C `sizeof(struct)` - bad order of evaluation

[h0nzZik]

The program

int main() {
        return sizeof(struct S{int x;}) + sizeof(struct S);
}

fails to compile, with the error message

empty0.c:2:2: warning: Trying to compute sizeof of an incomplete or function type. (Constraint violation)

However, if we swap the operands of +, the program compiles (with -fno-native-compilation):

int main() {
        return sizeof(struct S) + sizeof(struct S{int x;});
}

I guess that the problem is that the right side of + is evaluated first. For -,*, and /, the order of evaluation seem to be right.

[SamB]

Surely order of evaluation isn't supposed to matter here?

Clang's Type::isIncompleteType method cites C99 6.2.5p22, which says:

[…] A structure or union type of unknown content (as described in 6.7.2.3) is an incomplete type. It is completed, for all declarations of that type, by declaring the same structure or union tag with its defining content later in the same scope.

Other than getting pushed a few paragraphs down, this seems the same in C23 6.2.5p27 (reference bumped to 6.7.3.4).

C99 6.7.2.3p4 elaborates:

All declarations of structure, union, or enumerated types that have the same scope and use the same tag declare the same type. The type is incomplete111) until the closing brace of the list defining the content, and complete thereafter.

This is only slightly changed in C23 6.7.3.4p4-6:

All declarations of structure, union, or enumerated types that have the same scope and use the same tag declare the same type.

Irrespective of whether there is a tag or what other declarations of the type are in the same translation unit, the type (except enumerated types with a fixed underlying type) is incomplete until immediately after the closing brace of the list defining the content for the first time and complete thereafter.

Enumerated types with fixed underlying type (6.7.3.3) are complete immediately after their first associated enum type specifier ends.

Even if we weren't already inclined to treat the completeness of types as based on source order, the reference to the closing brace makes it difficult to interpret any other way.

Thus, whether a type is complete or not should probably be calculated syntactically before evaluation begins, and it shouldn't matter which order the sizeof expressions are evaluated in.