C++ integral constant expression definition

Question

In the current C++ standard there is the following paragraph (expr.const#5) (emphasis mine):

An integral constant expression is an expression of integral or unscoped enumeration type, implicitly converted to a prvalue, where the converted expression is a core constant expression. [ Note: Such expressions may be used as bit-field lengths, as enumerator initializers if the underlying type is not fixed ([dcl.enum]), and as alignments. — end note ]

I have two questions regarding this definition:

1. Does the phrase "implicitly converted to a prvalue" mean that for an expression to be considered an "integral constant expression" it must appear in a context that forces it to be implicitly converted to a prvalue?

2. What does "the converted expression" refer to? I know that this question is addressed in Clarification of converted constant expression definition. The answer given there is that "the converted expression" is t, after the following initialization: T t = expr;. However, I do not see how evaluating that expression (t) would match any of the rules given in [expr.const#4] (paragraph describing required conditions for an expression to be considered a core constant expression) which would make it unqualified to be a core constant expression.

Thank you.

Answer 1

The statement that an integral constant expression is implicitly converted to a prvalue means that the lvalue-to-rvalue conversion is applied to any expression used as an integral constant expression. In the one case where an expression might be an integral constant expression—initializing a non-local object of const-qualified integer type that might be usable in constant expressions—the initializer is a prvalue anyway, so no change of interpretation can occur.

Beyond that, both of your questions have the same answer: whatever conversions are necessary to bring the expression (as written) to a prvalue integral type must also be allowed in a core constant expression (see, for example, /4.7 just before your citation and /6 just after it). The “converted expression” comprises the conversion in the T t=e; interpretation, not just the id-expression t (which would, for instance, always be an lvalue).