Constexpr constructors with virtual bases work in GCC 16 and Clang trunk

A class with a virtual base could not have a constexpr constructor until P3533R2, “constexpr virtual inheritance” (Hana Dusíková, 2025-02-17). On 2026-10-18, GCC 16.1 and 16.2 accept it with -std=c++26, and Clang trunk accepts it. GCC 15.3, GCC 14.1 and Clang 23.1 reject it. GCC 16 still refuses one thing that Clang trunk allows: a virtual call through a reference to the virtual base inside a constant expression.

In short

The paper removes the restriction on constexpr constructors and destructors in classes with virtual base classes. It names std::ios_base and the stream classes as later beneficiaries.
GCC 16.1, 16.2 and trunk define __cpp_constexpr_virtual_inheritance as 202506 under -std=c++26. Clang trunk accepts the code and defines no macro yet.
GCC 16 rejects the virtual call through a virtual base reference with a specific message. Clang trunk runs it.
I did not determine whether the standard requires that last case, so this reports behavior and not a bug.

The example

The base A is virtual. Both B and C inherit it. The class D inherits both of those. Every constructor is constexpr:

struct A {
  int v;
  constexpr A(int x) : v(x) {}
  constexpr virtual int f() const { return v; }
};
struct B : virtual A {
  constexpr B() : A(1) {}
  constexpr int f() const override { return 2; }
};
struct C : virtual A { constexpr C() : A(1) {} };
struct D : B, C { constexpr D() : A(5) {} };

The program checks two things at compile time. It reads the data member of the virtual base through a D, and it calls the virtual function on a complete B. The first embed runs it on GCC 16.1, and the second button runs the same file on Clang trunk.

Tested: GCC 16.1, -std=c++26 + Clang trunk

Run on Compiler ExplorerConstexpr constructors with a virtual basex86-64 gcc 16.1 · -std=c++26 · runs live Also on Clang trunkConstexpr constructors with a virtual basex86-64 clang (trunk) · -std=c++26 · runs live

Both print read_base = 5 and direct_call = 2. GCC reports the macro value 202506, and Clang reports it as undefined.

The case that differs

Compiling with -DCALL_THROUGH_BASE adds one more function. It makes a B and binds a reference to its virtual base. Then it calls the virtual function through that reference in a static_assert:

Tested: GCC 16.1, -std=c++26 + Clang trunk

Run on Compiler ExplorerA virtual call through a virtual base referencex86-64 gcc 16.1 · -std=c++26 -DCALL_THROUGH_BASE · fails to compile, exact error shown Also on Clang trunkA virtual call through a virtual base referencex86-64 clang (trunk) · -std=c++26 -DCALL_THROUGH_BASE · runs live

Compiler	Constructors, reads, direct call	Virtual call through the virtual base
GCC 14.1 (`-std=c++23`) and 15.3	error: the struct has virtual base classes	same error
GCC 16.1, 16.2 and trunk	compiles	error: calling a constexpr member function through a virtual base subobject
Clang 23.1	error: constexpr constructor not allowed in struct with virtual base class	same error
Clang trunk	compiles	compiles, prints `through_base = 2`

The GCC message names the function, B::f, and says it is called through a virtual base subobject. A smaller version compiled on GCC 16.1: the base had no data member and no constructor argument. So the trigger is not simply the virtual call, and I stopped at the version above.

What I did not test

Destructors, dynamic_cast and typeid in constant expressions were not run. Neither was any standard stream class, because the library types the paper mentions are not constexpr yet.

Sources: P3533R2, read 2026-10-18 · Compiler Explorer runs on 2026-10-18: GCC 14.1, 15.3, 16.1, 16.2 and trunk, Clang 23.1 and trunk.