types library: add coercedTo

lib/types.nix

@@ -352,6 +352,28 @@ rec {
       functor = (defaultFunctor name) // { wrapped = [ t1 t2 ]; };
     };
 
+    coercedTo = coercedType: coerceFunc: finalType:
+      assert coercedType.getSubModules == null;
+      mkOptionType rec {
+        name = "coercedTo";
+        description = "${finalType.description} or ${coercedType.description}";
+        check = x: finalType.check x || coercedType.check x;
+        merge = loc: defs:
+          let
+            coerceVal = val:
+              if finalType.check val then val
+              else let
+                coerced = coerceFunc val;
+              in assert finalType.check coerced; coerced;
+
+          in finalType.merge loc (map (def: def // { value = coerceVal def.value; }) defs);
+        getSubOptions = finalType.getSubOptions;
+        getSubModules = finalType.getSubModules;
+        substSubModules = m: coercedTo coercedType coerceFunc (finalType.substSubModules m);
+        typeMerge = t1: t2: null;
+        functor = (defaultFunctor name) // { wrapped = finalType; };
+      };
+
     # Obsolete alternative to configOf.  It takes its option
     # declarations from the ‘options’ attribute of containing option
     # declaration.