Prohibit not-Unit suspend functions

#KT-13560 Fixed

compiler/frontend/src/org/jetbrains/kotlin/resolve/SuspendModifierChecker.kt

@@ -80,6 +80,10 @@ object SuspendModifierChecker : SimpleDeclarationChecker {
         if (continuationParameterType?.arguments?.firstOrNull()?.isStarProjection == true) {
             report("Continuation<*> is prohibited as a last parameter of suspend function")
         }
+
+        if (functionDescriptor.returnType?.isUnit() != true) {
+            report("return type of suspension function must be a kotlin.Unit, but ${functionDescriptor.returnType} was found")
+        }
     }
 }
 

compiler/testData/diagnostics/tests/coroutines/suspendApplicability.kt

@@ -38,4 +38,7 @@ class Controller {
     suspend fun String.memberExtension(x: Continuation<Int>) {
 
     }
+
+    suspend fun returnsUnit(x: Continuation<Int>) = Unit
+    <!INAPPLICABLE_MODIFIER!>suspend<!> fun returnsNotUnit(x: Continuation<Int>) = 1
 }

compiler/testData/diagnostics/tests/coroutines/suspendApplicability.txt

@@ -10,6 +10,8 @@ public suspend fun kotlin.String.wrongExtension(/*0*/ x: kotlin.coroutines.Conti
     public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
     public final inline suspend fun inlineFun(/*0*/ x: kotlin.coroutines.Continuation<kotlin.Int>): kotlin.Unit
     public final suspend fun noParams(): kotlin.Unit
+    public final suspend fun returnsNotUnit(/*0*/ x: kotlin.coroutines.Continuation<kotlin.Int>): kotlin.Int
+    public final suspend fun returnsUnit(/*0*/ x: kotlin.coroutines.Continuation<kotlin.Int>): kotlin.Unit
     public final suspend fun starProjection(/*0*/ x: kotlin.coroutines.Continuation<*>): kotlin.Unit
     public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
     public final suspend fun valid(/*0*/ x: kotlin.coroutines.Continuation<kotlin.Int>): kotlin.Unit

idea/testData/checker/infos/suspendApplicability.kt

@@ -38,4 +38,6 @@ class Controller {
     <info descr="null">suspend</info> fun String.memberExtension(<warning descr="[UNUSED_PARAMETER] Parameter 'x' is never used">x</warning>: Continuation<Int>) {
 
     }
+
+    <error descr="[INAPPLICABLE_MODIFIER] 'suspend' modifier is inapplicable. The reason is that return type of suspension function must be a kotlin.Unit, but Int was found"><info descr="null">suspend</info></error> fun returnsNotUnit(<warning descr="[UNUSED_PARAMETER] Parameter 'x' is never used">x</warning>: Continuation<Int>) = 1
 }