Clang 9 emits the following error:

src/mongo/platform/decimal128_test.cpp:117:26: error: constexpr variable 'expected' must be initialized by a constant expression
constexpr Decimal128 expected(10);
^~~~~~~~~~~~
src/mongo/platform/decimal128.h:179:55: note: member call on object outside its lifetime is not allowed in a constant expression
: Decimal128(v < 0 ? 1 : 0, kExponentBias, 0, _makeCoefficientLow(v)) {}
^
src/mongo/platform/decimal128_test.cpp:117:26: note: in call to 'Decimal128(10)'
constexpr Decimal128 expected(10);