diff --git a/releasenotes.md b/releasenotes.md
index 013607d30..74550aaff 100644
--- a/releasenotes.md
+++ b/releasenotes.md
@@ -10,6 +10,7 @@
 - Improved error messages on missing qualifier on enum value. #2260
 - Add `--echo-prefix` to edit the prefix with `$echo` statements. Supports {FILE} and {LINE}
 - Catch accidental `foo == BAR;` where `foo = BAR;` was most likely intended. #2274
+- Improve error message when doing a rethrow in a function that doesn't return an optional.
 
 ### Fixes
 - mkdir/rmdir would not work properly with substring paths on non-windows platforms.
@@ -31,6 +32,7 @@
 - Make `to_float` more tolerant to spaces.
 - Fixes to thread local pointer handling.
 - Fixes to JSON parsing and Object.
+- Array indices are now using int64 internally.
 
 ### Stdlib changes
 
diff --git a/src/compiler/c_codegen.c b/src/compiler/c_codegen.c
index 75654010f..d02587438 100644
--- a/src/compiler/c_codegen.c
+++ b/src/compiler/c_codegen.c
@@ -257,7 +257,7 @@ static bool c_emit_type_decl(GenContext *c, Type *type)
 			if (prev) return false;
 			c_emit_type_decl(c, type->array.base);
 			int id = ++c->typename;
-			PRINTF("typedef struct { %s ptr[%u]; } __c3_array%d;\n", c_type_name(c, type->array.base), type->array.len, id);
+			PRINTF("typedef struct { %s ptr[%llu]; } __c3_array%d;\n", c_type_name(c, type->array.base), (unsigned long long)type->array.len, id);
 			scratch_buffer_clear();
 			scratch_buffer_printf(" __c3_array%d", id);
 			htable_set(&c->gen_decl, type, scratch_buffer_copy());
diff --git a/src/compiler/compiler_internal.h b/src/compiler/compiler_internal.h
index 6df722ed3..d149d86d5 100644
--- a/src/compiler/compiler_internal.h
+++ b/src/compiler/compiler_internal.h
@@ -16,11 +16,11 @@ typedef double Real;
 typedef uint64_t ByteSize;
 typedef uint32_t TypeSize;
 typedef int32_t IndexDiff;
-typedef int32_t ArrayIndex;
+typedef int64_t ArrayIndex;
 typedef uint16_t StructIndex;
 typedef uint32_t AlignSize;
 typedef int32_t ScopeId;
-typedef uint32_t ArraySize;
+typedef uint64_t ArraySize;
 typedef uint64_t BitSize;
 typedef uint16_t FileId;
 
@@ -34,7 +34,7 @@ typedef uint16_t FileId;
 #define UINT12_MAX        4095
 #define UINT20_MAX        1048575U
 
-#define MAX_ARRAYINDEX INT32_MAX
+#define MAX_ARRAYINDEX INT64_MAX
 #define MAX_FIXUPS 0xFFFFF
 #define MAX_HASH_SIZE (512 * 1024 * 1024)
 #define INVALID_SPAN ((SourceSpan){ .row = 0 })
@@ -72,6 +72,15 @@ typedef uint16_t FileId;
 #define TABLE_MAX_LOAD 0.5
 #define OUTF(...) do { if (!compiler.build.silent) printf(__VA_ARGS__); } while(0)
 #define OUTN(str__) do { if (!compiler.build.silent) puts(str__); } while(0)
+#ifdef NDEBUG
+#define ASSERT_SPANF(node__, check__, format__, ...) do { (void)(check__); } while(0)
+#define ASSERT_SPAN(node__, check__) do { (void)(check__); } while(0)
+#define ASSERT_AT(span__, check__) do { (void)(check__);} while(0)
+#else
+#define ASSERT_SPANF(node__, check__, format__, ...) do { if (!(check__)) { assert_print_line((node__)->span); eprintf(format__, __VA_ARGS__); ASSERT(check__); } } while(0)
+#define ASSERT_SPAN(node__, check__) do { if (!(check__)) { assert_print_line((node__)->span); ASSERT(check__); } } while(0)
+#define ASSERT_AT(span__, check__) do { if (!(check__)) { assert_print_line(span__); ASSERT(check__); } } while(0)
+#endif
 
 #define INVALID_PTR ((void*)(uintptr_t)0xAAAAAAAAAAAAAAAA)
 
@@ -704,7 +713,7 @@ typedef enum RangeType
 typedef struct
 {
 	ResolveStatus status : 3;
-	RangeType range_type;
+	RangeType range_type : 4;
 	bool start_from_end : 1;
 	bool end_from_end : 1;
 	bool is_len : 1;
@@ -2021,6 +2030,8 @@ INLINE bool compile_asserts(void)
 	return safe_mode_enabled() || compiler.build.testing;
 }
 
+void assert_print_line(SourceSpan span);
+
 bool ast_is_not_empty(Ast *ast);
 
 bool ast_is_compile_time(Ast *ast);
@@ -2945,6 +2956,7 @@ INLINE TypeInfoId type_info_id_new_base(Type *type, SourceSpan span)
 	return type_infoid(type_info_new_base(type, span));
 }
 
+
 INLINE Type *type_new(TypeKind kind, const char *name)
 {
 	Type *type = CALLOCS(Type);
@@ -4236,16 +4248,12 @@ INLINE bool check_module_name(Path *path)
 	return true;
 }
 
-#ifdef NDEBUG
-#define ASSERT_SPANF(node__, check__, format__, ...) do { (void)(check__); } while(0)
-#define ASSERT_SPAN(node__, check__) do { (void)(check__); } while(0)
-#define ASSERT_AT(span__, check__) do { (void)(check__);} while(0)
-#else
-#define ASSERT_SPANF(node__, check__, format__, ...) do { if (!(check__)) { assert_print_line((node__)->span); eprintf(format__, __VA_ARGS__); ASSERT(check__); } } while(0)
-#define ASSERT_SPAN(node__, check__) do { if (!(check__)) { assert_print_line((node__)->span); ASSERT(check__); } } while(0)
-#define ASSERT_AT(span__, check__) do { if (!(check__)) { assert_print_line(span__); ASSERT(check__); } } while(0)
-#endif
-void assert_print_line(SourceSpan span);
+INLINE bool expr_is_valid_index(Expr *expr)
+{
+	ASSERT_SPAN(expr, expr_is_const_int(expr));
+	return int_fits(expr->const_expr.ixx, TYPE_I64);
+}
+
 
 const char *default_c_compiler(void);
 
diff --git a/src/compiler/headers.c b/src/compiler/headers.c
index 5de9beabb..0ea399c8f 100644
--- a/src/compiler/headers.c
+++ b/src/compiler/headers.c
@@ -203,7 +203,7 @@ static void header_print_type(HeaderContext *c, Type *type)
 		case TYPE_ARRAY:
 			PRINTF("struct { ");
 			header_print_type(c, type->array.base);
-			PRINTF(" arr[%d]; }", type->array.len);
+			PRINTF(" arr[%llu]; }", (unsigned long long)type->array.len);
 			return;
 		case TYPE_ANY:
 		case TYPE_INTERFACE:
@@ -227,7 +227,7 @@ static void header_print_type(HeaderContext *c, Type *type)
 				default:
 					UNREACHABLE;
 			}
-			PRINTF("%dx%d", (int)type_bit_size(type->array.base), type->array.len);
+			PRINTF("%dx%llu", (int)type_bit_size(type->array.base), (unsigned long long)type->array.len);
 			return;
 	}
 }
@@ -345,7 +345,7 @@ static void header_gen_members(HeaderContext *c, int indent, Decl **members)
 				{
 					case TYPE_ARRAY:
 						header_print_type(c, type->array.base);
-						PRINTF(" %s[%d];\n", member->name, type->array.len);
+						PRINTF(" %s[%llu];\n", member->name, (unsigned long long)type->array.len);
 						break;
 					case TYPE_FLEXIBLE_ARRAY:
 						header_print_type(c, type->array.base);
@@ -574,7 +574,7 @@ RETRY:
 			header_print_type(c, flat_type);
 			PRINTF(" ");
 			header_print_type(c, type);
-			PRINTF(" __attribute__((vector_size(%d)));\n", (int)type_size(flat_type) * type->array.len);
+			PRINTF(" __attribute__((vector_size(%llu)));\n", (int)type_size(flat_type) * (unsigned long long)type->array.len);
 			return;
 		}
 	}
diff --git a/src/compiler/number.c b/src/compiler/number.c
index 5c7ce8e4f..4607fb778 100644
--- a/src/compiler/number.c
+++ b/src/compiler/number.c
@@ -491,7 +491,7 @@ const char *expr_const_to_error_string(const ExprConst *expr)
 		case CONST_FLOAT:
 			return str_printf("%g", expr->fxx.f);
 		case CONST_STRING:
-			return str_printf("\"%*.s\"", expr->bytes.len, expr->bytes.ptr);
+			return str_printf("\"%*.s\"", (int)expr->bytes.len, expr->bytes.ptr);
 		case CONST_BYTES:
 			return "<binary data>";
 		case CONST_REF:
diff --git a/src/compiler/sema_expr.c b/src/compiler/sema_expr.c
index 255b98f18..a94238f50 100644
--- a/src/compiler/sema_expr.c
+++ b/src/compiler/sema_expr.c
@@ -3989,8 +3989,7 @@ INLINE bool sema_expr_analyse_range_internal(SemaContext *context, Range *range,
 		{
 			SourceSpan span = start->span;
 			span = extend_span_with_token(span, end->span);
-			sema_error_at(context, span, "No common type can be found between start and end index.");
-			return false;
+			RETURN_SEMA_ERROR_AT(span, "No common type can be found between start and end index.");
 		}
 		if (!cast_implicit(context, start, common, false) || !cast_implicit(context, end, common, false)) return false;
 	}
@@ -4013,12 +4012,12 @@ INLINE bool sema_expr_analyse_range_internal(SemaContext *context, Range *range,
 	if (end && sema_cast_const(end))
 	{
 		// Only ArrayIndex sized
-		if (!int_fits(end->const_expr.ixx, TYPE_I64))
+		if (!expr_is_valid_index(end))
 		{
 			RETURN_SEMA_ERROR(end, "The index cannot be stored in a 64-signed integer, which isn't supported.");
 		}
 
-		int64_t end_index = int_to_i64(end->const_expr.ixx);
+		ArrayIndex end_index = int_to_i64(end->const_expr.ixx);
 
 		if (range->end_from_end)
 		{
@@ -4026,11 +4025,11 @@ INLINE bool sema_expr_analyse_range_internal(SemaContext *context, Range *range,
 			// Something like  1 .. ^4 with an unknown length.
 			if (len < 0) return true;
 			// Otherwise we fold the "from end"
-			end_index = len - end_index;
-			if (end_index < 0)
+			if (end_index > len)
 			{
-				RETURN_SEMA_ERROR(end, "An index may only be negative for pointers (it was: %lld).", end_index);
+				RETURN_SEMA_ERROR(end, "An index may only be negative for pointers (it was: %lld).", len - end_index);
 			}
+			end_index = len - end_index;
 			range->end_from_end = false;
 		}
 		if (end_index < 0 && env != RANGE_PTR)
@@ -4038,8 +4037,7 @@ INLINE bool sema_expr_analyse_range_internal(SemaContext *context, Range *range,
 			RETURN_SEMA_ERROR(end, "An index may only be negative for pointers (it was: %lld).", end_index);
 		}
 		// No more analysis
-		if (end_index > MAX_ARRAYINDEX || end_index < -MAX_ARRAYINDEX) return true;
-		range->const_end = (ArrayIndex)end_index;
+		range->const_end = end_index;
 		range->range_type = range->is_len ? RANGE_CONST_LEN : RANGE_CONST_END;
 	}
 	else if (!end && len > 0)
@@ -4052,24 +4050,23 @@ INLINE bool sema_expr_analyse_range_internal(SemaContext *context, Range *range,
 	if (sema_cast_const(start))
 	{
 		// Only ArrayIndex sized
-		if (!int_fits(start->const_expr.ixx, TYPE_I64))
+		if (!expr_is_valid_index(start))
 		{
 			RETURN_SEMA_ERROR(end, "The index cannot be stored in a 64-signed integer, which isn't supported.");
 		}
 		// Only ArrayIndex sized
-		int64_t start_index = int_to_i64(start->const_expr.ixx);
+		ArrayIndex start_index = int_to_i64(start->const_expr.ixx);
 		if (range->start_from_end)
 		{
 			if (start_index < 0) RETURN_SEMA_ERROR(end, "Negative numbers are not allowed when indexing from the end.");
 			// Something like  ^1 .. 4 with an unknown length.
 			if (len < 0) return true;
 			// Otherwise we fold the "from end"
-			start_index = len - start_index;
-			if (start_index < 0)
+			if (len < start_index)
 			{
-				RETURN_SEMA_ERROR(start, "An index may only be negative for pointers (it was: %lld).", start_index);
+				RETURN_SEMA_ERROR(start, "An index may only be negative for pointers (it was: %lld).", len - start_index);
 			}
-			if (start_index > MAX_ARRAYINDEX || start_index < -MAX_ARRAYINDEX) return true;
+			start_index = len - start_index;
 			range->start_from_end = false;
 		}
 		if (start_index < 0 && env != RANGE_PTR)
@@ -4083,20 +4080,19 @@ INLINE bool sema_expr_analyse_range_internal(SemaContext *context, Range *range,
 		}
 		if (range->range_type == RANGE_CONST_END)
 		{
-			int64_t end_index = range->const_end;
+			ArrayIndex end_index = range->const_end;
 			if (end_index < start_index) RETURN_SEMA_ERROR(start, "The start index (%lld) should not be greater than the end index (%lld).",
 														   start_index, end_index);
-			if (start_index > MAX_ARRAYINDEX) return true;
-			range->const_end = (ArrayIndex)(end_index + 1 - start_index);
+			range->const_end = end_index + 1 - start_index;
 			range->range_type = RANGE_CONST_LEN;
 			range->is_len = true;
 		}
 		if (range->range_type == RANGE_CONST_LEN)
 		{
-			int64_t end_index = range->const_end;
+			ArrayIndex end_index = range->const_end;
 			range->range_type = RANGE_CONST_RANGE;
-			range->start_index = (ArrayIndex)start_index;
-			range->len_index = (ArrayIndex)end_index;
+			range->start_index = start_index;
+			range->len_index = end_index;
 		}
 	}
 	if (len > -1)
@@ -9002,8 +8998,10 @@ static inline bool sema_expr_analyse_rethrow(SemaContext *context, Expr *expr)
 		expr->rethrow_expr.in_block = NULL;
 		if (context->rtype && context->rtype->type_kind != TYPE_OPTIONAL)
 		{
-			RETURN_SEMA_ERROR(expr, "This expression implicitly returns with an optional result, "
-									"but the function does not allow optional results. Did you mean to use '!!' instead?");
+			RETURN_SEMA_ERROR(expr, "This expression is doing a rethrow, "
+									"but '%s' returns %s, which isn't an optional type. Did you intend to use '!!' instead?",
+									context->call_env.current_function->name,
+									type_quoted_error_string(context->rtype));
 		}
 	}
 	return true;
diff --git a/src/compiler/types.c b/src/compiler/types.c
index 6286d07cf..c86b6f860 100644
--- a/src/compiler/types.c
+++ b/src/compiler/types.c
@@ -1130,13 +1130,13 @@ static Type *type_create_array(Type *element_type, ArraySize len, bool vector, b
 	Type *vec_arr;
 	if (vector)
 	{
-		vec_arr = type_new(TYPE_VECTOR, str_printf("%s[<%u>]", element_type->name, len));
+		vec_arr = type_new(TYPE_VECTOR, str_printf("%s[<%llu>]", element_type->name, (unsigned long long)len));
 		vec_arr->array.base = element_type;
 		vec_arr->array.len = len;
 	}
 	else
 	{
-		vec_arr = type_new(TYPE_ARRAY, str_printf("%s[%u]", element_type->name, len));
+		vec_arr = type_new(TYPE_ARRAY, str_printf("%s[%llu]", element_type->name, (unsigned long long)len));
 		vec_arr->array.base = element_type;
 		vec_arr->array.len = len;
 	}
diff --git a/test/test_suite/errors/rethrow_no_err.c3 b/test/test_suite/errors/rethrow_no_err.c3
index 9244ddf8c..1763e67ff 100644
--- a/test/test_suite/errors/rethrow_no_err.c3
+++ b/test/test_suite/errors/rethrow_no_err.c3
@@ -5,8 +5,8 @@ fn void test()
     int i = 0;
     if (i!) return; // #error: No optional to rethrow before '!' in the expression, please remove '!'
     int? j = 0;
-    if (j!) return; // #error: This expression implicitly returns with an optional result, but the function
-    if ((j!)!) return; // #error: This expression implicitly returns with an optional result, but the function
+    if (j!) return; // #error: This expression is doing a rethrow
+    if ((j!)!) return; // #error: This expression is doing a rethrow
 }
 
 fn void? test2()
diff --git a/test/test_suite/errors/rethrow_optional_in_non_opt.c3 b/test/test_suite/errors/rethrow_optional_in_non_opt.c3
new file mode 100644
index 000000000..5efe904eb
--- /dev/null
+++ b/test/test_suite/errors/rethrow_optional_in_non_opt.c3
@@ -0,0 +1,9 @@
+module test;
+import std;
+fn int? test() => 1;
+
+fn int main()
+{
+	test()!; // #error: This expression is doing a rethrow
+	return 0;
+}