Add IDEA data-flow analysis to guess nullability

Add "if return..." folding to "return if"

j2k/testData/fileOrElement/callChainExpression/libraryMethodCallFromInstance-settings.kt

@@ -3,7 +3,7 @@
 internal class Library {
     fun call() {}
 
-    val string: String?
+    val string: String
         get() = ""
 }
 
@@ -11,9 +11,9 @@ internal class User {
     fun main() {
         val lib: Library = Library()
         lib.call()
-        lib.string!!.isEmpty()
+        lib.string.isEmpty()
 
         Library().call()
-        Library().string!!.isEmpty()
+        Library().string.isEmpty()
     }
 }

j2k/testData/fileOrElement/detectProperties/GetterTypeNotMatch.kt

@@ -1,7 +1,6 @@
-// ERROR: Type mismatch: inferred type is String? but Any was expected
 internal class A {
     private val s: String? = null
 
-    val value: Any
+    val value: Any?
         get() = s
 }

j2k/testData/fileOrElement/ifStatement/singleLine.kt

@@ -1 +1 @@
-if (true) return 1 else return 0
+return if (true) 1 else 0

j2k/testData/fileOrElement/issues/kt-824-isDir.kt

@@ -11,9 +11,9 @@ object Test {
             return false
         }
         val result = true
-        if (parent.isDirectory) {
-            return true
+        return if (parent.isDirectory) {
+            true
         } else
-            return false
+            false
     }
 }

j2k/testData/fileOrElement/issues/kt-852.kt

@@ -8,9 +8,7 @@ internal class Test {
         if (s1.isEmpty() && s2.isEmpty())
             return "OK"
 
-        if (s1.isEmpty() && s2.isEmpty() && s3.isEmpty())
-            return "OOOK"
+        return if (s1.isEmpty() && s2.isEmpty() && s3.isEmpty()) "OOOK" else ""
 
-        return ""
     }
 }

j2k/testData/fileOrElement/nullability/MethodReturnsNull.kt

@@ -1,9 +1,9 @@
 internal class C {
     fun foo(b: Boolean): String? {
-        if (b) {
-            return "abc"
+        return if (b) {
+            "abc"
         } else {
-            return null
+            null
         }
     }
 }

j2k/testData/fileOrElement/nullability/NullableVariableDotAccess.kt

@@ -1,5 +1,4 @@
 fun foo(s: String?, b: Boolean): Int {
     if (s == null) println("null")
-    if (b) return s!!.length
-    return 10
+    return if (b) s!!.length else 10
 }

j2k/testData/fileOrElement/nullability/Overrides.kt

@@ -25,7 +25,7 @@ internal class C : Base(), I {
         return ""
     }
 
-    override fun zoo(o: Any?): String? {
+    override fun zoo(o: Any?): String {
         return ""
     }
 }

j2k/testData/fileOrElement/nullability/autoNotNull.kt

@@ -1,8 +1,5 @@
 internal class A {
     fun foo(s: String?): Int {
-        if (s != null) {
-            return s.length
-        }
-        return -1
+        return s?.length ?: -1
     }
 }

j2k/testData/fileOrElement/nullability/nullableField.kt

@@ -1,4 +1,3 @@
-// ERROR: Type mismatch: inferred type is String? but String was expected
 // ERROR: Type mismatch: inferred type is Int? but Int was expected
 // ERROR: Type inference failed. Please try to specify type arguments explicitly.
 // ERROR: Using 'remove(Int): T' is an error. Use removeAt(index) instead.
@@ -50,7 +49,7 @@ class Test {
 
     }
 
-    fun foo2(s: String) {
+    fun foo2(s: String?) {
 
     }
 

j2k/testData/fileOrElement/nullability/nullableInitializer1.kt

@@ -1,20 +1,18 @@
-// ERROR: Type mismatch: inferred type is String? but String was expected
-// ERROR: Type mismatch: inferred type is String? but String was expected
 class Test {
     fun nullableString(p: Int): String? {
         return if (p > 0) "response" else null
     }
 
-    private var nullableInitializerField: String = nullableString(3)
+    private var nullableInitializerField = nullableString(3)
     private val nullableInitializerFieldFinal = nullableString(3)
-    var nullableInitializerPublicField: String = nullableString(3)
+    var nullableInitializerPublicField = nullableString(3)
 
     fun testProperty() {
         nullableInitializerField = "aaa"
 
-        nullableInitializerField[0]
+        nullableInitializerField!![0]
         nullableInitializerFieldFinal!![0]
-        nullableInitializerPublicField[0]
+        nullableInitializerPublicField!![0]
     }
 
     fun testLocalVariable() {

j2k/testData/fileOrElement/switch/nestedIf.kt

@@ -1,9 +1,9 @@
 fun foo(i: Int, j: Int): String {
     when (i) {
-        0 -> if (j > 0) {
-            return "1"
+        0 -> return if (j > 0) {
+            "1"
         } else {
-            return "2"
+            "2"
         }
         1 -> return "3"
         else -> return "4"

j2k/testData/fileOrElement/switch/nestedIfFallback.kt

@@ -1,10 +1,9 @@
 fun foo(i: Int, j: Int): String {
     when (i) {
         0 -> {
-            if (j > 0) {
-                return "1"
-            }
-            return "2"
+            return if (j > 0) {
+                "1"
+            } else "2"
         }
         1 -> return "2"
         else -> return "3"

j2k/testData/fileOrElement/typeCastExpression/castNullable.kt

@@ -1,7 +1,6 @@
 internal class A {
     private fun foo(o: Any?, b: Boolean): String? {
-        if (b) return o as String?
-        return ""
+        return if (b) o as String? else ""
     }
 
     fun bar() {