standard_validator: warn on custom default value in union

Summary: Defaults on `optional` fields cause jank. Explicit `optional` is banned in `union`, but all union fields are implicitly optional, so we should discourage defaults on union fields.

Reviewed By: ahilger

Differential Revision: D74677815

fbshipit-source-id: 654f4fd206b0202f5382a2712b0a75a659641d58

Author: aristidisp

third-party/thrift/src/thrift/compiler/sema/standard_validator.cc

@@ -667,6 +667,7 @@ std::string_view field_qualifier_to_string(t_field_qualifier qualifier) {
 
 void validate_field_default_value(sema_context& ctx, const t_field& field) {
   if (field.default_value() == nullptr) {
+    // Field does not have a default value => nothing to validate.
     return;
   }
 
@@ -680,6 +681,16 @@ void validate_field_default_value(sema_context& ctx, const t_field& field) {
   const t_structured& parent_node =
       dynamic_cast<const t_structured&>(*ctx.parent());
 
+  if (parent_node.is_union()) {
+    ctx.warning(
+        field,
+        "Union field is implicitly optional and should not have custom "
+        "default value: `{}` (in union `{}`).",
+        field.name(),
+        parent_node.name());
+    return;
+  }
+
   const t_field_qualifier field_qualifier = field.qualifier();
 
   if (ctx.sema_parameters().warn_on_redundant_custom_default_values &&

third-party/thrift/src/thrift/compiler/test/compiler_test.cc

@@ -2325,6 +2325,16 @@ TEST(Compilertest, custom_default_values) {
         # expected-warning@-1: Explicit default value is redundant for (optional) field: `j` (in `Widget`).
         # expected-warning@-2: Optional field should not have custom default value: `j` (in `Widget`).
     }
+
+    union UnionWidget {
+      1: i32 a;
+
+      2: i32 b = 0;
+        # expected-warning@-1: Union field is implicitly optional and should not have custom default value: `b` (in union `UnionWidget`).
+
+      3: bool c = true;
+        # expected-warning@-1: Union field is implicitly optional and should not have custom default value: `c` (in union `UnionWidget`).
+    }
   )",
       {"--extra-validation", "warn_on_redundant_custom_default_values"});
 }