Clang-cl can't use the never-value field optimization

Since offsetof() is not a constant expression on clang-cl,
though it is on other compilers.

llvm/llvm-project#59689

Author: danakj

macros/__private/compiler_bugs.h

@@ -125,3 +125,13 @@
 #define sus_clang_bug_49358(...)
 #define sus_clang_bug_49358_else(...) __VA_ARGS__
 #endif
+
+// TODO: https://github.com/llvm/llvm-project/issues/59689
+// offsetof() is not constant evaluable in clang-cl.
+#if _MSC_VER && __clang_major__ > 0  // TODO: Update when the bug is fixed.
+#define sus_clang_bug_59689(...) __VA_ARGS__
+#define sus_clang_bug_59689_else(...)
+#else
+#define sus_clang_bug_59689(...)
+#define sus_clang_bug_59689_else(...) __VA_ARGS__
+#endif

mem/never_value.h

@@ -179,7 +179,8 @@ concept NeverValueField = never_value_access<T>::has_field;
                                                                                \
   template <class SusUnsafeNeverValueOuter,                                    \
             bool SusUnsafeNeverValueStandardLayout =                           \
-                std::is_standard_layout_v<SusUnsafeNeverValueOuter>>           \
+                sus_clang_bug_59689(false) sus_clang_bug_59689_else(           \
+                    std::is_standard_layout_v<SusUnsafeNeverValueOuter>)>      \
   struct SusUnsafeNeverValueOverlay;                                           \
                                                                                \
   template <class SusUnsafeNeverValueOuter>                                    \

option/__private/storage.h

@@ -211,6 +211,10 @@ struct [[sus_trivial_abi]] StoragePointer<T&> {
 
 // This must be true in order for StoragePointer to be useful with the
 // never-value field optimization.
-static_assert(::sus::mem::NeverValueField<StoragePointer<int&>>);
+// clang-format off
+sus_clang_bug_59689_else(
+  static_assert(::sus::mem::NeverValueField<StoragePointer<int&>>);
+)
+// clang-format on
 
 }  // namespace sus::option::__private