https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112840
Bug ID: 112840 Summary: feature request: warn on incorrect tagged union value access Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: matheus.a.m.moreira at gmail dot com Target Milestone: --- It would be useful if GCC could warn the programmer if the value of a tagged union that doesn't correspond to its type tag is accessed. Here's an example that illustrates the kind of mistake such a warning would prevent: #include <stdio.h> enum T { I, F }; union U { int i; float f; }; struct S { enum T t; union U u; }; int main(void) { struct S s = { .t = F, .u.f = 12345.67890f }; switch (s.t) { case I: printf("%d\n", s.u.i); break; case F: // copied the above case // but neglected to update the code printf("%d\n", s.u.i); break; } } I understand that unions are typically used for type punning and that such accesses are often intended by the programmer but compiler checks would still be beneficial when that's not the case. A compiler mechanism to establish a relationship between the union values and their corresponding enum tags would be extremely useful. Something like this, perhaps: struct S { enum T t; union U { int i __attribute__((tag(t, I))); float f __attribute__((tag(t, F))); } u; }; Then gcc would be able to warn when union values are accessed in a context where their specified tags are not known to be the correct value: switch (s.t) { case I: // i is accessed // the tag of i is t // t is supposed to equal I // compiler knows t equals I because of switch case // correct, no warning is emitted printf("%d\n", s.u.i); break; case F: // i is accessed // the tag of i is t // t is supposed to equal I // compiler knows t equals F because of switch case // incorrect, a warning is emitted printf("%d\n", s.u.i); break; } Such a feature would make C less error prone. I've also seen support for safe tagged unions in newer languages like Zig. People have created C preprocessor solutions to use tagged unions safely in C due to the lack of this safety: https://github.com/Hirrolot/datatype99 Relevant clang issue: https://github.com/llvm/llvm-project/issues/74205