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3993. The parse function of a BasicFormatter type needs to be constexpr

Section: 28.5.6.1 [formatter.requirements] Status: New Submitter: Jiang An Opened: 2023-10-10 Last modified: 2023-10-30

Priority: 3

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Discussion:

Format strings need to be checkable at compile-time since P2216R3. In order to check a compile-time format string, the parse functions of the formatters need to be called. However, currently there is seemingly no requirement for the parse function to be constexpr, and hence whether a format string is correct for formatted types may be not well-defined at compile-time.

[2023-10-30; Reflector poll]

Set priority to 3 after reflector poll. "Should be OK to provide a non-constexpr parse if you only use it with std::vformat and not std::format." "Only the std::formatter specializations in the standard library should be required to have constexpr parse." "No need to require this, but a note with this text explaining when a call to parse can be a constant expression might be nice."

Proposed resolution:

This wording is relative to N4958.

  1. Modify 28.5.6.1 [formatter.requirements], Table 73, BasicFormatter requirements [tab:formatter.basic] as indicated:

    Table 73: BasicFormatter requirements [tab:formatter.basic]
    Expression Return type Requirement
    g.parse(pc) PC::iterator Parses format-spec (28.5.2 [format.string]) for type T in the
    range [pc.begin(), pc.end()) until the first unmatched
    character. Throws format_error unless
    the whole range is parsed or the unmatched
    character is }.
    [Note 1: This allows formatters to emit meaningful
    error messages. — end note]
    Stores the parsed format specifiers in *this and
    returns an iterator past the end of the parsed
    range. The expression shall be a core constant expression if g,
    pc.begin(), pc.end(), and lvalue-to-rvalue conversion
    for each element in [pc.begin(), pc.end()) are core constant
    expressions, and no exception is thrown.
    …