Am Donnerstag, dem 16.07.2026 um 09:17 +1200 schrieb Ashton Warner via Gcc:
> Hello,
>
> I would like to discuss the possibility of adding a GCC attribute for
> describing tagged unions (discriminated unions) to improve static
> analysis diagnostics.
>
> The motivation is to allow programmers to explicitly describe a
> relationship between an enum discriminator and a union member. C has a
> common pattern of representing variants using a struct containing an
> enum and a union:
>
> enum num_type {
> T_INT,
> T_FLOAT,
> };
>
> struct number {
> enum num_type type;
>
> union {
> int ival;
> float fval;
> };
> };
>
> I propose the GCC attribute __attribute__((tagged_by(...))) with the
> following syntax:
>
> tagged_by(discriminator, mapping-list)
>
> where:
>
> - discriminator is an identifier
> - mapping-list is a comma-separated list of one or more mappings.
> - Each mapping has the form:
> (enumerator, union-member)
>
> For example:
>
> struct number {
> enum num_type type;
>
> union {
> int ival;
> float fval;
> } __attribute__((tagged_by(type,
> (T_INT, ival),
> (T_FLOAT, fval)
> )));
> };
>
> The mapping is intentionally explicit rather than inferred from the
> declaration order of enum values and union members. This avoids
> changing the meaning of the attribute if either the enum or union
> members are reordered.
>
> The attribute does not change the representation or runtime behaviour
> of the union. It provides additional information that GCC can use for
> diagnostics and static analysis.
>
> For example:
>
> struct number n;
>
> n.type = T_INT;
> n.fval = 1.0f;
>
> could produce a diagnostic because fval is not the member associated
> with the current discriminator value.
>
> The implementation should diagnose invalid mappings, such as an
> enumerator that is not part of the discriminator's enum type or a
> member that does not exist in the union.
>
> The attribute is intended to provide semantic information in a similar
> way to existing attributes such as counted_by, where the compiler is
> given information about a relationship that already exists in the
> program.
>
I agree with this proposal. There is an existing enhancement
request: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112840
I like an attribute on the member more than the attribute on the
union type.
> Open questions:
>
> - How should enumerators without an associated union member be
> represented? One possibility is allowing a mapping without a member,
> for example (T_UNKNOWN), to explicitly indicate that an enumerator
> represents a valid discriminator state with no active union member.
> Enumerators that are not mentioned in the attribute could then be
> diagnosed.
> - Should diagnostics based on this attribute be implemented as part of
> existing warning infrastructure, -fanalyzer, or another analysis
> pass?
> - Could this information be useful to future runtime checking tools?
Yes, I think this would be useful for checking at run-time similar
to sanitizers. This would be easy to implement.
> - Are there existing GCC mechanisms that overlap with this
> functionality?
> - Are there additional constraints or semantics that would be needed
> for GCC to implement this attribute?
The issue similar to counted_by are the exact semantics for when
the constraints are in effect relative to when the members are
accessed / changed.
Martin
