KT-8743 Propose class names for extension receiver in completion after 'fun' if a capital letter was entered

#KT-8743 Fixed

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

@@ -39,6 +39,7 @@ import org.jetbrains.kotlin.psi.*
 import org.jetbrains.kotlin.psi.psiUtil.*
 import org.jetbrains.kotlin.resolve.bindingContextUtil.getReferenceTargets
 import org.jetbrains.kotlin.resolve.lazy.BodyResolveMode
+import org.jetbrains.kotlin.utils.addToStdlib.check
 import org.jetbrains.kotlin.utils.addToStdlib.firstIsInstanceOrNull
 
 public class KotlinCompletionContributor : CompletionContributor() {
@@ -275,21 +276,29 @@ public class KotlinCompletionContributor : CompletionContributor() {
         if (invocationCount == 0 && prefixMatcher.getPrefix().isEmpty() && AFTER_INTEGER_LITERAL_AND_DOT.accepts(position)) return true
 
         // no auto-popup on typing after "val", "var" and "fun" because it's likely the name of the declaration which is being typed by user
-        if (invocationCount == 0 && isInExtensionReceiver(position)) return true
+        if (invocationCount == 0) {
+            val callable = isInExtensionReceiverOf(position)
+            if (callable != null) {
+                return when (callable) {
+                    is JetNamedFunction -> prefixMatcher.prefix.let { it.isEmpty() || it[0].isLowerCase() /* function name usually starts with lower case letter */ }
+                    else -> true
+                }
+            }
+        }
 
         return false
     }
 
-    private fun isInExtensionReceiver(position: PsiElement): Boolean {
-        val nameRef = position.getParent() as? JetNameReferenceExpression ?: return false
-        val userType = nameRef.getParent() as? JetUserType ?: return false
-        val typeRef = userType.getParent() as? JetTypeReference ?: return false
-        if (userType != typeRef.getTypeElement()) return false
+    private fun isInExtensionReceiverOf(position: PsiElement): JetCallableDeclaration? {
+        val nameRef = position.getParent() as? JetNameReferenceExpression ?: return null
+        val userType = nameRef.getParent() as? JetUserType ?: return null
+        val typeRef = userType.getParent() as? JetTypeReference ?: return null
+        if (userType != typeRef.getTypeElement()) return null
         val parent = typeRef.getParent()
         return when (parent) {
-            is JetNamedFunction -> typeRef == parent.getReceiverTypeReference()
-            is JetProperty -> typeRef == parent.getReceiverTypeReference()
-            else -> false
+            is JetNamedFunction -> parent.check { typeRef == it.receiverTypeReference }
+            is JetProperty -> parent.check { typeRef == it.receiverTypeReference }
+            else -> null
         }
     }
 

idea/idea-completion/testData/basic/common/AutopopupInFunExtensionReceiver.kt

@@ -0,0 +1,6 @@
+class Xyz
+
+fun X<caret>
+
+// INVOCATION_COUNT: 0
+// EXIST: Xyz

idea/idea-completion/tests/org/jetbrains/kotlin/idea/completion/test/JSBasicCompletionTestGenerated.java

@@ -67,6 +67,12 @@ public class JSBasicCompletionTestGenerated extends AbstractJSBasicCompletionTes
             JetTestUtils.assertAllTestsPresentByMetadata(this.getClass(), new File("idea/idea-completion/testData/basic/common"), Pattern.compile("^(.+)\\.kt$"), true);
         }
 
+        @TestMetadata("AutopopupInFunExtensionReceiver.kt")
+        public void testAutopopupInFunExtensionReceiver() throws Exception {
+            String fileName = JetTestUtils.navigationMetadata("idea/idea-completion/testData/basic/common/AutopopupInFunExtensionReceiver.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("BasicAny.kt")
         public void testBasicAny() throws Exception {
             String fileName = JetTestUtils.navigationMetadata("idea/idea-completion/testData/basic/common/BasicAny.kt");

idea/idea-completion/tests/org/jetbrains/kotlin/idea/completion/test/JvmBasicCompletionTestGenerated.java

@@ -67,6 +67,12 @@ public class JvmBasicCompletionTestGenerated extends AbstractJvmBasicCompletionT
             JetTestUtils.assertAllTestsPresentByMetadata(this.getClass(), new File("idea/idea-completion/testData/basic/common"), Pattern.compile("^(.+)\\.kt$"), true);
         }
 
+        @TestMetadata("AutopopupInFunExtensionReceiver.kt")
+        public void testAutopopupInFunExtensionReceiver() throws Exception {
+            String fileName = JetTestUtils.navigationMetadata("idea/idea-completion/testData/basic/common/AutopopupInFunExtensionReceiver.kt");
+            doTest(fileName);
+        }
+
         @TestMetadata("BasicAny.kt")
         public void testBasicAny() throws Exception {
             String fileName = JetTestUtils.navigationMetadata("idea/idea-completion/testData/basic/common/BasicAny.kt");