library ieee;
use ieee.std_logic_1164.all;

entity reproducer is
end entity reproducer;

architecture rtl of reproducer is
  type slv_vector_t is array (natural range <>) of std_logic_vector;
  type rec is record
    vector : slv_vector_t;
    -- vector : slv_vector_t(0 TO 1)(0 DOWNTO 0); -- constraint record works
  end record;
  function get_rec return rec is
    variable a : slv_vector_t(0 TO 0)(0 DOWNTO 0) := (0 => "0");
    variable b : slv_vector_t(0 TO 0)(0 DOWNTO 0) := (0 => "1");
  begin
    return (vector => (slv_vector_t'(0 => "0") & slv_vector_t'(0 => "1"))); -- crash when 'vector' is unconstrained
    -- return (vector => a & b); -- crash when 'vector' is unconstrained
  end function;  
begin
    main : process
        variable val : std_logic_vector(0 DOWNTO 0);
    begin
        val := get_rec.vector(0);
        report to_string(val);
        wait;        
    end process;
end architecture;

$ nvc -a reproducer.vhd -e reproducer -r
*** Caught signal 11 (SEGV_MAPERR) [address=0x562b42b30b60, ip=0x562969e4cba8] ***

[0x562969da4c0d] 
[0x7f6d1b3ca32f] (/usr/lib/x86_64-linux-gnu/libc.so.6) 
[0x562969e4cba8] 
[0x562969e4cded] 
[0x562969e4cf78] 
[0x562969e2b822] 
[0x562969e2bc7c] 
[0x562969e349f9] 
[0x562969e35314] 
[0x562969e34d0a] 
[0x562969e5222a] 
[0x562969e374f1] 
[0x562969e38acf] 
[0x562969e0b4a8] 
[0x562969e028fe] 
[0x562969ef25c3] 
[0x562969da0003] 
[0x562969d9d158] 
[0x562969d9da32] 
[0x562969d98cb1] 
[0x7f6d1b3af1c9] (/usr/lib/x86_64-linux-gnu/libc.so.6) 
[0x7f6d1b3af28a] (/usr/lib/x86_64-linux-gnu/libc.so.6) __libc_start_main
[0x562969d9a304] 

nvc 1.15.2 (2fe12d4) (Using LLVM 18.1.3) [x86_64-pc-linux-gnu]

Please report this bug at https://github.com/nickg/nvc/issues

$ nvc -a reproducer.vhd -e reproducer -r
** Fatal: 0ms+0: value length -1480589314 does not match variable VAL length 2
    > reproducer.vhd:22
    |
 22 |         val := get_rec.vector(0); -- not okay
    |         ^^^