Do not insert redundant space

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

@@ -29,6 +29,7 @@ import org.jetbrains.kotlin.builtins.extractParameterNameFromFunctionTypeArgumen
 import org.jetbrains.kotlin.builtins.getValueParameterTypesFromFunctionType
 import org.jetbrains.kotlin.builtins.isFunctionType
 import org.jetbrains.kotlin.idea.caches.resolve.getResolutionFacade
+import org.jetbrains.kotlin.idea.completion.handlers.isCharAt
 import org.jetbrains.kotlin.idea.core.ExpectedInfos
 import org.jetbrains.kotlin.idea.core.KotlinNameSuggester
 import org.jetbrains.kotlin.idea.core.fuzzyType
@@ -62,8 +63,17 @@ object LambdaSignatureTemplates {
             context.project.executeWriteCommand(commandName, groupId = commandGroupId) {
                 try {
                     if (rangeMarker.isValid) {
-                        context.document.deleteString(rangeMarker.startOffset, rangeMarker.endOffset)
+                        val startOffset = rangeMarker.startOffset
-                        context.editor.caretModel.moveToOffset(rangeMarker.startOffset)
+                        context.document.deleteString(startOffset, rangeMarker.endOffset)
+
+                        if (signatureOnly) {
+                            val spaceAhead = context.document.charsSequence.isCharAt(startOffset, ' ')
+                            if (!spaceAhead) {
+                                context.document.insertString(startOffset, " ")
+                            }
+                        }
+
+                        context.editor.caretModel.moveToOffset(startOffset)
                         val template = buildTemplate(lambdaType, signatureOnly, explicitParameterTypes, context.project)
                         TemplateManager.getInstance(context.project).startTemplate(context.editor, template)
                     }
@@ -181,9 +191,6 @@ object LambdaSignatureTemplates {
         if (!signatureOnly) {
             template.addTextSegment(" }")
         }
-        else {
-            template.addTextSegment(" ") //TODO: no additional space if space ahead
-        }
 
         return template
     }

idea/idea-completion/testData/handlers/smart/lambdaSignature/NoAdditionalSpace.kt

@@ -0,0 +1,7 @@
+fun foo(p: (x: Char, String) -> Unit){}
+
+fun bar() {
+    foo { <caret> }
+}
+
+// ELEMENT: "x, s ->"

idea/idea-completion/testData/handlers/smart/lambdaSignature/NoAdditionalSpace.kt.after

@@ -0,0 +1,7 @@
+fun foo(p: (x: Char, String) -> Unit){}
+
+fun bar() {
+    foo { x, s -> <caret> }
+}
+
+// ELEMENT: "x, s ->"

idea/idea-completion/testData/handlers/smart/lambdaSignature/Simple.kt

@@ -0,0 +1,7 @@
+fun foo(p: (x: Char, String) -> Unit){}
+
+fun bar() {
+    foo { <caret>}
+}
+
+// ELEMENT: "x, s ->"

idea/idea-completion/testData/handlers/smart/lambdaSignature/Simple.kt.after

@@ -0,0 +1,7 @@
+fun foo(p: (x: Char, String) -> Unit){}
+
+fun bar() {
+    foo { x, s -> <caret> }
+}
+
+// ELEMENT: "x, s ->"

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

@@ -926,4 +926,25 @@ public class SmartCompletionHandlerTestGenerated extends AbstractSmartCompletion
             doTest(fileName);
         }
     }
+
+    @TestMetadata("idea/idea-completion/testData/handlers/smart/lambdaSignature")
+    @TestDataPath("$PROJECT_ROOT")
+    @RunWith(JUnit3RunnerWithInners.class)
+    public static class LambdaSignature extends AbstractSmartCompletionHandlerTest {
+        public void testAllFilesPresentInLambdaSignature() throws Exception {
+            KotlinTestUtils.assertAllTestsPresentByMetadata(this.getClass(), new File("idea/idea-completion/testData/handlers/smart/lambdaSignature"), Pattern.compile("^(.+)\\.kt$"), TargetBackend.ANY, true);
+        }
+
+        @TestMetadata("NoAdditionalSpace.kt")
+        public void testNoAdditionalSpace() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("idea/idea-completion/testData/handlers/smart/lambdaSignature/NoAdditionalSpace.kt");
+            doTest(fileName);
+        }
+
+        @TestMetadata("Simple.kt")
+        public void testSimple() throws Exception {
+            String fileName = KotlinTestUtils.navigationMetadata("idea/idea-completion/testData/handlers/smart/lambdaSignature/Simple.kt");
+            doTest(fileName);
+        }
+    }
 }