fix: qualify external ref schema types in default response codes (oapi-codegen#2241)

When a response uses a $ref to an external components/responses
definition, the generated strict-server code for non-fixed status
codes (e.g. default) used the inner schema type without the external
package qualifier, causing a compile error. Apply
ensureExternalRefsInSchema to response content schemas after
resolving external response refs.

Closes oapi-codegen#2113

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: mromaszewicz

internal/test/issues/issue-2113/common/spec.yaml

@@ -0,0 +1,22 @@
+openapi: "3.0.4"
+info:
+  title: Common
+  version: "0.0.1"
+paths: {}
+components:
+  schemas:
+    ProblemDetails:
+      type: object
+      properties:
+        title:
+          type: string
+        status:
+          type: integer
+      required: [title, status]
+  responses:
+    StandardError:
+      description: Error response
+      content:
+        application/json:
+          schema:
+            $ref: "#/components/schemas/ProblemDetails"

internal/test/issues/issue-2113/config.api.yaml

@@ -0,0 +1,11 @@
+# yaml-language-server: $schema=../../../../configuration-schema.json
+package: api
+generate:
+  chi-server: true
+  strict-server: true
+  models: true
+import-mapping:
+  ./common/spec.yaml: github.com/oapi-codegen/oapi-codegen/v2/internal/test/issues/issue-2113/gen/common
+output: gen/api/api.gen.go
+output-options:
+  skip-prune: true

internal/test/issues/issue-2113/config.common.yaml

@@ -0,0 +1,7 @@
+# yaml-language-server: $schema=../../../../configuration-schema.json
+package: common
+generate:
+  models: true
+output: gen/common/common.gen.go
+output-options:
+  skip-prune: true

internal/test/issues/issue-2113/doc.go

@@ -0,0 +1,4 @@
+package issue2113
+
+//go:generate go run github.com/oapi-codegen/oapi-codegen/v2/cmd/oapi-codegen --config=config.common.yaml common/spec.yaml
+//go:generate go run github.com/oapi-codegen/oapi-codegen/v2/cmd/oapi-codegen --config=config.api.yaml spec.yaml

internal/test/issues/issue-2113/gen/api/api.gen.go

@@ -0,0 +1,22 @@
+package issue2113
+
+import (
+	"testing"
+
+	"github.com/oapi-codegen/oapi-codegen/v2/internal/test/issues/issue-2113/gen/api"
+	"github.com/oapi-codegen/oapi-codegen/v2/internal/test/issues/issue-2113/gen/common"
+)
+
+// TestExternalRefInResponse verifies that a $ref to an external
+// components/responses object correctly qualifies the schema type
+// with the external package import. See
+// https://github.com/oapi-codegen/oapi-codegen/issues/2113
+func TestExternalRefInResponse(t *testing.T) {
+	// This will fail to compile if the generated code uses
+	// ProblemDetails instead of common.ProblemDetails (via the
+	// externalRef alias) in the default response type.
+	_ = api.ListThingsdefaultJSONResponse{
+		Body:       common.ProblemDetails{Title: "err", Status: 500},
+		StatusCode: 500,
+	}
+}

internal/test/issues/issue-2113/gen/common/common.gen.go

@@ -0,0 +1,21 @@
+openapi: "3.0.4"
+info:
+  title: API
+  version: "0.0.1"
+paths:
+  /things:
+    get:
+      operationId: listThings
+      responses:
+        "200":
+          description: OK
+          content:
+            application/json:
+              schema:
+                type: array
+                items:
+                  type: string
+        "400":
+          $ref: "./common/spec.yaml#/components/responses/StandardError"
+        default:
+          $ref: "./common/spec.yaml#/components/responses/StandardError"

internal/test/issues/issue-2113/issue_test.go

@@ -914,6 +914,12 @@ func GenerateResponseDefinitions(operationID string, responses map[string]*opena
 				rd.Ref = refType
 				refSet[refType] = struct{}{}
 			}
+			// Ensure content schemas get the external ref qualifier so that
+			// non-fixed status code paths (e.g. "default") emit the qualified type.
+			for i, rcd := range rd.Contents {
+				ensureExternalRefsInSchema(&rcd.Schema, responseOrRef.Ref)
+				rd.Contents[i] = rcd
+			}
 		}
 		responseDefinitions = append(responseDefinitions, rd)
 	}