Standard Library Modules: Add compiler workarounds for `<random>` (#4906)

stl/inc/__msvc_int128.hpp

@@ -707,9 +707,9 @@ struct _Unsigned128 : _Base128 {
     using _Signed_type   = _Signed128;
     using _Unsigned_type = _Unsigned128;
 
-#if !_HAS_CXX17
+#if !_HAS_CXX17 || (_HAS_CXX20 && !defined(__clang__) && !defined(__EDG__)) // TRANSITION, DevCom-10729775
     constexpr _Unsigned128() noexcept : _Base128{} {}
-#endif // !_HAS_CXX17
+#endif // ^^^ workaround for C++20 MSVC modules and header units; should be guarded for !_HAS_CXX17 only ^^^
 
     using _Base128::_Base128;
     constexpr explicit _Unsigned128(const _Base128& _That) noexcept : _Base128{_That} {}
@@ -1024,9 +1024,9 @@ struct _Signed128 : _Base128 {
     using _Signed_type   = _Signed128;
     using _Unsigned_type = _Unsigned128;
 
-#if !_HAS_CXX17
+#if !_HAS_CXX17 || (_HAS_CXX20 && !defined(__clang__) && !defined(__EDG__)) // TRANSITION, DevCom-10729775
     constexpr _Signed128() noexcept : _Base128{} {}
-#endif // !_HAS_CXX17
+#endif // ^^^ workaround for C++20 MSVC modules and header units; should be guarded for !_HAS_CXX17 only ^^^
 
     using _Base128::_Base128;
     constexpr explicit _Signed128(const _Base128& _That) noexcept : _Base128{_That} {}

tests/std/include/test_header_units_and_modules.hpp

@@ -593,6 +593,14 @@ void test_random() {
     minstd_rand0 lcg;
     lcg.discard(9999);
     assert(lcg() == 1043618065); // N4868 [rand.predef]/1
+
+#ifndef _MSVC_INTERNAL_TESTING // TRANSITION, VSO-2226569
+    // Test coverage for GH-4899 "Standard Library Modules: uniform_real_distribution emits
+    // error C2512: 'std::_Unsigned128': no appropriate default constructor available":
+    const double val = generate_canonical<double, 53>(lcg);
+    assert(val >= 0.0);
+    assert(val < 1.0);
+#endif // ^^^ no workaround ^^^
 }
 
 void test_ranges() {