[Backport 9.2] Add OpenAPI mapping to internal variants

compiler-rs/clients_schema_to_openapi/src/schemas.rs

@@ -404,10 +404,56 @@ impl<'a> TypesAndComponents<'a> {
                 // TODO: typed-keys: add an extension to identify it?
             }
             Some(TypeAliasVariants::InternalTag(tag)) => {
-                // TODO: add tag.default_tag as an extension
+                // outputs a map of discriminator values to schema references
+                // e.g. { "type1": "#/components/schemas/Type1", "type2": "#/components/schemas/Type2" }
+                let mut disc_mapping = IndexMap::new();
+                let ValueOf::UnionOf(union) = &alias.typ else {
+                    bail!("InternalTag type alias {} does not wrap a union", alias.base.name);
+                };
+                // Extract union members and build mapping
+                for item in &union.items {
+                    let ValueOf::InstanceOf(instance) = item else {
+                        bail!(
+                            "InternalTag union member in type alias {} is not an instance_of",
+                            alias.base.name
+                        );
+                    };
+
+                    match self.model.get_type(&instance.typ) {
+                        Ok(TypeDefinition::Interface(variant_itf)) => {
+                            // Find the discriminator property in the variant
+                            let Some(disc_prop) = variant_itf.properties.iter().find(|p| p.name == tag.tag) else {
+                                bail!(
+                                    "InternalTag union member in type alias {} does not have discriminator property {}",
+                                    alias.base.name,
+                                    tag.tag
+                                );
+                            };
+
+                            // Extract the literal value
+                            let ValueOf::LiteralValue(literal) = &disc_prop.typ else {
+                                bail!(
+                                    "InternalTag union member in type alias {} has non-literal discriminator property {}",
+                                    alias.base.name,
+                                    tag.tag
+                                );
+                            };
+                            let discriminator_value = literal.value.to_string();
+                            let schema_ref = format!("#/components/schemas/{}", instance.typ.schema_name());
+                            disc_mapping.insert(discriminator_value, schema_ref);
+                        }
+                        _ => bail!(
+                            "InternalTag union member in type alias {} is not an interface",
+                            alias.base.name
+                        ),
+                    }
+                }
+                disc_mapping.sort_unstable_keys();
+
                 schema.schema_data.discriminator = Some(Discriminator {
                     property_name: tag.tag.clone(),
-                    mapping: Default::default(),
+                    mapping: disc_mapping,
+                    // TODO: add tag.default_tag as an extension
                     extensions: Default::default(),
                 });
             }