Fix translation of signed array indices (#4113)

* cast only if the index is long long or signed
* cast long long to usize rather than c_uint

closes #4075

2 files changed, 60 insertions, 1 deletions

diff --git a/test/run_translated_c.zig b/test/run_translated_c.zig
index 5d8233ec96..dd34713ee3 100644
--- a/test/run_translated_c.zig
+++ b/test/run_translated_c.zig
@@ -149,4 +149,24 @@ pub fn addCases(cases: *tests.RunTranslatedCContext) void {
         \\  return 0;
         \\}
     , "");
+
+    cases.add("cast signed array index to unsigned",
+        \\#include <stdlib.h>
+        \\int main(int argc, char **argv) {
+        \\  int a[10], i = 0;
+        \\  a[i] = 0;
+        \\  if (a[i] != 0) abort();
+        \\  return 0;
+        \\}
+    , "");
+
+    cases.add("cast long long array index to unsigned",
+        \\#include <stdlib.h>
+        \\int main(int argc, char **argv) {
+        \\  long long a[10], i = 0;
+        \\  a[i] = 0;
+        \\  if (a[i] != 0) abort();
+        \\  return 0;
+        \\}
+    , "");
 }
diff --git a/test/translate_c.zig b/test/translate_c.zig
index 2ad4a1564c..f8b6c33dcd 100644
--- a/test/translate_c.zig
+++ b/test/translate_c.zig
@@ -1843,12 +1843,51 @@ pub fn addCases(cases: *tests.TranslateCContext) void {
         \\pub export var array: [100]c_int = .{0} ** 100;
         \\pub export fn foo(arg_index: c_int) c_int {
         \\    var index = arg_index;
-        \\    return array[index];
+        \\    return array[@intCast(c_uint, index)];
         \\}
     ,
         \\pub const ACCESS = array[2];
     });
 
+    cases.add("cast signed array index to unsigned",
+        \\void foo() {
+        \\  int a[10], i = 0;
+        \\  a[i] = 0;
+        \\}
+    , &[_][]const u8{
+        \\pub export fn foo() void {
+        \\    var a: [10]c_int = undefined;
+        \\    var i: c_int = 0;
+        \\    a[@intCast(c_uint, i)] = 0;
+        \\}
+    });
+
+    cases.add("long long array index cast to usize",
+        \\void foo() {
+        \\  long long a[10], i = 0;
+        \\  a[i] = 0;
+        \\}
+    , &[_][]const u8{
+        \\pub export fn foo() void {
+        \\    var a: [10]c_longlong = undefined;
+        \\    var i: c_longlong = @bitCast(c_longlong, @as(c_longlong, @as(c_int, 0)));
+        \\    a[@intCast(usize, i)] = @bitCast(c_longlong, @as(c_longlong, @as(c_int, 0)));
+        \\}
+    });
+
+    cases.add("unsigned array index skips cast",
+        \\void foo() {
+        \\  unsigned int a[10], i = 0;
+        \\  a[i] = 0;
+        \\}
+    , &[_][]const u8{
+        \\pub export fn foo() void {
+        \\    var a: [10]c_uint = undefined;
+        \\    var i: c_uint = @bitCast(c_uint, @as(c_int, 0));
+        \\    a[i] = @bitCast(c_uint, @as(c_int, 0));
+        \\}
+    });
+
     cases.add("macro call",
         \\#define CALL(arg) bar(arg)
     , &[_][]const u8{