FoldInitializerAndIfToElvisInspection: don't report when 'var' variable has no explicit type and it's used as not nullable type

#KT-38349 Fixed

idea/src/org/jetbrains/kotlin/idea/inspections/FoldInitializerAndIfToElvisInspection.kt

@@ -29,14 +29,19 @@ import org.jetbrains.kotlin.idea.util.hasComments
 import org.jetbrains.kotlin.lexer.KtTokens
 import org.jetbrains.kotlin.psi.*
 import org.jetbrains.kotlin.psi.psiUtil.PsiChildRange
+import org.jetbrains.kotlin.psi.psiUtil.endOffset
 import org.jetbrains.kotlin.psi.psiUtil.siblings
+import org.jetbrains.kotlin.psi.psiUtil.startOffset
 import org.jetbrains.kotlin.resolve.BindingContext
+import org.jetbrains.kotlin.resolve.calls.callUtil.getType
 import org.jetbrains.kotlin.resolve.lazy.BodyResolveMode
 import org.jetbrains.kotlin.types.isError
+import org.jetbrains.kotlin.types.isNullable
 import org.jetbrains.kotlin.types.typeUtil.isNothing
 import org.jetbrains.kotlin.types.typeUtil.isSubtypeOf
 import org.jetbrains.kotlin.types.typeUtil.makeNotNullable
 import org.jetbrains.kotlin.utils.addToStdlib.firstIsInstanceOrNull
+import org.jetbrains.kotlin.utils.addToStdlib.safeAs
 
 class FoldInitializerAndIfToElvisInspection : AbstractApplicabilityBasedInspection<KtIfExpression>(KtIfExpression::class.java) {
     override fun inspectionText(element: KtIfExpression): String = KotlinBundle.message("if.null.return.break.foldable.to")
@@ -137,6 +142,15 @@ class FoldInitializerAndIfToElvisInspection : AbstractApplicabilityBasedInspecti
                 val checkedType = ifExpression.analyze(BodyResolveMode.PARTIAL)[BindingContext.TYPE, typeReference]
                 val variableType = (prevStatement.resolveToDescriptorIfAny() as? VariableDescriptor)?.type
                 if (checkedType != null && variableType != null && !checkedType.isSubtypeOf(variableType)) return null
+            } else if (prevStatement.isVar && operationExpression is KtBinaryExpression) {
+                val ifEndOffset = ifExpression.endOffset
+                val context = ifExpression.analyze()
+                val isUsedAsNotNullable = ReferencesSearch.search(prevStatement, LocalSearchScope(prevStatement.parent)).any {
+                    if (it.element.startOffset <= ifEndOffset) return@any false
+                    val type = it.element.safeAs<KtExpression>()?.getType(context) ?: return@any false
+                    !type.isNullable()
+                }
+                if (isUsedAsNotNullable) return null
             }
 
             val statement = if (then is KtBlockExpression) then.statements.singleOrNull() else then

idea/testData/inspectionsLocal/foldInitializerAndIfToElvis/VarUsedAsNotNullable.kt

@@ -0,0 +1,8 @@
+// PROBLEM: none
+fun test(foo: Int?, bar: Int): Int {
+    var i = foo
+    <caret>if (i == null) {
+        return bar
+    }
+    return i
+}

idea/testData/inspectionsLocal/foldInitializerAndIfToElvis/VarUsedAsNotNullable2.kt

@@ -0,0 +1,12 @@
+// PROBLEM: none
+fun test(foo: Int?, bar: Int): Int {
+    var i = foo
+    <caret>if (i == null) {
+        return bar
+    }
+    baz(i)
+    val j = i + 1
+    return j
+}
+
+fun baz(i: Int?) {}

idea/testData/inspectionsLocal/foldInitializerAndIfToElvis/VarUsedAsNullable.kt

@@ -0,0 +1,7 @@
+fun test(foo: Int?, bar: Int): Int? {
+    var i = foo
+    <caret>if (i == null) {
+        return bar
+    }
+    return i
+}

idea/testData/inspectionsLocal/foldInitializerAndIfToElvis/VarUsedAsNullable.kt.after

@@ -0,0 +1,4 @@
+fun test(foo: Int?, bar: Int): Int? {
+    var i: Int? = foo ?: return bar
+    return i
+}

idea/testData/inspectionsLocal/foldInitializerAndIfToElvis/VarUsedAsNullable2.kt

@@ -0,0 +1,9 @@
+fun test(foo: Int?, bar: Int): Int {
+    var i = foo
+    <caret>if (i == null) {
+        return bar
+    }
+    return baz(i)
+}
+
+fun baz(i: Int?) = 1

idea/testData/inspectionsLocal/foldInitializerAndIfToElvis/VarUsedAsNullable2.kt.after

@@ -0,0 +1,6 @@
+fun test(foo: Int?, bar: Int): Int {
+    var i: Int? = foo ?: return bar
+    return baz(i)
+}
+
+fun baz(i: Int?) = 1

idea/tests/org/jetbrains/kotlin/idea/inspections/LocalInspectionTestGenerated.java

@@ -4923,6 +4923,26 @@ public class LocalInspectionTestGenerated extends AbstractLocalInspectionTest {
         public void testVar() throws Exception {
             runTest("idea/testData/inspectionsLocal/foldInitializerAndIfToElvis/Var.kt");
         }
+
+        @TestMetadata("VarUsedAsNotNullable.kt")
+        public void testVarUsedAsNotNullable() throws Exception {
+            runTest("idea/testData/inspectionsLocal/foldInitializerAndIfToElvis/VarUsedAsNotNullable.kt");
+        }
+
+        @TestMetadata("VarUsedAsNotNullable2.kt")
+        public void testVarUsedAsNotNullable2() throws Exception {
+            runTest("idea/testData/inspectionsLocal/foldInitializerAndIfToElvis/VarUsedAsNotNullable2.kt");
+        }
+
+        @TestMetadata("VarUsedAsNullable.kt")
+        public void testVarUsedAsNullable() throws Exception {
+            runTest("idea/testData/inspectionsLocal/foldInitializerAndIfToElvis/VarUsedAsNullable.kt");
+        }
+
+        @TestMetadata("VarUsedAsNullable2.kt")
+        public void testVarUsedAsNullable2() throws Exception {
+            runTest("idea/testData/inspectionsLocal/foldInitializerAndIfToElvis/VarUsedAsNullable2.kt");
+        }
     }
 
     @TestMetadata("idea/testData/inspectionsLocal/forEachParameterNotUsed")