FIR: more accurate nullability of intersection of flexible types

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/types/ConeTypeIntersector.kt

@@ -145,11 +145,7 @@ object ConeTypeIntersector {
         protected fun ConeKotlinType.resultNullability(context: ConeTypeContext): ResultNullability =
             when {
                 isMarkedNullable -> ACCEPT_NULL
-                // Technically, upper bound's nullability is more accurate, e.g., a type from Java with @NotNull.
-                // However, it requires one more recursive call here. In contrast, UNKNOWN here is a safe approximation: for even such
-                // flexible type case, as long as the upper bound is still used in the intersection computation (and it indeed is),
-                // we will still get the same nullability from the resulting intersected type.
-                this is ConeFlexibleType -> UNKNOWN
+                this is ConeFlexibleType -> upperBound.resultNullability(context)
                 ConeNullabilityChecker.isSubtypeOfAny(context, this) -> NOT_NULL
                 else -> UNKNOWN
             }

compiler/testData/diagnostics/tests/smartCasts/varnotnull/toFlexibleType.fir.kt

@@ -1,7 +1,12 @@
 // FILE: J.java
+import org.jetbrains.annotations.*;
+import java.util.List;
 
 class J {
     static String foo() { return "abc"; }
+
+    @NotNull
+    static List<String> bar() { return new List<String>(); }
 }
 
 // FILE: test.kt
@@ -11,6 +16,10 @@ fun bar() {
     v = J.foo()
     v<!UNSAFE_CALL!>.<!>length
     <!INAPPLICABLE_CANDIDATE!>gav<!>(v)
+
+    var l: List<String>?
+    l = J.bar()
+    l.isEmpty()
 }
 
 fun gav(v: String) = v

compiler/testData/diagnostics/tests/smartCasts/varnotnull/toFlexibleType.kt

@@ -1,7 +1,12 @@
 // FILE: J.java
+import org.jetbrains.annotations.*;
+import java.util.List;
 
 class J {
     static String foo() { return "abc"; }
+
+    @NotNull
+    static List<String> bar() { return new List<String>(); }
 }
 
 // FILE: test.kt
@@ -11,6 +16,10 @@ fun bar() {
     v = J.foo()
     <!DEBUG_INFO_SMARTCAST!>v<!>.length
     gav(<!DEBUG_INFO_SMARTCAST!>v<!>)
+
+    var l: List<String>?
+    l = J.bar()
+    <!DEBUG_INFO_SMARTCAST!>l<!>.isEmpty()
 }
 
-fun gav(v: String) = v
+fun gav(v: String) = v

compiler/testData/diagnostics/tests/smartCasts/varnotnull/toFlexibleType.txt

@@ -10,5 +10,6 @@ public/*package*/ open class J {
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
 
     // Static members
+    @org.jetbrains.annotations.NotNull public/*package*/ open fun bar(): kotlin.collections.(Mutable)List<kotlin.String!>
     public/*package*/ open fun foo(): kotlin.String!
 }