ICE in `FunctionDefinition::type` when an `external` free function is bound via `using for`

[jubnzv]

The compiler panics with Solidity assertion failed in libsolidity/ast/AST.cpp:490 inside FunctionDefinition::type() when a free function declared external is listed in a using { f } for T directive.

MRE:

function f(int) external;
using {f} for int global;

Reproduce: save as mre.sol, run solc --bin mre.sol.

Output:

Internal compiler error:
/solidity/libsolidity/ast/AST.cpp(490): Throw in function virtual const solidity::frontend::Type* solidity::frontend::FunctionDefinition::type() const
Dynamic exception type: boost::wrapexcept<solidity::langutil::InternalCompilerError>
std::exception::what: Solidity assertion failed
[solidity::util::tag_comment*] = Solidity assertion failed

Expected behavior: a type error rejecting external free functions in using for bindings, since only internal and public free functions make sense as bound methods.

Git commit: 9be6619
solc 0.8.35-develop.2026.4.18+commit.9be66192.Linux.g++