Allow a function to declare a Unit return type when it uses a generic…

… function initializer (#4371)

* Allow a function to declare a Unit return type when it uses a generic function initializer

* Added check for when redundant function return typing and generic typing are provided; Slight optimization in OptionalUnit for determining whether a function has an explicit return type

* Added test to expand code coverage

* Optimizing check for whether a function contains an initializer

* Adding more tests to improve code coverage

* Adding another test to improve code coverage

* Removing test due to it failing compilation check

detekt-rules-style/src/main/kotlin/io/gitlab/arturbosch/detekt/rules/style/optional/OptionalUnit.kt

@@ -9,17 +9,21 @@ import io.gitlab.arturbosch.detekt.api.Rule
 import io.gitlab.arturbosch.detekt.api.Severity
 import io.gitlab.arturbosch.detekt.rules.isOverride
 import org.jetbrains.kotlin.cfg.WhenChecker
+import org.jetbrains.kotlin.js.translate.callTranslator.getReturnType
 import org.jetbrains.kotlin.psi.KtBlockExpression
 import org.jetbrains.kotlin.psi.KtExpression
 import org.jetbrains.kotlin.psi.KtIfExpression
 import org.jetbrains.kotlin.psi.KtNameReferenceExpression
 import org.jetbrains.kotlin.psi.KtNamedFunction
+import org.jetbrains.kotlin.psi.KtTypeReference
 import org.jetbrains.kotlin.psi.KtWhenExpression
 import org.jetbrains.kotlin.psi.psiUtil.siblings
 import org.jetbrains.kotlin.resolve.BindingContext
 import org.jetbrains.kotlin.resolve.bindingContextUtil.isUsedAsExpression
+import org.jetbrains.kotlin.resolve.calls.callUtil.getResolvedCall
 import org.jetbrains.kotlin.resolve.calls.callUtil.getType
 import org.jetbrains.kotlin.types.typeUtil.isNothing
+import org.jetbrains.kotlin.types.typeUtil.isTypeParameter
 import org.jetbrains.kotlin.types.typeUtil.isUnit
 import org.jetbrains.kotlin.utils.addToStdlib.firstIsInstanceOrNull
 
@@ -56,8 +60,9 @@ class OptionalUnit(config: Config = Config.empty) : Rule(config) {
     )
 
     override fun visitNamedFunction(function: KtNamedFunction) {
-        if (function.hasDeclaredReturnType()) {
-            checkFunctionWithExplicitReturnType(function)
+        val typeReference = function.typeReference
+        if (typeReference != null) {
+            checkFunctionWithExplicitReturnType(function, typeReference)
         } else if (!function.isOverride()) {
             checkFunctionWithInferredReturnType(function)
         }
@@ -105,11 +110,11 @@ class OptionalUnit(config: Config = Config.empty) : Rule(config) {
         }
     }
 
-    private fun checkFunctionWithExplicitReturnType(function: KtNamedFunction) {
-        val typeReference = function.typeReference
-        val typeElementText = typeReference?.typeElement?.text
+    private fun checkFunctionWithExplicitReturnType(function: KtNamedFunction, typeReference: KtTypeReference) {
+        val typeElementText = typeReference.typeElement?.text
         if (typeElementText == UNIT) {
-            if (function.initializer.isNothingType()) return
+            val initializer = function.initializer
+            if (initializer?.isGenericOrNothingType() == true) return
             report(CodeSmell(issue, Entity.from(typeReference), createMessage(function)))
         }
     }
@@ -124,8 +129,14 @@ class OptionalUnit(config: Config = Config.empty) : Rule(config) {
     private fun createMessage(function: KtNamedFunction) = "The function ${function.name} " +
         "defines a return type of Unit. This is unnecessary and can safely be removed."
 
-    private fun KtExpression?.isNothingType() =
-        bindingContext != BindingContext.EMPTY && this?.getType(bindingContext)?.isNothing() == true
+    private fun KtExpression.isGenericOrNothingType(): Boolean {
+        if (bindingContext == BindingContext.EMPTY) return false
+        val isGenericType = getResolvedCall(bindingContext)?.getReturnType()?.isTypeParameter() == true
+        val isNothingType = getType(bindingContext)?.isNothing() == true
+        // Either the function initializer returns Nothing or it is a generic function
+        // into which Unit is passed, but not both.
+        return (isGenericType && !isNothingType) || (isNothingType && !isGenericType)
+    }
 
     companion object {
         private const val UNIT = "Unit"

detekt-rules-style/src/test/kotlin/io/gitlab/arturbosch/detekt/rules/style/optional/OptionalUnitSpec.kt

@@ -18,6 +18,21 @@ class OptionalUnitSpec : Spek({
     val env: KotlinCoreEnvironment by memoized()
 
     describe("OptionalUnit rule") {
+        it("should report when a function has an explicit Unit return type with context") {
+            val code = """
+                fun foo(): Unit { }
+            """.trimIndent()
+            val findings = subject.compileAndLintWithContext(env, code)
+            assertThat(findings).hasSize(1)
+        }
+
+        it("should not report when a function has a non-unit body expression") {
+            val code = """
+                fun foo() = String
+            """.trimIndent()
+            val findings = subject.compileAndLintWithContext(env, code)
+            assertThat(findings).isEmpty()
+        }
 
         context("several functions which return Unit") {
 
@@ -296,14 +311,72 @@ class OptionalUnitSpec : Spek({
                 val findings = subject.compileAndLintWithContext(env, code)
                 assertThat(findings).hasSize(1)
             }
+
+            it("another object is used as the last expression") {
+                val code = """
+                    fun foo() {
+                        String
+                    }
+                """.trimIndent()
+                val findings = subject.compileAndLintWithContext(env, code)
+                assertThat(findings).isEmpty()
+            }
         }
 
-        it("should not report when function initializer is Nothing") {
-            val code = """
+        context("function initializers") {
+            it("should not report when function initializer is Nothing") {
+                val code = """
                     fun test(): Unit = throw UnsupportedOperationException()
                 """
-            val findings = subject.compileAndLintWithContext(env, code)
-            assertThat(findings).isEmpty()
+                val findings = subject.compileAndLintWithContext(env, code)
+                assertThat(findings).isEmpty()
+            }
+
+            it("should not report when the function initializer requires a type") {
+                val code = """
+                    fun <T> foo(block: (List<T>) -> Unit): T {
+                        val list = listOf<T>()
+                        block(list)
+                        return list.first()
+                    }
+    
+                    fun doFoo(): Unit = foo {}
+                """.trimIndent()
+                val findings = subject.compileAndLintWithContext(env, code)
+                assertThat(findings).isEmpty()
+            }
+
+            it("should report on function initializers when there is no context") {
+                val code = """
+                    fun test(): Unit = throw UnsupportedOperationException()
+                """
+                val findings = subject.compileAndLint(code)
+                assertThat(findings).hasSize(1)
+            }
+
+            it("should report when the function initializer takes in the type Nothing") {
+                val code = """
+                    fun <T> foo(block: (List<T>) -> Unit): T {
+                        val list = listOf<T>()
+                        block(list)
+                        return list.first()
+                    }
+    
+                    fun doFoo(): Unit = foo<Nothing> {}
+                """.trimIndent()
+                val findings = subject.compileAndLintWithContext(env, code)
+                assertThat(findings).hasSize(1)
+            }
+
+            it("should report when the function initializer does not provide a different type") {
+                val code = """
+                    fun foo() {}
+                    
+                    fun doFoo(): Unit = foo()
+                """.trimIndent()
+                val findings = subject.compileAndLintWithContext(env, code)
+                assertThat(findings).hasSize(1)
+            }
         }
     }
 })