[libc++] Fix implementation of iota_view::size (#67819)

We were incorrectly deducing the return type of size() because we were
not using ternary operators in the implementation (as the spec says).
Instead of deducing the common type of the expressions in the spec, we
would deduce potentially different return types and fail to compile.

Fixes #67551

Author: ldionne

libcxx/include/__ranges/iota_view.h

@@ -363,15 +363,14 @@ namespace ranges {
               (integral<_Start> && integral<_BoundSentinel>) || sized_sentinel_for<_BoundSentinel, _Start>
     {
       if constexpr (__integer_like<_Start> && __integer_like<_BoundSentinel>) {
-        if (__value_ < 0) {
-          if (__bound_sentinel_ < 0) {
-            return std::__to_unsigned_like(-__value_) - std::__to_unsigned_like(-__bound_sentinel_);
-          }
-          return std::__to_unsigned_like(__bound_sentinel_) + std::__to_unsigned_like(-__value_);
-        }
-        return std::__to_unsigned_like(__bound_sentinel_) - std::__to_unsigned_like(__value_);
+        return (__value_ < 0)
+                 ? ((__bound_sentinel_ < 0)
+                        ? std::__to_unsigned_like(-__value_) - std::__to_unsigned_like(-__bound_sentinel_)
+                        : std::__to_unsigned_like(__bound_sentinel_) + std::__to_unsigned_like(-__value_))
+                 : std::__to_unsigned_like(__bound_sentinel_) - std::__to_unsigned_like(__value_);
+      } else {
+        return std::__to_unsigned_like(__bound_sentinel_ - __value_);
       }
-      return std::__to_unsigned_like(__bound_sentinel_ - __value_);
     }
   };
 

libcxx/test/std/ranges/range.factories/range.iota.view/size.pass.cpp

@@ -10,9 +10,10 @@
 
 // constexpr auto size() const requires see below;
 
-#include <ranges>
 #include <cassert>
+#include <concepts>
 #include <limits>
+#include <ranges>
 
 #include "test_macros.h"
 #include "types.h"
@@ -89,6 +90,15 @@ constexpr bool test() {
     assert(io.size() == 0);
   }
 
+  // Make sure iota_view<short, short> works properly. For details,
+  // see https://github.com/llvm/llvm-project/issues/67551.
+  {
+    static_assert(std::ranges::sized_range<std::ranges::iota_view<short, short>>);
+    std::ranges::iota_view<short, short> io(10, 20);
+    std::same_as<unsigned int> auto sz = io.size();
+    assert(sz == 10);
+  }
+
   return true;
 }