Add IDEA data-flow analysis to guess nullability

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

j2k/testData/multiFile/NullabilityByDFa/SomeServiceUsage.java

@@ -0,0 +1,53 @@
+
+public class SomeServiceUsage {
+
+    public SomeService getService() {
+        return SomeService.getInstanceNotNull();
+    }
+
+    public SomeService getServiceNullable() {
+        return SomeService.getInstanceNullable();
+    }
+
+    // elvis
+    public SomeService getServiceNotNullByDataFlow() {
+        SomeService s = SomeService.getInstanceNullable();
+        return s == null ? SomeService.getInstanceNotNull() : s;
+    }
+
+    // nullable, bang-bang
+    public String aString1() {
+        return getServiceNullable().nullableString();
+    }
+
+    // nullable
+    public String aString2() {
+        return getService().nullableString();
+    }
+
+    // not nullable
+    public String aString3() {
+        return getService().notNullString();
+    }
+
+    // nullable, no bang-bang
+    public String aString4() {
+        return getServiceNotNullByDataFlow().nullableString();
+    }
+
+    // not nullable, no bang-bang
+    public String aString5() {
+        return getServiceNotNullByDataFlow().notNullString();
+    }
+
+    // nullable, safe-call
+    public String aString6() {
+        SomeService s = getServiceNullable();
+        if (s != null) {
+            return s.nullableString();
+        } else {
+            return null;
+        }
+    }
+
+}

j2k/testData/multiFile/NullabilityByDFa/SomeServiceUsage.kt

@@ -0,0 +1,47 @@
+class SomeServiceUsage {
+
+    val service: SomeService
+        get() = SomeService.getInstanceNotNull()
+
+    val serviceNullable: SomeService?
+        get() = SomeService.getInstanceNullable()
+
+    // elvis
+    val serviceNotNullByDataFlow: SomeService
+        get() {
+            val s = SomeService.getInstanceNullable()
+            return s ?: SomeService.getInstanceNotNull()
+        }
+
+    // nullable, bang-bang
+    fun aString1(): String? {
+        return serviceNullable!!.nullableString()
+    }
+
+    // nullable
+    fun aString2(): String? {
+        return service.nullableString()
+    }
+
+    // not nullable
+    fun aString3(): String {
+        return service.notNullString()
+    }
+
+    // nullable, no bang-bang
+    fun aString4(): String? {
+        return serviceNotNullByDataFlow.nullableString()
+    }
+
+    // not nullable, no bang-bang
+    fun aString5(): String {
+        return serviceNotNullByDataFlow.notNullString()
+    }
+
+    // nullable, safe-call
+    fun aString6(): String? {
+        val s = serviceNullable
+        return s?.nullableString()
+    }
+
+}

j2k/testData/multiFile/NullabilityByDFa/external/JavaFile.java

@@ -0,0 +1,26 @@
+
+public class SomeService {
+
+    public static SomeService getInstanceNotNull() {
+        return new SomeService();
+    }
+
+    public static SomeService getInstanceNullable() {
+        if (Math.random() > 0.5)
+            return null;
+        return new SomeService();
+    }
+
+    public String nullableString() {
+        if (Math.random() < 0.5)
+            return null;
+        return Math.random() + "";
+    }
+
+    public String notNullString() {
+        String s = nullableString();
+        if (s != null)
+            return s;
+        return "null";
+    }
+}

j2k/testData/multiFile/NullabilityByDFa/external/JavaFile.java.expected

@@ -0,0 +1,26 @@
+
+public class SomeService {
+
+    public static SomeService getInstanceNotNull() {
+        return new SomeService();
+    }
+
+    public static SomeService getInstanceNullable() {
+        if (Math.random() > 0.5)
+            return null;
+        return new SomeService();
+    }
+
+    public String nullableString() {
+        if (Math.random() < 0.5)
+            return null;
+        return Math.random() + "";
+    }
+
+    public String notNullString() {
+        String s = nullableString();
+        if (s != null)
+            return s;
+        return "null";
+    }
+}