Support ConstraintSystem.Builder.add, untested and unused at the moment

compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/inference/ConstraintSystem.kt

@@ -56,8 +56,8 @@ interface ConstraintSystem {
 
     interface Builder {
         /**
-         * Registers variables in a constraint system. Returns a substitutor which maps type parameter descriptors given as parameters
-         * to the corresponding type variables of the system. Use that substitutor to provide constraints to the system
+         * Registers variables in a constraint system. Returns a substitutor which maps type parameter descriptors passed as parameters
+         * to the corresponding types of variables of the system. Use that substitutor to provide constraints to the system
          */
         fun registerTypeVariables(
                 call: CallHandle, typeParameters: Collection<TypeParameterDescriptor>, external: Boolean = false
@@ -79,6 +79,12 @@ interface ConstraintSystem {
          */
         val typeVariableSubstitutors: Map<CallHandle, TypeSubstitutor>
 
+        /**
+         * Add all variables and constraints from the other system to this one. The other system may not have any common variables
+         * with this one, or even variables registered for calls, for which this system also has some registered variables
+         */
+        fun add(other: Builder)
+
         fun fixVariables()
 
         fun build(): ConstraintSystem

compiler/frontend/src/org/jetbrains/kotlin/resolve/calls/inference/ConstraintSystemBuilderImpl.kt

@@ -359,6 +359,30 @@ class ConstraintSystemBuilderImpl : ConstraintSystem.Builder {
         addBound(typeVariable, value, TypeBounds.BoundKind.EXACT_BOUND, ConstraintContext(ConstraintPositionKind.FROM_COMPLETER.position()))
     }
 
+    override fun add(other: ConstraintSystem.Builder) {
+        if (other !is ConstraintSystemBuilderImpl) {
+            throw IllegalArgumentException("Unknown constraint system builder implementation: $other")
+        }
+        if (!Collections.disjoint(typeVariableSubstitutors.keys, other.typeVariableSubstitutors.keys)) {
+            throw IllegalArgumentException(
+                    "Combining two constraint systems only makes sense when they were created for different calls. " +
+                    "Calls of the first system: ${typeVariableSubstitutors.keys}, second: ${other.typeVariableSubstitutors.keys}"
+            )
+        }
+        if (!Collections.disjoint(other.allTypeParameterBounds.keys, allTypeParameterBounds.keys)) {
+            throw IllegalArgumentException(
+                    "Combining two constraint systems only makes sense when they have no common variables. " +
+                    "First system variables: ${allTypeParameterBounds.keys}, second: ${other.allTypeParameterBounds.keys}"
+            )
+        }
+
+        allTypeParameterBounds.putAll(other.allTypeParameterBounds)
+        usedInBounds.putAll(other.usedInBounds)
+        errors.addAll(other.errors)
+        initialConstraints.addAll(other.initialConstraints)
+        typeVariableSubstitutors.putAll(other.typeVariableSubstitutors)
+    }
+
     override fun fixVariables() {
         // todo variables should be fixed in the right order
         val (external, functionTypeParameters) = allTypeParameterBounds.keys.partition { it.isExternal }