From a91d26192d1bdc8757ce7e2913e22fd600a99f50 Mon Sep 17 00:00:00 2001
From: Rangi <sylvie.oukaour+rangi42@gmail.com>
Date: Tue, 6 Jan 2026 17:53:47 -0500
Subject: [PATCH] Fix `SECTION UNION` alignment depending on piece order

---
 src/link/section.cpp                       |   1 +
 test/link/section-union/compat/a.asm       |   4 ++++
 test/link/section-union/compat/b.asm       |  17 +++++++++++++++++
 test/link/section-union/compat/ref.out.bin | Bin 0 -> 16384 bytes
 test/link/section-union/compat/script.link |   5 +++++
 test/link/test.sh                          |   9 +++++++++
 6 files changed, 36 insertions(+)
 create mode 100644 test/link/section-union/compat/a.asm
 create mode 100644 test/link/section-union/compat/b.asm
 create mode 100644 test/link/section-union/compat/ref.out.bin
 create mode 100644 test/link/section-union/compat/script.link

diff --git a/src/link/section.cpp b/src/link/section.cpp
index 10146714..5abccdd8 100644
--- a/src/link/section.cpp
+++ b/src/link/section.cpp
@@ -96,6 +96,7 @@ static void checkSectUnionCompat(Section &target, Section &other) {
 		if (checkAgainstFixedAlign(target, other, other.alignOfs)) {
 			target.isAlignFixed = true;
 			target.alignMask = other.alignMask;
+			target.alignOfs = other.alignOfs;
 		}
 	}
 }
diff --git a/test/link/section-union/compat/a.asm b/test/link/section-union/compat/a.asm
new file mode 100644
index 00000000..d7b901a0
--- /dev/null
+++ b/test/link/section-union/compat/a.asm
@@ -0,0 +1,4 @@
+SECTION "ROM", ROM0
+LOAD UNION "U", WRAM0
+        ds 1
+ENDL
diff --git a/test/link/section-union/compat/b.asm b/test/link/section-union/compat/b.asm
new file mode 100644
index 00000000..7428f1fb
--- /dev/null
+++ b/test/link/section-union/compat/b.asm
@@ -0,0 +1,17 @@
+SECTION "R1", WRAM0
+        ds $77
+
+SECTION "R2", WRAM0
+        ds $ef
+
+SECTION UNION "U", WRAM0
+        ds $52e
+
+SECTION UNION "U", WRAM0
+wStart::
+        ds $89
+        assert @ & $FF == 0, "wContent must be 8-bit aligned"
+        align 8
+wContent::
+        ds $111
+wEnd::
diff --git a/test/link/section-union/compat/ref.out.bin b/test/link/section-union/compat/ref.out.bin
new file mode 100644
index 0000000000000000000000000000000000000000..294f4016d05bdd696670c4840f1f36a71f9239de
GIT binary patch
literal 16384
zcmeIuF#!Mo0K%a4Pi+hzh(KY$fB^#r3>YwAz<>b*1`HT5V8DO@0|pEjFkrxd0RsjM
q7%*VKfB^#r3>YwAz<>b*1`HT5V8DO@0|pEjFkrxd0RsjMyaxtA00031

literal 0
HcmV?d00001

diff --git a/test/link/section-union/compat/script.link b/test/link/section-union/compat/script.link
new file mode 100644
index 00000000..bb3b77c7
--- /dev/null
+++ b/test/link/section-union/compat/script.link
@@ -0,0 +1,5 @@
+WRAM0
+        align 8
+        "R1"
+        "U"
+        "R2"
diff --git a/test/link/test.sh b/test/link/test.sh
index 78533e7f..d098efa0 100755
--- a/test/link/test.sh
+++ b/test/link/test.sh
@@ -390,6 +390,15 @@ rgblinkQuiet "$otemp" "$gbtemp" 2>"$outtemp"
 tryDiff "$test"/out.err "$outtemp"
 evaluateTest
 
+test="section-union/compat"
+startTest
+"$RGBASM" -o "$otemp" "$test"/a.asm
+"$RGBASM" -o "$gbtemp2" "$test"/b.asm
+continueTest
+rgblinkQuiet -o "$gbtemp" -l "$test"/script.link "$otemp" "$gbtemp2"
+tryCmpRom "$test"/ref.out.bin
+evaluateTest
+
 test="section-union/good"
 startTest
 "$RGBASM" -o "$otemp" "$test"/a.asm