[typescript-fetch] Fix infinite recursion with self-referencing discriminator mappings (#21956)

* Add selfReferencingDiscriminatorMapping to ExtendedCodegenModel

* Add selfReferencingDiscriminatorMapping to template

Author: sermler

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/TypeScriptFetchClientCodegen.java

@@ -1499,11 +1499,16 @@ public class ExtendedCodegenModel extends CodegenModel {
         public Set<String> oneOfArrays = new TreeSet<>();
         @Getter @Setter
         public Set<CodegenProperty> oneOfPrimitives = new HashSet<>();
-
+        @Getter @Setter
+        public CodegenDiscriminator.MappedModel selfReferencingDiscriminatorMapping;
+      
         public boolean isEntity; // Is a model containing an "id" property marked as isUniqueId
         public String returnPassthrough;
         public boolean hasReturnPassthroughVoid;
-
+        
+        public boolean hasSelfReferencingDiscriminatorMapping(){
+            return selfReferencingDiscriminatorMapping != null;
+        }
         public boolean isDateType() {
             return isDate && "Date".equals(dataType);
         }
@@ -1597,6 +1602,15 @@ public ExtendedCodegenModel(CodegenModel cm) {
             this.setAdditionalProperties(cm.getAdditionalProperties());
             this.setIsModel(cm.getIsModel());
             this.setComposedSchemas(cm.getComposedSchemas());
+            if (this.discriminator != null) {
+                Set<CodegenDiscriminator.MappedModel> mappedModels = this.discriminator.getMappedModels();
+                for(CodegenDiscriminator.MappedModel mappedModel : mappedModels) {
+                    if(mappedModel.getModelName().equals(cm.classname)){
+                        this.discriminator.getMappedModels().remove(mappedModel);
+                        this.selfReferencingDiscriminatorMapping=mappedModel;
+                    }
+                }
+            }
         }
 
         @Override

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

@@ -47,6 +47,12 @@ export function {{classname}}FromJSONTyped(json: any, ignoreDiscriminator: boole
             return {{modelName}}FromJSONTyped(json, ignoreDiscriminator);
         }
 {{/discriminator.mappedModels}}
+    {{#hasSelfReferencingDiscriminatorMapping}}
+        if (json['{{discriminator.propertyBaseName}}'] === '{{selfReferencingDiscriminatorMapping.mappingName}}') {
+            return {{selfReferencingDiscriminatorMapping.modelName}}FromJSONTyped(json, true);
+        }
+    {{/hasSelfReferencingDiscriminatorMapping}}
+
     }
 {{/discriminator}}
     return {