Assignment between union object members of incompatible types

Newsgroups	comp.std.c
Date	2020-11-30 10:44 -0800
Message-ID	<272ec269-16e7-4e06-b8a8-e5f6534e66a8n@googlegroups.com> (permalink)
Subject	Assignment between union object members of incompatible types
From	Ian Abbott <ijabbott63@gmail.com>

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A question (not posted by myself) from https://stackoverflow.com/questions/65077630 :

Consider the following:

union { int i; char c; } x = {0};
x.c = x.i;

Does the assignment x.c = x.i result in undefined behavior?

C18 6.15.16.1/3 says:

| If the value being stored in an object is read from another object 
| that overlaps in any way the storage of the first object, then the 
| overlap shall be exact and the two objects shall have qualified or 
| unqualified versions of a compatible type; otherwise, the behavior is 
| undefined.

The objects x.c and x.i overlap, but have incompatible types, so on first glance it appears to be UB.

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Assignment between union object members of incompatible types Ian Abbott <ijabbott63@gmail.com> - 2020-11-30 10:44 -0800
  Re: Assignment between union object members of incompatible types James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-11-30 14:45 -0500
  Re: Assignment between union object members of incompatible types Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2020-11-30 12:20 -0800
    Re: Assignment between union object members of incompatible types Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-12-01 03:39 -0800
      Re: Assignment between union object members of incompatible types Francis Glassborow <francis.glassborow@btinternet.com> - 2020-12-05 14:27 +0000
        Re: Assignment between union object members of incompatible types Tim Rentsch <tr.17687@z991.linuxsc.com> - 2021-07-10 08:42 -0700
  Re: Assignment between union object members of incompatible types Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-12-01 02:49 -0800

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