From 76f680afaededf946335cf4854c0b43d85a5033f Mon Sep 17 00:00:00 2001
From: Roberto Raggi <roberto.raggi@gmail.com>
Date: Tue, 15 Jul 2025 22:35:26 +0200
Subject: [PATCH] Fix sizeof of structs with a trailing unbound array

Fixes #574

Signed-off-by: Roberto Raggi <roberto.raggi@gmail.com>
---
 src/parser/cxx/symbols.cc          | 20 +++++++++++++++++++-
 tests/unit_tests/sema/sizeof_01.cc |  9 ++++++++-
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/src/parser/cxx/symbols.cc b/src/parser/cxx/symbols.cc
index 5c8c49f2..e8ef097c 100644
--- a/src/parser/cxx/symbols.cc
+++ b/src/parser/cxx/symbols.cc
@@ -290,18 +290,34 @@ auto ClassSymbol::buildClassLayout(Control* control)
     alignment = std::max(alignment, baseClassSymbol->alignment());
   }
 
+  FieldSymbol* lastField = nullptr;
+
   for (auto member : scope()->symbols()) {
     auto field = symbol_cast<FieldSymbol>(member);
     if (!field) continue;
     if (field->isStatic()) continue;
 
+    if (lastField && control->is_unbounded_array(lastField->type())) {
+      // If the last field is an unbounded array, we cannot compute the offset
+      // of the next field, so we report an error.
+      return std::unexpected(
+          std::format("size of incomplete type '{}'",
+                      to_string(lastField->type(), lastField->name())));
+    }
+
     if (!field->alignment()) {
       return std::unexpected(
           std::format("alignment of incomplete type '{}'",
                       to_string(field->type(), field->name())));
     }
 
-    auto size = memoryLayout->sizeOf(field->type());
+    std::optional<std::size_t> size;
+
+    if (control->is_unbounded_array(field->type())) {
+      size = 0;
+    } else {
+      size = memoryLayout->sizeOf(field->type());
+    }
 
     if (!size.has_value()) {
       return std::unexpected(
@@ -318,6 +334,8 @@ auto ClassSymbol::buildClassLayout(Control* control)
     }
 
     alignment = std::max(alignment, field->alignment());
+
+    lastField = field;
   }
 
   offset = align_to(offset, alignment);
diff --git a/tests/unit_tests/sema/sizeof_01.cc b/tests/unit_tests/sema/sizeof_01.cc
index 259574b6..84118408 100644
--- a/tests/unit_tests/sema/sizeof_01.cc
+++ b/tests/unit_tests/sema/sizeof_01.cc
@@ -27,4 +27,11 @@ int elements[] = {1, 2, 3, 4, 5};
 static_assert(sizeof(elements) == 5 * sizeof(int));
 
 double d[]{1.0, 2.0, 3.0};
-static_assert(sizeof(d) == 3 * sizeof(double));
\ No newline at end of file
+static_assert(sizeof(d) == 3 * sizeof(double));
+
+struct X {
+  char i;
+  int dat[];
+};
+static_assert(sizeof(X) == 4);
+static_assert(alignof(X) == 4);
\ No newline at end of file