Should missing definition of object / function with non-reserved identifier lead to diagnostics?

Question

Sample code:

void accept(int x);

int main(void)
{
        accept(0);
        return 0;
}

Invocations:

$ gcc t719.c -std=c11 -pedantic -Wall -Wextra
<nothing>

$ clang t719.c -std=c11 -pedantic -Wall -Wextra
<nothing>

$ cl t719.c /std:c11 /Za
t719.obj : error LNK2019: unresolved external symbol accept referenced in function main

$ icc t719.c -std=c11 -pedantic -Wall -Wextra
<nothing>

Consider that the user forgot to define accept. We see that no diagnostics may be produced. Is diagnostics required?

UPD: Another example:

extern int y0;

int main(void)
{
        return y0;
}

# linux (begin)
$ gcc t719.c -std=c11 -pedantic -Wall -Wextra
undefined reference to `y0'

$ clang t719.c -std=c11 -pedantic -Wall -Wextra
undefined reference to `y0'

$ icc t719.c -std=c11 -pedantic -Wall -Wextra
<nothing>
# program returned: 243
# linux (end)

# windows (begin)
# gcc in cygwin
$ gcc t719.c -std=c11 -pedantic -Wall -Wextra
<nothing>
# program returned: 255

# clang in cygwin
$ clang t719.c -std=c11 -pedantic -Wall -Wextra
<nothing>
# program returned: 255

$ cl t719.c /std:c11 /Za
unresolved external symbol _y0 referenced in function _main

$ LLVM/12.0.0/bin/clang t719.c -std=c11 -pedantic -Wall -Wextra
<nothing>
# program returned: 72

$ icl -Qstd=c11 t719.c 
<nothing>
# program returned: 65
# windows (end)