fix: support backwards compatible changes in discriminators (#20983)

Closes https://github.com/OpenAPITools/openapi-generator/issues/20982

modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache

@@ -126,7 +126,7 @@ export function {{classname}}ToJSONTyped(value?: {{#hasReadOnly}}Omit<{{classnam
                 return {{modelName}}ToJSONTyped(value as {{modelName}}, ignoreDiscriminator);
         {{/discriminator.mappedModels}}
             default:
-                throw new Error(`No variant of {{classname}} exists with '{{discriminator.propertyName}}=${value['{{discriminator.propertyName}}']}'`);
+                return value;
         }
     }
     {{/discriminator}}

modules/openapi-generator/src/main/resources/typescript-fetch/modelOneOf.mustache

@@ -27,7 +27,7 @@ export function {{classname}}FromJSONTyped(json: any, ignoreDiscriminator: boole
             return Object.assign({}, {{modelName}}FromJSONTyped(json, true), { {{discriminator.propertyName}}: '{{mappingName}}' } as const);
 {{/discriminator.mappedModels}}
         default:
-            throw new Error(`No variant of {{classname}} exists with '{{discriminator.propertyName}}=${json['{{discriminator.propertyName}}']}'`);
+            return json;
     }
 {{/discriminator}}
 {{^discriminator}}
@@ -56,7 +56,7 @@ export function {{classname}}ToJSONTyped(value?: {{classname}} | null, ignoreDis
             return Object.assign({}, {{modelName}}ToJSON(value), { {{discriminator.propertyName}}: '{{mappingName}}' } as const);
 {{/discriminator.mappedModels}}
         default:
-            throw new Error(`No variant of {{classname}} exists with '{{discriminator.propertyName}}=${value['{{discriminator.propertyName}}']}'`);
+            return json;
     }
 {{/discriminator}}
 

samples/client/others/typescript-fetch/self-import-issue/models/AbstractUserDto.ts

@@ -96,7 +96,7 @@ export function AbstractUserDtoToJSONTyped(value?: AbstractUserDto | null, ignor
             case 'remote-authenticated':
                 return RemoteAuthenticatedUserDtoToJSONTyped(value as RemoteAuthenticatedUserDto, ignoreDiscriminator);
             default:
-                throw new Error(`No variant of AbstractUserDto exists with 'type=${value['type']}'`);
+                return value;
         }
     }
 

samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Animal.ts

@@ -82,7 +82,7 @@ export function AnimalToJSONTyped(value?: Animal | null, ignoreDiscriminator: bo
             case 'DOG':
                 return DogToJSONTyped(value as Dog, ignoreDiscriminator);
             default:
-                throw new Error(`No variant of Animal exists with 'className=${value['className']}'`);
+                return value;
         }
     }
 

samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ParentWithNullable.ts

@@ -85,7 +85,7 @@ export function ParentWithNullableToJSONTyped(value?: ParentWithNullable | null,
             case 'ChildWithNullable':
                 return ChildWithNullableToJSONTyped(value as ChildWithNullable, ignoreDiscriminator);
             default:
-                throw new Error(`No variant of ParentWithNullable exists with 'type=${value['type']}'`);
+                return value;
         }
     }
 

samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Animal.ts

@@ -82,7 +82,7 @@ export function AnimalToJSONTyped(value?: Animal | null, ignoreDiscriminator: bo
             case 'DOG':
                 return DogToJSONTyped(value as Dog, ignoreDiscriminator);
             default:
-                throw new Error(`No variant of Animal exists with 'className=${value['className']}'`);
+                return value;
         }
     }