Int => error
+    doA(<!TYPE_MISMATCH!>inv<!>)
+}

compiler/testData/diagnostics/tests/inference/constraints/subtypeConstraintOnNullableType.kt

@@ -0,0 +1,24 @@
+// !DIAGNOSTICS: -BASE_WITH_NULLABLE_UPPER_BOUND
+class TypeOf<T>(t: T)
+
+trait A<T>
+
+trait Out<out T>
+
+fun <T> foo(a: A<T>, o: Out<T?>): T = throw Exception("$a $o")
+
+fun <T> doOut(o: Out<T?>): T = throw Exception("$o")
+
+fun test(a: A<Int>, aN: A<Int?>, o: Out<Int?>) {
+    val out = doOut(o)
+    //T? >: Int? => T >: Int
+    TypeOf(out): TypeOf<Int>
+
+    val nullable = foo(aN, o)
+    //T = Int?, T? >: Int? => T = Int?
+    TypeOf(nullable): TypeOf<Int?>
+
+    val notNullable = foo(a, o)
+    //T = Int, T? >: Int? => T = Int
+    TypeOf(notNullable): TypeOf<Int>
+}

compiler/testData/diagnostics/tests/inference/constraints/supertypeConstraintOnNullableType.kt

@@ -0,0 +1,24 @@
+// !DIAGNOSTICS: -BASE_WITH_NULLABLE_UPPER_BOUND
+class TypeOf<T>(t: T)
+
+trait A<T>
+
+trait In<in T>
+
+fun <T> foo(a: A<T>, i: In<T>): T = throw Exception("$a $i")
+
+fun <T> doIn(i: In<T?>): T = throw Exception("$i")
+
+fun test(a: A<Int>, aN: A<Int?>, i: In<Int?>) {
+    val _in = doIn(i)
+    //T? <: Int? => T <: Int?
+    TypeOf(_in): TypeOf<Int?>
+
+    val notNullable = foo(a, i)
+    //T = Int, T? <: Int? => T = Int
+    TypeOf(notNullable): TypeOf<Int>
+
+    val nullable = foo(aN, i)
+    //T = Int?, T? <: Int? => T = Int?
+    TypeOf(nullable): TypeOf<Int?>
+}