https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92399
Bug ID: 92399
Summary: Rejects valid: reading of an inactive union member
that shares common initial sequence with the active on
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Keywords: rejects-valid
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: nok.raven at gmail dot com
Target Milestone: ---
The wording is in [class.union]/1 and [class.mem]/25.
[class.mem]/25 contains an example:
struct T1 { int a, b; };
struct T2 { int c; double d; };
union U { T1 t1; T2 t2; };
int f() {
U u = { { 1, 2 } }; // active member is t1
return u.t2.c; // OK, as if u.t1.a were nominated
}
Running this example in constexpr context produces an error:
<source>:8:20: in 'constexpr' expansion of 'f()'
<source>:8:21: error: accessing 'U::t2' member instead of initialized 'U::t1'
member in constant expression
8 | constexpr int i = f();
| ^
Compiler returned: 1
https://godbolt.org/z/ZNK716
