Fix underflowing SECTION FRAGMENT fixed address (#2091)

src/asm/section.cpp

@@ -214,10 +214,8 @@ static void mergeFragments(Section &sect, uint32_t org, uint8_t alignment, uint1
 	// combination of both.
 	// The merging is however performed at the *end* of the original section!
 	if (org != UINT32_MAX) {
-		uint16_t curOrg = org - sect.size;
-
-		// If both are fixed, they must be the same
-		if (sect.org != UINT32_MAX && sect.org != curOrg) {
+		// If both are fixed, they must be compatible
+		if (uint16_t curOrg = org - sect.size; sect.org != UINT32_MAX && sect.org != curOrg) {
 			sectError(
 			    "Section \"%s\" already declared as fixed at incompatible address $%04" PRIx32,
 			    sect.name.c_str(),
@@ -231,6 +229,16 @@ static void mergeFragments(Section &sect, uint32_t org, uint8_t alignment, uint1
 			    sectAlignSize,
 			    sect.alignOfs
 			);
+		} else if (org < sect.size) {
+			// Check that `curOrg` did not underflow. Note that it's safe for the above checks to
+			// use an underflowed value, since their reported errors will still be accurate.
+			sectError(
+			    "Section \"%s\" already contains %" PRIu32
+			    " bytes, higher than this fragment's fixed address $%04" PRIx32,
+			    sect.name.c_str(),
+			    sect.size,
+			    org
+			);
 		} else {
 			// Otherwise, just override
 			sect.org = curOrg;

test/asm/fragment-org-beyond-size.asm

@@ -0,0 +1,4 @@
+SECTION FRAGMENT "test", ROM0
+ds $11, 42
+
+SECTION FRAGMENT "test", ROM0[$10]

test/asm/fragment-org-beyond-size.err

@@ -0,0 +1,2 @@
+FATAL: Section "test" already contains 17 bytes, higher than this fragment's fixed address $0010
+    at fragment-org-beyond-size.asm(4)