Add -Wsingle-union warning for UNION without NEXTU (#2170)

src/asm/section.cpp

@@ -38,6 +38,7 @@ using namespace std::literals;
 struct UnionStackEntry {
 	uint32_t start;
 	uint32_t size;
+	bool hasNextu;
 };
 
 struct SectionStackEntry {
@@ -782,7 +783,7 @@ void sect_StartUnion() {
 		return;
 	}
 
-	currentUnionStack.push({.start = curOffset, .size = 0});
+	currentUnionStack.push({.start = curOffset, .size = 0, .hasNextu = false});
 }
 
 static void endUnionMember() {
@@ -801,6 +802,7 @@ void sect_NextUnionMember() {
 		return;
 	}
 	endUnionMember();
+	currentUnionStack.top().hasNextu = true;
 }
 
 void sect_EndUnion() {
@@ -808,6 +810,9 @@ void sect_EndUnion() {
 		error("Found `ENDU` outside of a `UNION` construct");
 		return;
 	}
+	if (!currentUnionStack.top().hasNextu) {
+		warning(WARNING_SINGLE_UNION, "`UNION` construct has no `NEXTU` alternatives");
+	}
 	endUnionMember();
 	curOffset += currentUnionStack.top().size;
 	currentUnionStack.pop();

src/asm/warning.cpp

@@ -37,6 +37,7 @@ Diagnostics<WarningLevel, WarningID> warnings = {
         {"obsolete",             LEVEL_DEFAULT   },
         {"shift",                LEVEL_EVERYTHING},
         {"shift-amount",         LEVEL_EVERYTHING},
+        {"single-union",         LEVEL_EVERYTHING},
         {"unmatched-directive",  LEVEL_EXTRA     },
         {"unterminated-load",    LEVEL_EXTRA     },
         {"user",                 LEVEL_DEFAULT   },