Fixed Code completion does not work inside function variable invocation

#KT-4978 Fixed

idea/src/org/jetbrains/jet/plugin/completion/CompletionSession.java

@@ -18,10 +18,7 @@ package org.jetbrains.jet.plugin.completion;
 
 import com.google.common.base.Predicate;
 import com.google.common.collect.Collections2;
-import com.intellij.codeInsight.completion.CompletionParameters;
-import com.intellij.codeInsight.completion.CompletionResultSet;
-import com.intellij.codeInsight.completion.CompletionType;
-import com.intellij.codeInsight.completion.JavaCompletionContributor;
+import com.intellij.codeInsight.completion.*;
 import com.intellij.codeInsight.lookup.LookupElement;
 import com.intellij.openapi.project.Project;
 import com.intellij.openapi.util.Condition;
@@ -69,18 +66,20 @@ class CompletionSession {
         JetScope scope = expressionBindingContext.get(BindingContext.RESOLUTION_SCOPE, jetReference.getExpression());
 
         inDescriptor = scope != null ? scope.getContainingDeclaration() : null;
-
         Condition<DeclarationDescriptor> descriptorFilter = new Condition<DeclarationDescriptor>() {
             @Override
             public boolean value(DeclarationDescriptor descriptor) {
                 return isVisibleDescriptor(descriptor);
             }
         };
-        this.jetResult = new JetCompletionResultSet(
-                WeigherPackage.addJetSorting(result, parameters),
-                resolveSession,
-                expressionBindingContext,
-                descriptorFilter);
+
+        // set prefix matcher here to override default one which relies on CompletionUtil.findReferencePrefix()
+        // which sometimes works incorrectly for Kotlin
+        result = result.withPrefixMatcher(CompletionUtil.findJavaIdentifierPrefix(parameters));
+
+        result = WeigherPackage.addJetSorting(result, parameters);
+
+        this.jetResult = new JetCompletionResultSet(result, resolveSession, expressionBindingContext, descriptorFilter);
     }
 
     public void completeForReference() {

idea/src/org/jetbrains/jet/plugin/completion/JetTypesCompletionHelper.java

@@ -65,12 +65,10 @@ public class JetTypesCompletionHelper {
             @NotNull CompletionParameters parameters,
             @NotNull final JetCompletionResultSet jetCompletionResult
     ) {
-        CompletionResultSet tempResult = jetCompletionResult.getResult().withPrefixMatcher(
-                CompletionUtil.findReferenceOrAlphanumericPrefix(parameters));
         JavaClassNameCompletionContributor.addAllClasses(
                 parameters,
                 false,
-                JavaCompletionSorting.addJavaSorting(parameters, tempResult).getPrefixMatcher(),
+                JavaCompletionSorting.addJavaSorting(parameters, jetCompletionResult.getResult()).getPrefixMatcher(),
                 new Consumer<LookupElement>() {
                     @Override
                     public void consume(LookupElement lookupElement) {

idea/testData/completion/basic/common/FunctionVariableCallArgument.kt

@@ -0,0 +1,6 @@
+fun f(p: (String) -> Unit, s: String) {
+    p(<caret>)
+}
+
+// EXIST: s
+// EXIST: StringBuilder

idea/testData/completion/smart/InvokeFunctionVariable.kt

@@ -0,0 +1,5 @@
+fun f(p: (String) -> Unit, s: String) {
+    p(<caret>)
+}
+
+// EXIST: s

idea/tests/org/jetbrains/jet/completion/JSBasicCompletionTestGenerated.java

@@ -189,6 +189,11 @@ public class JSBasicCompletionTestGenerated extends AbstractJSBasicCompletionTes
             doTest("idea/testData/completion/basic/common/FunctionCompletionFormatting.kt");
         }
         
+        @TestMetadata("FunctionVariableCallArgument.kt")
+        public void testFunctionVariableCallArgument() throws Exception {
+            doTest("idea/testData/completion/basic/common/FunctionVariableCallArgument.kt");
+        }
+        
         @TestMetadata("HigherOrderFunction1.kt")
         public void testHigherOrderFunction1() throws Exception {
             doTest("idea/testData/completion/basic/common/HigherOrderFunction1.kt");

idea/tests/org/jetbrains/jet/completion/JvmBasicCompletionTestGenerated.java

@@ -189,6 +189,11 @@ public class JvmBasicCompletionTestGenerated extends AbstractJvmBasicCompletionT
             doTest("idea/testData/completion/basic/common/FunctionCompletionFormatting.kt");
         }
         
+        @TestMetadata("FunctionVariableCallArgument.kt")
+        public void testFunctionVariableCallArgument() throws Exception {
+            doTest("idea/testData/completion/basic/common/FunctionVariableCallArgument.kt");
+        }
+        
         @TestMetadata("HigherOrderFunction1.kt")
         public void testHigherOrderFunction1() throws Exception {
             doTest("idea/testData/completion/basic/common/HigherOrderFunction1.kt");

idea/tests/org/jetbrains/jet/completion/JvmSmartCompletionTestGenerated.java

@@ -266,6 +266,11 @@ public class JvmSmartCompletionTestGenerated extends AbstractJvmSmartCompletionT
         doTest("idea/testData/completion/smart/InsideIdentifier.kt");
     }
     
+    @TestMetadata("InvokeFunctionVariable.kt")
+    public void testInvokeFunctionVariable() throws Exception {
+        doTest("idea/testData/completion/smart/InvokeFunctionVariable.kt");
+    }
+    
     @TestMetadata("JavaEnumMembers.kt")
     public void testJavaEnumMembers() throws Exception {
         doTest("idea/testData/completion/smart/JavaEnumMembers.kt");