Minimal reproducer for https://stackoverflow.com/q/69520633/627587

minimal-repro.sh

#!/usr/bin/env bash

cat <<EOF | clang++ -xc++ - -c -o string.o
template <class T>
struct basic_string {
    __attribute__((internal_linkage))
    void shrink_to_fit();
};

template <class T>
void basic_string<T>::shrink_to_fit() { }

template class basic_string<char>;
EOF

cat <<EOF | clang++ -xc++ - -c -o main.o
template <class T>
struct basic_string {
    __attribute__((internal_linkage))
    void shrink_to_fit();
};

template <class T>
void basic_string<T>::shrink_to_fit() { }

extern template class basic_string<char>;

int main() {
    basic_string<char> s;
    s.shrink_to_fit();
}
EOF

clang++ string.o main.o

rm string.o main.o