Clang disallows brace elision in designated initializer of member array when constructing a temporary function parameter

[wangbo15]

Clang rejects the following valid code:

struct B {
  int b[10];
};

struct C
{
  C (B x) {}
};

//OK
//B bb{.b = 42};
//OK
//C c ({.b = {42}});
//OK
//C c({42});
//Fails
C c ({.b = 42});

The question seems to be related to both the designated initializer and the brace elision operation because separately removing the former( like C c({42}); ) or the latter( like C c ({.b = {42}}); ) can both make the code compile.

GCC, MSVC, and EDG always accept it.

The diagnostic of clang:

<source>:17:3: error: no matching constructor for initialization of 'C'
   17 | C c ({.b = 42});
      |   ^  ~~~~~~~~~
<source>:5:8: note: candidate constructor (the implicit copy constructor) not viable: cannot convert initializer list argument to 'const C'
    5 | struct C
      |        ^
<source>:5:8: note: candidate constructor (the implicit move constructor) not viable: cannot convert initializer list argument to 'C'
    5 | struct C
      |        ^
<source>:7:3: note: candidate constructor not viable: cannot convert initializer list argument to 'B'
    7 |   C (B x) {}
      |   ^  ~~~
1 error generated.

https://godbolt.org/z/eve54hcx3

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Bo Wang (wangbo15)

Clang rejects the following valid code:

struct B {
  int b[10];
};

struct C
{
  C (B x) {}
};

//OK
//B bb{.b = 42};
//OK
//C c ({.b = {42}});
//OK
//C c({42});
//Fails
C c ({.b = 42});

The question seems to be related to both the designated initializer and the brace elision operation because separately removing the former( like C c({42}); ) or the latter( like C c ({.b = {42}}); ) can both make the code compile.

GCC, MSVC, and EDG always accept it.

The diagnostic of clang:

<source>:17:3: error: no matching constructor for initialization of 'C'
   17 | C c ({.b = 42});
      |   ^  ~~~~~~~~~
<source>:5:8: note: candidate constructor (the implicit copy constructor) not viable: cannot convert initializer list argument to 'const C'
    5 | struct C
      |        ^
<source>:5:8: note: candidate constructor (the implicit move constructor) not viable: cannot convert initializer list argument to 'C'
    5 | struct C
      |        ^
<source>:7:3: note: candidate constructor not viable: cannot convert initializer list argument to 'B'
    7 |   C (B x) {}
      |   ^  ~~~
1 error generated.

https://godbolt.org/z/eve54hcx3

[Endilll]

It seems that Clang tries to do https://eel.is/c++draft/dcl.init.general#16.9 "Otherwise, the initial value of the object being initialized is the (possibly converted) value of the initializer expression" fallback and fails. I can't find anything in [dcl.init] that says how arrays are initialized from a single expression not enclosed in parentheses or braces*. So I believe Clang is correct to reject your example per existing wording.

//OK
//B bb{.b = 42};

You say it's OK, but actually Clang warns that "brace elision for designated initializer is a C99 extension". This might hint to a reason why other compilers accept this.

(*) There is provision https://eel.is/c++draft/dcl.init#string-3 that takes care of string literals that are shorter than a character array they initialize. Clang implements that: https://godbolt.org/z/1hK64zrf1