Add support for if/then/else conditional schemas

Author: cubahno

README.md

@@ -16,7 +16,7 @@ on the real value-add for your organization.
 - **Smart pruning** - Automatically removes unused types by default (configurable)
 
 ### Type System
-- **Union types** - Full support for `oneOf`, `anyOf`, and `allOf` with intelligent type merging and `runtime.Either[A, B]` for two-element unions
+- **Union types** - Full support for `oneOf`, `anyOf`, `allOf`, and `if`/`then`/`else` with intelligent type merging, `runtime.Either[A, B]` for two-element unions, and `runtime.Conditional[T, E]` for conditional schemas
 - **Additional properties** - Handle dynamic fields with `map[string]interface{}` or custom types
 - **Validation** - Built-in validation using [go-playground/validator](https://github.com/go-playground/validator) with `Validate()` methods on generated types
 - **Custom extensions** - for fine-grained control over code generation

docs/union-types.md

@@ -1,12 +1,13 @@
 # Union Types
 
-Union types in OpenAPI allow schemas to accept multiple different types or structures. The `oapi-codegen` generator handles `allOf`, `anyOf`, and `oneOf` with intelligent type generation based on the number and nature of the variants.
+Union types in OpenAPI allow schemas to accept multiple different types or structures. The `oapi-codegen` generator handles `allOf`, `anyOf`, `oneOf`, and `if`/`then`/`else` with intelligent type generation based on the number and nature of the variants.
 
 ## Overview
 
 - **`allOf`**: Merges all schemas into a single struct with all fields combined
 - **`anyOf`**: Can match any of the specified schemas
 - **`oneOf`**: Must match exactly one of the specified schemas
+- **`if`/`then`/`else`**: Conditional schemas (OpenAPI 3.1) - uses `runtime.Conditional[T, E]`
 
 The generator applies smart optimizations based on the union structure:
 
@@ -167,3 +168,83 @@ The `runtime.JSONMerge` function combines all the JSON parts into a single objec
 
 [View complex union example](https://github.com/doordash-oss/oapi-codegen-dd/tree/main/examples/union/allof-anyof-oneof/){:target="_blank"}
 
+---
+
+## Conditional Schemas (`if`/`then`/`else`)
+
+OpenAPI 3.1 supports JSON Schema conditional keywords. The generator
+treats `then` and `else` as structural branches. The `if` schema is
+ignored - it's a validation predicate (e.g. "if kind equals typeA")
+with no structural properties. `if` is also a reserved keyword in Go.
+
+### Both Branches - `runtime.Conditional`
+
+When both `then` and `else` are present, the generator creates a
+`runtime.Conditional[T, E]` wrapper with named variant types. Unlike
+`runtime.Either` (`.A`/`.B`), `Conditional` uses `.Then`/`.Else`
+fields and `.IsThen()`/`.IsElse()` methods.
+
+**OpenAPI Spec:**
+
+```yaml
+--8<-- "ifthenelse/basic/api.yaml:20:42"
+```
+
+**Generated Go Code:**
+
+```go
+--8<-- "ifthenelse/basic/gen.go:16:19"
+```
+
+The variant types use `_Then` and `_Else` suffixes:
+
+```go
+--8<-- "ifthenelse/basic/gen.go:86:98"
+```
+
+Usage:
+
+```go
+if resource.Resource_IfThenElse.IsThen() {
+    fmt.Println(resource.Resource_IfThenElse.Then.FieldA)
+}
+if resource.Resource_IfThenElse.IsElse() {
+    fmt.Println(resource.Resource_IfThenElse.Else.FieldB)
+}
+```
+
+[View basic example](https://github.com/doordash-oss/oapi-codegen-dd/tree/main/examples/ifthenelse/basic/){:target="_blank"}
+
+### Single Branch - Flat Merge
+
+When only `then` or only `else` is present, the branch properties
+are flat-merged into the parent struct. No wrapper type is created.
+
+**OpenAPI Spec:**
+
+```yaml
+--8<-- "ifthenelse/then-only/api.yaml:20:35"
+```
+
+**Generated Go Code:**
+
+```go
+--8<-- "ifthenelse/then-only/gen.go:12:16"
+```
+
+The `timeout` and `retries` fields from the `then` branch appear
+directly on the `Config` struct.
+
+[View then-only example](https://github.com/doordash-oss/oapi-codegen-dd/tree/main/examples/ifthenelse/then-only/){:target="_blank"}
+
+### With `$ref` Branches
+
+When `then`/`else` reference component schemas, the generated
+`Conditional` uses the referenced type names directly:
+
+```go
+--8<-- "ifthenelse/with-refs/gen.go:98:100"
+```
+
+[View with-refs example](https://github.com/doordash-oss/oapi-codegen-dd/tree/main/examples/ifthenelse/with-refs/){:target="_blank"}
+

examples/ifthenelse/basic/api.yaml

@@ -0,0 +1,42 @@
+openapi: "3.1.0"
+info:
+  version: 1.0.0
+  title: Basic if/then/else
+  description: Both then and else branches produce a union (Either) type
+paths:
+  /resources:
+    get:
+      operationId: listResources
+      responses:
+        '200':
+          description: OK
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Resource'
+
+components:
+  schemas:
+    Resource:
+      type: object
+      properties:
+        kind:
+          type: string
+      if:
+        properties:
+          kind:
+            const: "typeA"
+      then:
+        type: object
+        properties:
+          fieldA:
+            type: string
+          valueA:
+            type: integer
+      else:
+        type: object
+        properties:
+          fieldB:
+            type: boolean
+          valueB:
+            type: number

examples/ifthenelse/basic/cfg.yaml

@@ -0,0 +1,3 @@
+# yaml-language-server: $schema=../../configuration-schema.json
+package: gen
+skip-prune: true

examples/ifthenelse/basic/gen.go

@@ -0,0 +1,3 @@
+package gen
+
+//go:generate go run github.com/doordash-oss/oapi-codegen-dd/v3/cmd/oapi-codegen -config cfg.yaml api.yaml

examples/ifthenelse/basic/generate.go

@@ -0,0 +1,48 @@
+openapi: "3.1.0"
+info:
+  version: 1.0.0
+  title: if/then/else inside allOf
+  description: Conditional branches nested inside an allOf composition
+paths:
+  /resources:
+    get:
+      operationId: listResources
+      responses:
+        '200':
+          description: OK
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/SpecialResource'
+
+components:
+  schemas:
+    BaseResource:
+      type: object
+      properties:
+        id:
+          type: string
+        name:
+          type: string
+
+    SpecialResource:
+      allOf:
+        - $ref: '#/components/schemas/BaseResource'
+        - type: object
+          properties:
+            category:
+              type: string
+          if:
+            properties:
+              category:
+                const: "premium"
+          then:
+            type: object
+            properties:
+              premiumFeature:
+                type: string
+          else:
+            type: object
+            properties:
+              standardLimit:
+                type: integer

examples/ifthenelse/inside-allof/api.yaml

@@ -0,0 +1,3 @@
+# yaml-language-server: $schema=../../configuration-schema.json
+package: gen
+skip-prune: true