[clang] [clang] [SemaCXX] Implement CWG2627 Bit-fields and narrowing conversions (PR #78112)

[Vlad Serebrennikov via cfe-commits]

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@@ -24,6 +44,103 @@ using enum E;
 #endif
 }
 
+namespace cwg2627 { // cwg2627: 19
+#if __cplusplus >= 202002L
+struct C {
+  long long i : 8;
+  friend auto operator<=>(C, C) = default;
+};
+
+void f() {
+  C x{1}, y{2};
+  static_cast<void>(x <=> y);
+  static_cast<void>(x.i <=> y.i);
+}
+
+template<typename T>
+struct CDependent {
+  T i : 8;
+  friend auto operator<=>(CDependent, CDependent) = default;
+};
+
+template<typename T>
+concept three_way_comparable = requires(T t) { { t <=> t }; };
+template<typename T>
+concept bf_three_way_comparable = requires(T t) { { t.i <=> t.i }; };
+static_assert(three_way_comparable<CDependent<long long>>);
+static_assert(bf_three_way_comparable<CDependent<long long>>);
+#endif
+
+#if __cplusplus >= 201103L
+template<int W>
+struct D {
+  __int128 i : W;
+};
+
+template<int W>
+std::int64_t f(D<W> d) {
+    return std::int64_t{ d.i }; // #cwg2627-f
+}
+
+template std::int64_t f(D<63>);
+template std::int64_t f(D<64>);
+template std::int64_t f(D<65>);
+// since-cxx11-error-re@#cwg2627-f {{non-constant-expression cannot be narrowed from type '__int128' to 'std::int64_t' (aka '{{.+}}') in initializer list}}
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Endilll wrote:

That's unexpected.
I like `using int64_t = long long; static_assert(sizeof(int64_t) == 8)` more than regular expressions.

https://github.com/llvm/llvm-project/pull/78112