Constraint system now may return partial substitutor

core/descriptors/src/org/jetbrains/kotlin/resolve/calls/inference/ConstraintSystem.kt

@@ -82,4 +82,9 @@ public interface ConstraintSystem {
      * If there is no information for type parameter, returns type projection for DONT_CARE type.
      */
     public fun getCurrentSubstitutor(): TypeSubstitutor
+
+    /**
+     * Returns a substitution only for type parameters that have result values, otherwise returns a type parameter itself.
+     */
+    public fun getPartialSubstitutor(): TypeSubstitutor
 }

core/descriptors/src/org/jetbrains/kotlin/resolve/calls/inference/ConstraintSystemImpl.kt

@@ -108,16 +108,17 @@ public class ConstraintSystemImpl : ConstraintSystem {
             substituteOriginal: Boolean
     ): Map<TypeParameterDescriptor, TypeProjection> {
         val substitutionContext = HashMap<TypeParameterDescriptor, TypeProjection>()
-        for ((typeParameter, typeBounds) in typeParameterBounds) {
+        for ((variable, typeBounds) in typeParameterBounds) {
             val typeProjection: TypeProjection
             val value = typeBounds.value
+            val typeParameter = if (substituteOriginal) variablesToOriginal[variable]!! else variable
             if (value != null && !TypeUtils.containsSpecialType(value, DONT_CARE)) {
                 typeProjection = TypeProjectionImpl(value)
             }
             else {
                 typeProjection = getDefaultTypeProjection(typeParameter)
             }
-            substitutionContext.put(if (substituteOriginal) variablesToOriginal[typeParameter]!! else typeParameter, typeProjection)
+            substitutionContext.put(typeParameter, typeProjection)
         }
         return substitutionContext
     }
@@ -449,6 +450,9 @@ public class ConstraintSystemImpl : ConstraintSystem {
     override fun getCurrentSubstitutor() =
             getSubstitutor(substituteOriginal = true) { TypeProjectionImpl(TypeUtils.DONT_CARE) }
 
+    override fun getPartialSubstitutor() =
+            getSubstitutor(substituteOriginal = true) { TypeProjectionImpl(it.correspondingType) }
+
     private fun getSubstitutor(substituteOriginal: Boolean, getDefaultValue: (TypeParameterDescriptor) -> TypeProjection) =
             replaceUninferredBy(getDefaultValue, substituteOriginal).setApproximateCapturedTypes()