display what accouting would create

common/proto/table_definition.proto

@@ -458,6 +458,31 @@ message ListColumnTypesResponse {
     // "temporal" for the date and time types, "gtin" for the GTIN lengths.
     // Empty when the type stands on its own.
     string group = 9;
+
+    // One column a compound type expands into.
+    message GeneratedColumn {
+      // Name the server gives the column. Fixed, and reserved: a table
+      // declaring a compound type may not also declare a column of this name.
+      string name = 1;
+
+      // The generated column's own logical type, as this same catalog
+      // describes it.
+      string field_type = 2;
+
+      // Whether the generated column carries the currency and rounding
+      // declared on the definition row.
+      bool inherits_currency = 3;
+    }
+
+    // What this type expands into, in the order the server creates the
+    // columns. Populated for compound types only, where the generated names
+    // are fixed; empty for every other type, including the ones whose
+    // companions are named after the declared column (phone, iban).
+    //
+    // A picker offering a compound type shows these, so choosing it is not a
+    // blind choice. The columns stay the server's to create — none of them may
+    // be declared.
+    repeated GeneratedColumn generated_columns = 10;
   }
 
   // Every column type, declarable or not, ordered by name.