This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 117a. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2025-04-13
1824. Completeness of return type vs point of instantiationSection: 9.3.4.6 [dcl.fct] Status: CD4 Submitter: Steve Clamage Date: 2013-12-19[Moved to DR at the November, 2014 meeting.]
Consider the following example:
template<typename T> struct A {
T t;
};
struct S {
A<S> f() { return A<S>(); }
};
According to 9.3.4.6 [dcl.fct] paragraph 9,
The type of a parameter or the return type for a function definition shall not be an incomplete class type (possibly cv-qualified) unless the function is deleted (9.6.3 [dcl.fct.def.delete]) or the definition is nested within the member-specification for that class (including definitions in nested classes defined within the class).
Thus type A<S> must be a complete type. The requirement for a complete type triggers the instantiation of the template, which requires that its template argument be complete in order to use it as the type of a non-static data member.
According to 13.8.4.1 [temp.point] paragraph 4, the point of instantiation of A<S> is “immediately preced[ing] the namespace scope declaration or definition that refers to the specialization.” Thus the point of instantiation precedes the definition of S, making this example ill-formed. Most or all current implementations accept the example, however.
Perhaps the specification in 9.3.4.6 [dcl.fct] ought to say that the completeness of the type is checked from the context of the function body (at which S is a complete type)?
Proposed resolution (February, 2014):
Change 9.3.4.6 [dcl.fct] paragraph 9 as follows:
Types shall not be defined in return or parameter types. The type of a parameter or the return type for a function definition shall not be an incomplete class type (possibly cv-qualified) in the context of the function definition unless the function is deleted (9.6.3 [dcl.fct.def.delete]) or the definition is nested within the member-specification for that class (including definitions in nested classes defined within the class).
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