The following code does not trigger any error in GCC:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

struct single
{
};

struct derived : virtual single
{
    template <typename Arg>
    constexpr derived(Arg arg){} 
};

auto obj = derived{1};

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Note that a normal ctor without template can be correctly rejected.

MSVC, EDG, and Clang reject it by giving diagnostics like:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

<source>:9:15: error: constexpr constructor not allowed in struct with virtual base class
    9 |     constexpr derived(Arg arg){} 
      |               ^
<source>:6:18: note: virtual base class declared here
    6 | struct derived : virtual single
      |                  ^~~~~~~~~~~~~~
<source>:12:12: error: no matching constructor for initialization of 'derived'
   12 | auto obj = derived{1};
      |            ^      ~~~
<source>:6:8: note: candidate constructor (the implicit copy constructor) not viable: no known conversion from 'int' to 'const derived' for 1st argument
    6 | struct derived : virtual single
      |        ^~~~~~~
<source>:6:8: note: candidate constructor (the implicit move constructor) not viable: no known conversion from 'int' to 'derived' for 1st argument
    6 | struct derived : virtual single
      |        ^~~~~~~
2 errors generated.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Please see https://godbolt.org/z/bYaeEYE3a