What Determines Whether a C 11 Template Substitution Error Is a Hard or Soft Error?

Immediate Context in C 11 Template Substitution

The C 11 Standard introduces Substitution Failure Is Not An Error (SFINAE), which allows for the suppression of hard compilation errors during template argument substitution. However, the standard defines a specific "immediate context" for which SFINAE applies, leaving some uncertainty about when a substitution error results in a hard or soft error.

Immediate Context Definition

The Standard defines the immediate context as:

"Only invalid types and expressions in the immediate context of the function type and its template parameter types can result in a deduction failure."

This definition is vague, but a non-normative note provides a hint:

"The evaluation of the substituted types and expressions can result in side effects such as... Such side effects are not in the “immediate context” and can result in the program being ill-formed."

Decision Procedure

To determine whether a substitution error occurs in the immediate context, consider the following decision procedure:

• Step 1: Identify all templates and implicitly-defined functions required to determine the result of the substitution.
• Step 2: Imagine these entities are generated before substitution starts.
• Step 3: If errors occur during this hypothetical generation, they are not in the immediate context and will result in hard errors.
• Step 4: If all entities are generated without error, any subsequent errors during substitution are not errors but result in argument deduction failures.

Examples

• Hard error: func> - Instantiation of A fails because pointers to references are not allowed.
• Argument deduction failure: func> - Instantiation of A succeeds, but A::type does not exist.
• Argument deduction failure: class B { void f(int); }; func::f - Implicit definition of f yields a deleted member function.