KT-6780 Completion inserts incorrect comma in end of parameter list when lambda parameter exists

#KT-6780 Fixed

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

@@ -208,11 +208,21 @@ class ExpectedInfos(
                 }
             }
             else {
+                val lastNonOptionalParam = parameters.lastOrNull { !it.hasDefaultValue() }
+
+                fun needCommaForParameter(parameter: ValueParameterDescriptor): Boolean {
+                    if (parameter.hasDefaultValue()) return false // parameter is optional
+                    if (parameter.getVarargElementType() != null) return false // vararg arguments list can be empty
+                    // last non-optional parameter of functional type can be placed outside parenthesis:
+                    if (parameter == lastNonOptionalParam && KotlinBuiltIns.isFunctionOrExtensionFunctionType(parameter.getType())) return false
+                    return true
+                }
+
                 val tail = if (isFunctionLiteralArgument)
                     null
                 else if (parameterIndex == parameters.lastIndex)
                     Tail.RPARENTH //TODO: support square brackets
-                else if (parameters.drop(parameterIndex + 1).all { it.hasDefaultValue() || it.getVarargElementType() != null })
+                else if (parameters.drop(parameterIndex + 1).none(::needCommaForParameter))
                     null
                 else
                     Tail.COMMA

idea/idea-completion/testData/handlers/smart/FunctionLiteralParamAlreadyExist.kt

@@ -0,0 +1,7 @@
+fun foo(p: Int, handler: () -> Unit){}
+
+fun bar(p: Int) {
+    foo(<caret>) { doIt() }
+}
+
+// ELEMENT: p

idea/idea-completion/testData/handlers/smart/FunctionLiteralParamAlreadyExist.kt.after

@@ -0,0 +1,7 @@
+fun foo(p: Int, handler: () -> Unit){}
+
+fun bar(p: Int) {
+    foo(p<caret>) { doIt() }
+}
+
+// ELEMENT: p

idea/idea-completion/testData/handlers/smart/LastNonOptionalParamIsFunction.kt

@@ -0,0 +1,7 @@
+fun foo(p: Int, handler: () -> Unit, optional: String = ""){}
+
+fun bar(p: Int) {
+    foo(<caret>)
+}
+
+// ELEMENT: p

idea/idea-completion/testData/handlers/smart/LastNonOptionalParamIsFunction.kt.after

@@ -0,0 +1,7 @@
+fun foo(p: Int, handler: () -> Unit, optional: String = ""){}
+
+fun bar(p: Int) {
+    foo(p<caret>)
+}
+
+// ELEMENT: p

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

@@ -329,6 +329,12 @@ public class SmartCompletionHandlerTestGenerated extends AbstractSmartCompletion
         doTest(fileName);
     }
 
+    @TestMetadata("FunctionLiteralParamAlreadyExist.kt")
+    public void testFunctionLiteralParamAlreadyExist() throws Exception {
+        String fileName = JetTestUtils.navigationMetadata("idea/idea-completion/testData/handlers/smart/FunctionLiteralParamAlreadyExist.kt");
+        doTest(fileName);
+    }
+
     @TestMetadata("FunctionReference1.kt")
     public void testFunctionReference1() throws Exception {
         String fileName = JetTestUtils.navigationMetadata("idea/idea-completion/testData/handlers/smart/FunctionReference1.kt");
@@ -461,6 +467,12 @@ public class SmartCompletionHandlerTestGenerated extends AbstractSmartCompletion
         doTest(fileName);
     }
 
+    @TestMetadata("LastNonOptionalParamIsFunction.kt")
+    public void testLastNonOptionalParamIsFunction() throws Exception {
+        String fileName = JetTestUtils.navigationMetadata("idea/idea-completion/testData/handlers/smart/LastNonOptionalParamIsFunction.kt");
+        doTest(fileName);
+    }
+
     @TestMetadata("MergeTail1.kt")
     public void testMergeTail1() throws Exception {
         String fileName = JetTestUtils.navigationMetadata("idea/idea-completion/testData/handlers/smart/MergeTail1.kt");