Designated initialization with ranges would not error on overflow by 1.

releasenotes.md

@@ -48,6 +48,7 @@
 - `String.tokenize_all` would yield one too many empty tokens at the end.
 - `String.replace` no longer depends on `String.split`.
 - Fix the case where `\u<unicode char>` could crash the compiler on some platforms.
+- Designated initialization with ranges would not error on overflow by 1.
 
 ### Stdlib changes
 - Add `ThreadPool` join function to wait for all threads to finish in the pool without destroying the threads.

src/compiler/sema_initializers.c

@@ -1377,10 +1377,10 @@ static Type *sema_find_type_of_element(SemaContext *context, Type *type, Designa
 				*did_report_error = true;
 				return NULL;
 			}
-			if (end_index > (ArrayIndex)len)
+			if (end_index >= (ArrayIndex)len)
 			{
 				*did_report_error = true;
-				SEMA_ERROR(element->index_expr, "The index may must be less than the array length (which was %llu).", (unsigned long long)len);
+				SEMA_ERROR(element->index_end_expr, "The index must be less than the array length (which was %llu).", (unsigned long long)len);
 				return NULL;
 			}
 			element->index_end = end_index;

test/test_suite/bitstruct/designated_init_range_overflow.c3

@@ -0,0 +1,16 @@
+enum Row : inline int { A, B, C, D, E, F }
+
+int[3][6] m = {
+    [Row.E] = {
+        [0..3] = 1, // #error: The index must be less than the array length (which was 3)
+    },
+    [Row.F] = {
+        [1] = 2,
+    },
+};
+
+fn int main()
+{
+	(void)m;
+	return 0;
+}