fix: wrap optional scalar reference types in Option<T> (#122)

src/SwaggerProvider.DesignTime/Provider.OpenApiClient.fs

@@ -119,6 +119,7 @@ type public OpenApiClientTypeProvider(cfg: TypeProviderConfig) as this =
                             |> Seq.toList
 
                         let useDateOnly = cfg.SystemRuntimeAssemblyVersion.Major >= 6
+
                         let defCompiler = DefinitionCompiler(schema, preferNullable, useDateOnly)
 
                         let opCompiler =

src/SwaggerProvider.DesignTime/v3/DefinitionCompiler.fs

@@ -582,6 +582,8 @@ type DefinitionCompiler(schema: OpenApiDocument, provideNullable, useDateOnly: b
                     typedefof<Option<obj>>
 
             ProvidedTypeBuilder.MakeGenericType(baseGenTy, [ tyType ])
+        else if tyType = typeof<string> then
+            ProvidedTypeBuilder.MakeGenericType(typedefof<Option<obj>>, [ tyType ])
         else
             tyType
 

tests/SwaggerProvider.Tests/v3/Schema.TypeMappingTests.fs

@@ -130,14 +130,15 @@ let ``optional Guid maps to Option<Guid>``() =
 
     ty |> shouldEqual(typedefof<Option<_>>.MakeGenericType(typeof<Guid>))
 
-// ── Optional reference types are NOT wrapped (they are already nullable) ─────
+// ── Optional string is wrapped in Option<string> ─────────────────────────────
 
 [<Fact>]
-let ``optional string is not wrapped in Option``() =
+let ``optional string maps to Option<string>``() =
     let ty = compilePropertyType "          type: string\n" false
 
-    // string is a reference type — not wrapped in Option<T> even when non-required
-    ty |> shouldEqual typeof<string>
+    // plain string fields are wrapped in Option<string> when non-required
+    ty
+    |> shouldEqual(typedefof<Option<_>>.MakeGenericType(typeof<string>))
 
 [<Fact>]
 let ``optional byte array is not wrapped in Option``() =
@@ -403,8 +404,7 @@ let ``PreferNullable: required integer is not wrapped (Nullable only for optiona
     ty |> shouldEqual typeof<int32>
 
 [<Fact>]
-let ``PreferNullable: optional string is not wrapped (reference type)``() =
-    // Reference types like string are not wrapped in Nullable<T> since they are
-    // already nullable by nature — same behaviour as Option mode.
+let ``PreferNullable: optional string maps to Option<string>``() =
+    // Strings are always wrapped in Option<string> when non-required, even with PreferNullable.
     let ty = compilePropertyTypeWith true "          type: string\n" false
-    ty |> shouldEqual typeof<string>
+    ty |> shouldEqual typeof<string option>

tests/SwaggerProvider.Tests/v3/Schema.V2SchemaCompilationTests.fs

@@ -143,8 +143,8 @@ let ``v2 petstore Pet type has correct property types``() =
     idProp.PropertyType |> shouldEqual typeof<int64>
     // required string
     nameProp.PropertyType |> shouldEqual typeof<string>
-    // optional string — string is a reference type, not wrapped in Option
-    tagProp.PropertyType |> shouldEqual typeof<string>
+    // optional string — now wrapped in Option<string>
+    tagProp.PropertyType |> shouldEqual typeof<string option>
 
 [<Fact>]
 let ``v2 petstore schema generates API client types with operations``() =