std: non-byte-multiple sized integers have no definite representation

Closes #7445

lib/std/meta/trait.zig

@@ -481,10 +481,12 @@ pub fn hasUniqueRepresentation(comptime T: type) bool {
         .Enum,
         .ErrorSet,
         .Fn,
-        .Int, // TODO check that it is still true
         .Pointer,
         => return true,
 
+        // The padding bits are undefined.
+        .Int => |info| return (info.bits % 8) == 0,
+
         .Array => |info| return comptime hasUniqueRepresentation(info.child),
 
         .Struct => |info| {
@@ -525,14 +527,10 @@ test "std.meta.trait.hasUniqueRepresentation" {
 
     testing.expect(hasUniqueRepresentation(TestStruct3));
 
-    testing.expect(hasUniqueRepresentation(i1));
-    testing.expect(hasUniqueRepresentation(u2));
-    testing.expect(hasUniqueRepresentation(i3));
-    testing.expect(hasUniqueRepresentation(u4));
-    testing.expect(hasUniqueRepresentation(i5));
-    testing.expect(hasUniqueRepresentation(u6));
-    testing.expect(hasUniqueRepresentation(i7));
-    testing.expect(hasUniqueRepresentation(u8));
-    testing.expect(hasUniqueRepresentation(i9));
-    testing.expect(hasUniqueRepresentation(u10));
+    inline for ([_]type{ i0, u8, i16, u32 }) |T| {
+        testing.expect(hasUniqueRepresentation(T));
+    }
+    inline for ([_]type{ i1, u9, i17, u33 }) |T| {
+        testing.expect(!hasUniqueRepresentation(T));
+    }
 }