Add a utility to substitute generic arguments into a canonical

generic function type.

Author: rjmccall

include/swift/AST/Types.h

@@ -3208,6 +3208,8 @@ BEGIN_CAN_TYPE_WRAPPER(GenericFunctionType, AnyFunctionType)
     return cast<GenericFunctionType>(fnType->getCanonicalType());
   }
 
+  CanFunctionType substGenericArgs(SubstitutionMap subs) const;
+
   CanGenericSignature getGenericSignature() const {
     return CanGenericSignature(getPointer()->getGenericSignature());
   }

lib/AST/Type.cpp

@@ -2683,6 +2683,12 @@ GenericFunctionType::substGenericArgs(SubstitutionMap subs) {
                            substFn->getResult(), getExtInfo());
 }
 
+CanFunctionType
+CanGenericFunctionType::substGenericArgs(SubstitutionMap subs) const {
+  return cast<FunctionType>(
+           getPointer()->substGenericArgs(subs)->getCanonicalType());
+}
+
 static Type getMemberForBaseType(LookupConformanceFn lookupConformances,
                                  Type origBase,
                                  Type substBase,