[Voted into WP at August, 2010 meeting.]

Proposed resolution (February, 2010):

Change 9.2.6 [dcl.constexpr] paragraph 4 as follows:

The definition of a constexpr constructor shall satisfy the following constraints:

• each of its parameter types shall be a literal type or a reference to a literal type

• its function-body shall not be a function-try-block

• the compound-statement of its function-body shall be empty

• every non-static data member and base class sub-object shall be initialized (11.9.3 [class.base.init])

• every constructor involved in initializing non-static data members and base class sub-objects invoked by a mem-initializer shall be a constexpr constructor.

• every constructor argument and full-expression in a mem-initializer shall be a potential constant expression

• every assignment-expression that is an initializer-clause appearing directly or indirectly within a brace-or-equal-initializer for a non-static data member that is not named by a mem-initializer-id shall be a constant expression

• every implicit conversion used in converting a constructor argument to the corresponding parameter type and converting a full-expression to the corresponding member type shall be one of those allowed in a constant expression.

The rules for constexpr constructors are missing some necessary requirements. In particular, there is no requirement that a brace-or-equal-initializer for a non-static data member be a constant expression, and the requirement for constexpr constructors for initializing non-static data members applies only to members named in a mem-initializer, allowing a non-constexpr default constructor to be invoked.