[NI] Fix lambda return type when expected type is not functional type

Introduce a fresh type variable for lambda return type.

We can't set expected lambda return type to 'Any?', because we can't
infer the actual type from return expressions in lambda in that case.

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

@@ -24,6 +24,7 @@ import org.jetbrains.kotlin.resolve.calls.components.CreateDescriptorWithFreshTy
 import org.jetbrains.kotlin.resolve.calls.inference.ConstraintSystemBuilder
 import org.jetbrains.kotlin.resolve.calls.inference.addSubtypeConstraintIfCompatible
 import org.jetbrains.kotlin.resolve.calls.inference.model.ArgumentConstraintPosition
+import org.jetbrains.kotlin.resolve.calls.inference.model.TypeVariableForLambdaReturnType
 import org.jetbrains.kotlin.resolve.calls.model.*
 import org.jetbrains.kotlin.resolve.calls.tower.isSuccess
 import org.jetbrains.kotlin.types.UnwrappedType
@@ -82,7 +83,7 @@ internal object CheckArguments : ResolutionPart {
     }
 
     // if expected type isn't function type, then may be it is Function<R>, Any or just `T`
-    fun processLambdaArgument(
+    private fun processLambdaArgument(
             kotlinCall: KotlinCall,
             csBuilder: ConstraintSystemBuilder,
             argument: LambdaKotlinCallArgument,
@@ -117,7 +118,7 @@ internal object CheckArguments : ResolutionPart {
             parameters = argument.parametersTypes?.map { it ?: builtIns.nothingType } ?: emptyList()
             returnType = (argument as? FunctionExpression)?.returnType ?:
                          expectedType.arguments.singleOrNull()?.type?.unwrap()?.takeIf { isFunctionSupertype } ?:
-                         builtIns.nullableAnyType
+                         createFreshTypeVariableForLambdaReturnType(csBuilder, argument, builtIns)
 
             // what about case where expected type is type variable? In old TY such cases was not supported. => do nothing for now. todo design
         }
@@ -130,7 +131,17 @@ internal object CheckArguments : ResolutionPart {
         return null
     }
 
-    fun checkCallableExpectedType(
+    private fun createFreshTypeVariableForLambdaReturnType(
+            csBuilder: ConstraintSystemBuilder,
+            argument: LambdaKotlinCallArgument,
+            builtIns: KotlinBuiltIns
+    ): UnwrappedType {
+        val typeVariable = TypeVariableForLambdaReturnType(argument, builtIns, "_L")
+        csBuilder.registerVariable(typeVariable)
+        return typeVariable.defaultType
+    }
+
+    private fun checkCallableExpectedType(
             csBuilder: ConstraintSystemBuilder,
             argument: CallableReferenceKotlinCallArgument,
             expectedType: UnwrappedType

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

@@ -21,6 +21,7 @@ import org.jetbrains.kotlin.descriptors.ClassifierDescriptor
 import org.jetbrains.kotlin.descriptors.TypeParameterDescriptor
 import org.jetbrains.kotlin.descriptors.annotations.Annotations
 import org.jetbrains.kotlin.resolve.calls.model.KotlinCall
+import org.jetbrains.kotlin.resolve.calls.model.LambdaKotlinCallArgument
 import org.jetbrains.kotlin.resolve.descriptorUtil.builtIns
 import org.jetbrains.kotlin.resolve.scopes.MemberScope
 import org.jetbrains.kotlin.types.*
@@ -55,3 +56,9 @@ class TypeVariableFromCallableDescriptor(
         val originalTypeParameter: TypeParameterDescriptor,
         val call: KotlinCall? = null
 ) : NewTypeVariable(originalTypeParameter.builtIns, originalTypeParameter.name.identifier)
+
+class TypeVariableForLambdaReturnType(
+        val lambdaArgument: LambdaKotlinCallArgument,
+        builtIns: KotlinBuiltIns,
+        name: String
+) : NewTypeVariable(builtIns, name)