[clang] [clang-cl] Add support for [[msvc::constexpr]] C++11 attribute (PR #71300)

[Richard Dzenis via cfe-commits]

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@@ -0,0 +1,37 @@
+// RUN: %clang_cc1 -fms-compatibility -fms-compatibility-version=19.33 -std=c++20 -verify %s
+// expected-no-diagnostics
+
+[[msvc::constexpr]] int log2(int x) { [[msvc::constexpr]] return x > 1 ? 1 + log2(x / 2) : 0; }
+constexpr bool test_log2() { [[msvc::constexpr]] return log2(32) == 5; }
+static_assert(test_log2());
+
+[[msvc::constexpr]] int get_value(int x)
+{
+  switch (x)
+  {
+    case 42: return 1337;
+    default:
+             if (x < 0) [[msvc::constexpr]] return log2(-x);
+             else return x;
+  }
+}
+
+constexpr bool test_complex_expr() {
+  [[msvc::constexpr]] return get_value(get_value(42) - 1337 + get_value(-32) - 5 + (get_value(1) ? get_value(0) : get_value(2))) == get_value(0);
+}
+static_assert(test_complex_expr());
+
+constexpr bool get_constexpr_true() { return true; }
+[[msvc::constexpr]] bool get_msconstexpr_true() { return get_constexpr_true(); }
+constexpr bool test_get_msconstexpr_true() { [[msvc::constexpr]] return get_msconstexpr_true(); }
+static_assert(test_get_msconstexpr_true());
+
+/*
+// TODO: Add support for [[msvc::constexpr]] constructor
----------------
RIscRIpt wrote:

Created a follow-up #72149.

> However, i think we should enable the test, and record the diagnostic to ensure we show it is invalid for now (along with better showing the TODO).

I hope I understood you correctly: I enabled the test, and added `expected-*` comments alongside with `TODO` mentioning 72149.

https://github.com/llvm/llvm-project/pull/71300