Fix defaults on `#[synthesized]` fields (#2825)

Fixes #2821

2 files changed, 23 insertions, 5 deletions

diff --git a/crates/typst-macros/src/elem.rs b/crates/typst-macros/src/elem.rs
index e531a7ed..b3f86117 100644
--- a/crates/typst-macros/src/elem.rs
+++ b/crates/typst-macros/src/elem.rs
@@ -723,7 +723,7 @@ fn create_native_elem_impl(element: &Elem) -> TokenStream {
             quote! {
                 <#elem as #foundations::ElementFields>::Fields::#name => None,
             }
-        } else if field.inherent() {
+        } else if field.inherent() || (field.synthesized && field.default.is_some()) {
             quote! {
                 <#elem as #foundations::ElementFields>::Fields::#name => Some(
                     #foundations::IntoValue::into_value(self.#field_ident.clone())
@@ -748,7 +748,7 @@ fn create_native_elem_impl(element: &Elem) -> TokenStream {
             quote! {
                 <#elem as #foundations::ElementFields>::Fields::#name => false,
             }
-        } else if field.inherent() {
+        } else if field.inherent() || (field.synthesized && field.default.is_some()) {
             quote! {
                 <#elem as #foundations::ElementFields>::Fields::#name => true,
             }
@@ -867,13 +867,22 @@ fn create_native_elem_impl(element: &Elem) -> TokenStream {
                 quote! { ::ecow::EcoString::inline(#name).into() }
             };
 
-            quote! {
-                if let Some(value) = &self.#field_ident {
+            if field.synthesized && field.default.is_some() {
+                quote! {
                     fields.insert(
                         #field_call,
-                        #foundations::IntoValue::into_value(value.clone())
+                        #foundations::IntoValue::into_value(self.#field_ident.clone())
                     );
                 }
+            } else {
+                quote! {
+                    if let Some(value) = &self.#field_ident {
+                        fields.insert(
+                            #field_call,
+                            #foundations::IntoValue::into_value(value.clone())
+                        );
+                    }
+                }
             }
         });
 
diff --git a/tests/typ/bugs/2821-missing-fields.typ b/tests/typ/bugs/2821-missing-fields.typ
new file mode 100644
index 00000000..0fec2043
--- /dev/null
+++ b/tests/typ/bugs/2821-missing-fields.typ
@@ -0,0 +1,9 @@
+// Issue #2821: Setting a figure's supplement to none removes the field
+// Ref: false
+
+---
+#show figure.caption: it => {
+  assert(it.has("supplement"))
+  assert(it.supplement == none)
+}
+#figure([], caption: [], supplement: none)