[Bug c++/55227] designated initializer for char array by string constant

["wjwray at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55227

Will Wray <wjwray at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |wjwray at gmail dot com

--- Comment #6 from Will Wray <wjwray at gmail dot com> ---
Hit this in implementing P1997 array copy semantics (which generalizes
the char-array <- string-literal case to array <- array initialization).

Compiler Explorer link https://godbolt.org/z/vednE3bYz

The reshape_init_r code that ensadc points to is one problem.
The code effectively special-cases the C rule that allows optional
braces around a string-literal initializer:

  A brace-enclosed string-literal initializer has its braces stripped,
  but then the code does not account at all for nested designators
  (as a recursive call to reshape_init would or should do).

This can be seen in the way that bogus nested designators are ignored:

    struct C {char a[2];}

    C y = {{.bogus="y"}}; // Invalid, g++ accepts
    C z = {{[0]="z"}};    // Invalid, g++ accepts (pedantic warning GNU [0])

(so the 'solution' of adding braces is misleading, non-standard, non-portable).

Then, in placing the designator correctly outside the braces, it is not
matched with the brace-enclosed initializer (haven't tracked down why yet):

    C w = {.a={"w"}};  // Valid, g++ rejects designated braced
                       // (gcc C accepts) (other C++ compilers accept)

A second problem is that the 'matching' of designator to initializer is
brittle even without optional braces; only the exactly-matching char-array
type (i.e. and extent) is accepted in an unbraced string literal:

    C u = {.a="u"}; // Valid, designated char[2] literal for char[2] field
    C r = {.a=""};  // Valid, g++ rejects designated char[1] for char[2]

A workaround is to match the size of the char array by padding with zeros:

    C r = {.a="\0"};

I've spent over a day trying to isolate the 'matching' issue -
could do with assist from a designated-initializer mechanic.