- Subscripting of constant slices would sometimes be considered non-constant #2635.

releasenotes.md

@@ -13,6 +13,7 @@
 - `Bytebuffer.grow` was broken #2622.
 - Hex escapes like `"\x80"` would be incorrectly lowered. #2623
 - Ignore const null check on deref in `$defined` and `$sizeof` #2633.
+- Subscripting of constant slices would sometimes be considered non-constant #2635.
 
 ### Stdlib changes
 - Add `ThreadPool` join function to wait for all threads to finish in the pool without destroying the threads.

src/compiler/sema_expr.c

@@ -12321,8 +12321,8 @@ bool sema_cast_const(Expr *expr)
 			if (!expr_is_const(exprptr(expr->subscript_expr.index.expr))) return false;
 			Expr *parent = exprptr(expr->subscript_expr.expr);
 			Type *flat_type = type_flatten(parent->type);
-			if (!type_is_any_arraylike(flat_type) || flat_type->type_kind == TYPE_UNTYPED_LIST) return false;
 			if (!sema_cast_const(parent)) return false;
+			if (flat_type->type_kind != TYPE_SLICE && !type_is_any_arraylike(flat_type) && flat_type->type_kind != TYPE_UNTYPED_LIST) return false;
 			return sema_expr_fold_to_index(expr, parent, expr->subscript_expr.index);
 		}
 		case EXPR_IDENTIFIER:

src/compiler/sema_stmts.c

@@ -3037,10 +3037,9 @@ bool sema_analyse_ct_echo_stmt(SemaContext *context, Ast *statement)
 {
 	Expr *message = statement->expr_stmt;
 	if (!sema_analyse_expr_rvalue(context, message)) return false;
-	if (message->expr_kind != EXPR_CONST)
+	if (!sema_cast_const(message))
 	{
-		SEMA_ERROR(message, "Expected a constant value.");
-		return false;
+		RETURN_SEMA_ERROR(message, "Expected a constant value.");
 	}
 	const char *prefix = compiler.build.echo_prefix ? compiler.build.echo_prefix : "c3c:";
 	while (prefix[0] != 0)

test/test_suite/struct/struct_slice_access.c3t

@@ -0,0 +1,24 @@
+module test;
+import std;
+
+struct OfStruct
+{
+    int type;
+}
+
+struct IContainAnArrayOfStructs
+{
+    OfStruct[] array;
+}
+
+macro reproduce(OfStruct[] $self)
+{
+    var a = $self;
+    var b = $self[0];
+}
+
+fn int main(String[] args)
+{
+    reproduce({{1},{2},{3}});
+    return 0;
+}