Do not report "redundant Unit" for generic calls coerced to Unit

So #KT-18999 Fixed

idea/src/org/jetbrains/kotlin/idea/inspections/RedundantUnitReturnTypeInspection.kt

@@ -20,12 +20,14 @@ import com.intellij.codeInspection.IntentionWrapper
 import com.intellij.codeInspection.ProblemHighlightType
 import com.intellij.codeInspection.ProblemsHolder
 import com.intellij.psi.PsiElementVisitor
-import org.jetbrains.kotlin.descriptors.FunctionDescriptor
+import org.jetbrains.kotlin.idea.caches.resolve.analyze
 import org.jetbrains.kotlin.idea.intentions.RemoveExplicitTypeIntention
-import org.jetbrains.kotlin.idea.search.usagesSearch.descriptor
 import org.jetbrains.kotlin.psi.KtCodeFragment
 import org.jetbrains.kotlin.psi.KtNamedFunction
 import org.jetbrains.kotlin.psi.KtVisitorVoid
+import org.jetbrains.kotlin.resolve.BindingContext
+import org.jetbrains.kotlin.resolve.calls.callUtil.getResolvedCall
+import org.jetbrains.kotlin.resolve.lazy.BodyResolveMode
 import org.jetbrains.kotlin.types.typeUtil.isUnit
 
 class RedundantUnitReturnTypeInspection : AbstractKotlinInspection() {
@@ -34,13 +36,24 @@ class RedundantUnitReturnTypeInspection : AbstractKotlinInspection() {
             override fun visitNamedFunction(function: KtNamedFunction) {
                 super.visitNamedFunction(function)
                 if (function.containingFile is KtCodeFragment) return
-                if ((function.descriptor as? FunctionDescriptor)?.returnType?.isUnit() == true) {
-                    function.typeReference?.typeElement?.let {
-                        holder.registerProblem(it,
-                                               "Redundant 'Unit' return type",
-                                               ProblemHighlightType.LIKE_UNUSED_SYMBOL,
-                                               IntentionWrapper(RemoveExplicitTypeIntention(), function.containingKtFile))
+                val typeElement = function.typeReference?.typeElement ?: return
+                val context = function.analyze(BodyResolveMode.PARTIAL)
+                val descriptor = context[BindingContext.FUNCTION, function] ?: return
+                if (descriptor.returnType?.isUnit() == true) {
+                    if (!function.hasBlockBody()) {
+                        val bodyExpression = function.bodyExpression
+                        if (bodyExpression != null) {
+                            val resolvedCall = bodyExpression.getResolvedCall(bodyExpression.analyze(BodyResolveMode.PARTIAL))
+                            if (resolvedCall != null) {
+                                if (resolvedCall.candidateDescriptor.returnType?.isUnit() != true) return
+                            }
+                        }
                     }
+
+                    holder.registerProblem(typeElement,
+                                           "Redundant 'Unit' return type",
+                                           ProblemHighlightType.LIKE_UNUSED_SYMBOL,
+                                           IntentionWrapper(RemoveExplicitTypeIntention(), function.containingKtFile))
                 }
             }
         }

idea/testData/inspections/redundantUnitReturnType/WithLambda.kt

@@ -0,0 +1,21 @@
+fun <T> run(f: () -> T) = f()
+
+fun foo(): Unit = run {
+    bar()
+}
+
+fun bar() = 1
+
+fun call(f: () -> Unit) = f()
+
+fun boo(): Unit = call {
+    baz()
+}
+
+fun baz() {}
+
+fun <T, R> T.let(f: (T) -> R) = f(this)
+
+fun goo(): Unit = 1.let {
+    bar()
+}

idea/testData/inspections/redundantUnitReturnType/inspectionData/expected.xml

@@ -23,4 +23,12 @@
         <problem_class severity="WARNING" attribute_key="NOT_USED_ELEMENT_ATTRIBUTES">Redundant 'Unit' return type</problem_class>
         <description>Redundant Unit return type</description>
     </problem>
+    <problem>
+        <file>WithLambda.kt</file>
+        <line>11</line>
+        <module>light_idea_test_case</module>
+        <entry_point TYPE="file" FQNAME="temp:///src/WithLambda.kt" />
+        <problem_class severity="WARNING" attribute_key="NOT_USED_ELEMENT_ATTRIBUTES">Redundant 'Unit' return type</problem_class>
+        <description>Redundant 'Unit' return type</description>
+    </problem>
 </problems>