[NI] Coerce to unit for empty lambda block

Fixes 'kt3903'

compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/components/KotlinCallCompleter.kt

@@ -26,6 +26,7 @@ import org.jetbrains.kotlin.resolve.calls.inference.components.FixationOrderCalc
 import org.jetbrains.kotlin.resolve.calls.inference.components.NewTypeSubstitutor
 import org.jetbrains.kotlin.resolve.calls.inference.components.ResultTypeResolver
 import org.jetbrains.kotlin.resolve.calls.inference.model.ExpectedTypeConstraintPosition
+import org.jetbrains.kotlin.resolve.calls.inference.model.LambdaArgumentConstraintPosition
 import org.jetbrains.kotlin.resolve.calls.inference.model.NewTypeVariable
 import org.jetbrains.kotlin.resolve.calls.inference.model.NotEnoughInformationForTypeParameter
 import org.jetbrains.kotlin.resolve.calls.inference.returnTypeOrNothing
@@ -36,6 +37,7 @@ import org.jetbrains.kotlin.resolve.calls.tower.ResolutionCandidateStatus
 import org.jetbrains.kotlin.types.TypeUtils
 import org.jetbrains.kotlin.types.UnwrappedType
 import org.jetbrains.kotlin.types.checker.KotlinTypeChecker
+import org.jetbrains.kotlin.types.typeUtil.builtIns
 import org.jetbrains.kotlin.utils.SmartList
 import org.jetbrains.kotlin.utils.addIfNotNull
 
@@ -264,6 +266,11 @@ class KotlinCallCompleter(
             // todo strange code -- why top-level kotlinCall? may be it isn't right outer call
             CheckArguments.checkArgument(topLevelCallContext, topLevelCall, c.getBuilder(), innerCall, lambda.returnType.let(::substitute))
         }
+
+        if (lambda.resultArguments.isEmpty()) {
+            val unitType = lambda.returnType.builtIns.unitType
+            c.getBuilder().addSubtypeConstraint(lambda.returnType.let(::substitute), unitType, LambdaArgumentConstraintPosition(lambda))
+        }
     }
 
     private fun canWeAnalyzeIt(c: Context, lambda: ResolvedLambdaArgument): Boolean {

compiler/resolution/src/org/jetbrains/kotlin/resolve/calls/inference/model/ConstraintPositionAndErrors.kt

@@ -43,6 +43,11 @@ class FixVariableConstraintPosition(val variable: NewTypeVariable) : ConstraintP
 class KnownTypeParameterConstraintPosition(val typeArgument: KotlinType) : ConstraintPosition() {
     override fun toString() = "TypeArgument $typeArgument"
 }
+class LambdaArgumentConstraintPosition(val lambdaArgument: ResolvedLambdaArgument) : ConstraintPosition() {
+    override fun toString(): String {
+        return "LambdaArgument $lambdaArgument"
+    }
+}
 
 class IncorporationConstraintPosition(val from: ConstraintPosition, val initialConstraint: InitialConstraint) : ConstraintPosition() {
     override fun toString() = "Incorporate $initialConstraint from position $from"