Override completion right after override modifier

#KT-25312 Fixed

idea/idea-completion/src/org/jetbrains/kotlin/idea/completion/OverridesCompletion.kt

@@ -112,12 +112,24 @@ class OverridesCompletion(
                         else -> "dummy() {}"
                     }
                     val dummyMemberText = dummyMemberHead + dummyMemberTail
-                    context.document.replaceString(context.startOffset, context.tailOffset, dummyMemberText)
+                    val override = KtTokens.OVERRIDE_KEYWORD.value
+
+                    tailrec fun calcStartOffset(startOffset: Int, diff: Int = 0): Int {
+                        if (context.document.text[startOffset - 1].isWhitespace()) {
+                            return calcStartOffset(startOffset - 1, diff + 1)
+                        } else if (context.document.text.substring(startOffset - override.length, startOffset) == override) {
+                            return startOffset - override.length
+                        } else return diff + startOffset
+                    }
+
+                    val startOffset = calcStartOffset(context.startOffset)
+                    val tailOffset = context.tailOffset
+                    context.document.replaceString(startOffset, tailOffset, dummyMemberText)
 
                     val psiDocumentManager = PsiDocumentManager.getInstance(context.project)
                     psiDocumentManager.commitAllDocuments()
 
-                    val dummyMember = context.file.findElementAt(context.startOffset)!!.getStrictParentOfType<KtNamedDeclaration>()!!
+                    val dummyMember = context.file.findElementAt(startOffset)!!.getStrictParentOfType<KtNamedDeclaration>()!!
 
                     // keep original modifiers
                     val modifierList = KtPsiFactory(context.project).createModifierList(dummyMember.modifierList!!.text)

idea/idea-completion/testData/handlers/basic/override/kt25312.kt

@@ -0,0 +1,8 @@
+interface Interface {
+    fun funA()
+}
+
+expect class SClass : Interface { // there is no error highlighting about unimplemented members, see KT-25044
+    override fu<caret>
+}
+// ELEMENT_TEXT: "override fun funA() {...}"

idea/idea-completion/testData/handlers/basic/override/kt25312.kt.after

@@ -0,0 +1,8 @@
+interface Interface {
+    fun funA()
+}
+
+expect class SClass : Interface { // there is no error highlighting about unimplemented members, see KT-25044
+    override fun funA()<caret>
+}
+// ELEMENT_TEXT: "override fun funA() {...}"

idea/idea-completion/tests/org/jetbrains/kotlin/idea/completion/test/handlers/BasicCompletionHandlerTestGenerated.java

@@ -521,6 +521,11 @@ public class BasicCompletionHandlerTestGenerated extends AbstractBasicCompletion
             runTest("idea/idea-completion/testData/handlers/basic/override/KeepModifiersBefore.kt");
         }
 
+        @TestMetadata("kt25312.kt")
+        public void testKt25312() throws Exception {
+            runTest("idea/idea-completion/testData/handlers/basic/override/kt25312.kt");
+        }
+
         @TestMetadata("OverrideFunction.kt")
         public void testOverrideFunction() throws Exception {
             runTest("idea/idea-completion/testData/handlers/basic/override/OverrideFunction.kt");