if discriminator is not defined with useOneOfDiscriminatorLookup set …

…to true, we generated as useOneOfDiscriminatorLookup false (OpenAPITools#11178)

modules/openapi-generator/src/main/resources/go/model_oneof.mustache

@@ -48,8 +48,38 @@ func (dst *{{classname}}) UnmarshalJSON(data []byte) error {
 	}
 
 	{{/mappedModels}}
-	{{/discriminator}}
 	return nil
+	{{/discriminator}}
+	{{^discriminator}}
+        match := 0
+        {{#oneOf}}
+        // try to unmarshal data into {{{.}}}
+        err = json.Unmarshal(data, &dst.{{{.}}})
+        if err == nil {
+                json{{{.}}}, _ := json.Marshal(dst.{{{.}}})
+                if string(json{{{.}}}) == "{}" { // empty struct
+                        dst.{{{.}}} = nil
+                } else {
+                        match++
+                }
+        } else {
+                dst.{{{.}}} = nil
+        }
+
+        {{/oneOf}}
+        if match > 1 { // more than 1 match
+                // reset to nil
+                {{#oneOf}}
+                dst.{{{.}}} = nil
+                {{/oneOf}}
+
+                return fmt.Errorf("Data matches more than one schema in oneOf({{classname}})")
+        } else if match == 1 {
+                return nil // exactly one match
+        } else { // no match
+                return fmt.Errorf("Data failed to match schemas in oneOf({{classname}})")
+        }
+	{{/discriminator}}
 	{{/useOneOfDiscriminatorLookup}}
 	{{^useOneOfDiscriminatorLookup}}
 	match := 0