add a function to retrieve the aggregate type from the underlying types

of a select (ap203 error)

src/express/resolve.c

@@ -236,6 +236,40 @@ RESOLVEinitialize( void ) {
 #endif
 }
 
+/**
+ * Retrieve the aggregate type from the underlying types of the select type t_select
+ * Parameters: t_select - the select type to retrieve the aggregate type from
+ *             t_agg - the current aggregate type
+ * Returns: the aggregate type
+ */
+Type TYPE_retrieve_aggregate(Type t_select, Type t_agg ) {
+    if ( TYPEis_select(t_select) ) {
+        // parse the underlying types
+        LISTdo_links( t_select->u.type->body->list, link )
+        // the current underlying type
+        Type t = (Type) link->data;
+        if ( TYPEis_select(t) ) {
+            t_agg = TYPE_retrieve_aggregate( t, t_agg );
+        } else if ( TYPEis_aggregate( t ) ) {
+            if ( t_agg ) {
+                if ( t_agg != t->u.type->body->base ) {
+                    // 2 underlying types do not have to the same base
+                    return 0;
+                }
+            } else {
+                t_agg = t->u.type->body->base;
+            }
+        } else {
+            // the underlying type is neither a select nor an aggregate
+            return 0;
+        }
+
+        LISTod;
+    }
+
+    return t_agg;
+}
+
 /*
 ** Procedure:   EXPresolve
 ** Parameters:  Expression expression   - expression to resolve
@@ -478,6 +512,14 @@ EXP_resolve( Expression expr, Scope scope, Type typecheck ) {
             }
             if( TYPEis_aggregate( expr->return_type ) ) {
                 t = expr->u.query->aggregate->return_type->u.type->body->base;
+            } else if ( TYPEis_select(expr->return_type)) {
+                // retrieve the common aggregate type
+                t = TYPE_retrieve_aggregate(expr->return_type, 0);
+                if (!t) {
+                    ERRORreport_with_symbol( ERROR_query_requires_aggregate, &expr->u.query->aggregate->symbol );
+                    resolve_failed( expr );
+                    break;
+                }
             } else if( TYPEis_runtime( expr->return_type ) ) {
                 t = Type_Runtime;
             } else {