Bitstruct cast to other bitstruct by way of underlying type would fail #1159.

releasenotes.md

@@ -13,6 +13,7 @@
 - $$memcpy_inline and $$memset_inline fixed.
 - `.$Type = ...` and `.$foo = ...` now works #1156.
 - `int.min` incorrect behaviour #1154.
+- Bitstruct cast to other bitstruct by way of underlying type would fail #1159.
 
 ### Stdlib changes
 - Added `new_aligned` and `alloc_aligned` functions to prevent accidental under-alignment when allocating simd.

src/compiler/sema_casts.c

@@ -1528,9 +1528,14 @@ static void cast_int_arr_to_bitstruct(SemaContext *context, Expr *expr, Type *ty
 {
 	if (expr->expr_kind == EXPR_CAST && expr->cast_expr.kind == CAST_BSINTARR)
 	{
-		expr_replace(expr, exprptr(expr->cast_expr.expr));
+		Expr *inner = exprptr(expr->cast_expr.expr);
+		if (type_flatten(inner->type) == type_flatten(type))
+		{
+			expr_replace(expr, inner);
+			expr->type = type;
 			return;
 		}
+	}
 	insert_runtime_cast(expr, CAST_INTARRBS, type);
 }
 

test/test_suite/bitstruct/bitstruct_cast_and_back.c3

@@ -0,0 +1,15 @@
+// See issue #1159
+
+bitstruct Foo : int {
+  bool a;
+}
+
+bitstruct Bar : int {
+  bool a;
+  bool b;
+}
+
+fn void bitstruct_cast() {
+  Bar bar;
+  Foo foo = (Foo)(int)bar;
+}