explorer has lifetime(?) bug with String associated constants

[zygoloid]

package Testcase api;

interface HasName {
    let Name:! String;
}

external impl i32 as HasName where .Name == "i32" {}
external impl String as HasName where .Name == "String" {}

fn Main() -> i32 {
    Print(String.(HasName.Name));
    return 0;
}

https://carbon.compiler-explorer.com/z/esP9c8zsn
prints:

����V
result: 0

(A different garbage string each run.) Looks like a use-after-free of a string to me.

[tkadur]

When I try to run explorer locally on that code I get the following compiler crash. Not sure why it doesn't match Compiler Explorer's behavior.

Error message

explorer: external/llvm-project/llvm/include/llvm/Support/Casting.h:566: decltype(auto) llvm::cast(const From &) [To = Carbon::StringValue, From = Carbon::Value]: Assertion `isa<To>(Val) && "cast<Ty>() argument of incompatible type!"' failed.
Please report issues to https://github.com/carbon-language/carbon-lang/issues and include the crash backtrace.
Stack dump:
0.	Program arguments: bazel_bin/explorer/explorer explorer/testdata/string/associated_constant.carbon
 #0 0x0000557204711e1b backtrace (bazel_bin/explorer/explorer+0x4d7e1b)
 #1 0x0000557204aa2feb llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) /proc/self/cwd/external/llvm-project/llvm/lib/Support/Unix/Signals.inc:569:13
 #2 0x0000557204aa328b PrintStackTraceSignalHandler(void*) /proc/self/cwd/external/llvm-project/llvm/lib/Support/Unix/Signals.inc:636:1
 #3 0x0000557204a9d8e8 llvm::sys::RunSignalHandlers() /proc/self/cwd/external/llvm-project/llvm/lib/Support/Signals.cpp:104:18
 #4 0x0000557204aa3f0e SignalHandler(int) /proc/self/cwd/external/llvm-project/llvm/lib/Support/Unix/Signals.inc:407:1
 #5 0x00007f153593e200 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x12200)
 #6 0x00007f15355e68a1 raise ./signal/../sysdeps/unix/sysv/linux/raise.c:50:1
 #7 0x00007f15355d0546 abort ./stdlib/abort.c:81:7
 #8 0x00007f15355d042f get_sysdep_segment_value ./intl/loadmsgcat.c:509:8
 #9 0x00007f15355d042f _nl_load_domain ./intl/loadmsgcat.c:970:34
#10 0x00007f15355df222 (/lib/x86_64-linux-gnu/libc.so.6+0x31222)
#11 0x00005572048c42ec decltype(auto) llvm::cast<Carbon::StringValue, Carbon::Value>(Carbon::Value const&) /proc/self/cwd/external/llvm-project/llvm/include/llvm/Support/Casting.h:566:3
#12 0x00005572048b945a Carbon::Interpreter::StepExp() /proc/self/cwd/explorer/interpreter/interpreter.cpp:1154:47
#13 0x00005572048ac3e3 index /usr/lib/llvm-13/bin/../include/c++/v1/variant:788:12
#14 0x00005572048ac3e3 index /usr/lib/llvm-13/bin/../include/c++/v1/variant:1438:59
#15 0x00005572048ac3e3 __holds_alternative<1UL, Carbon::Error, Carbon::Success> /usr/lib/llvm-13/bin/../include/c++/v1/variant:1464:14
#16 0x00005572048ac3e3 holds_alternative<Carbon::Success, Carbon::Error, Carbon::Success> /usr/lib/llvm-13/bin/../include/c++/v1/variant:1470:10
#17 0x00005572048ac3e3 ok /proc/self/cwd/./common/error.h:62:36
#18 0x00005572048ac3e3 Carbon::Interpreter::Step() /proc/self/cwd/explorer/interpreter/interpreter.cpp:1646:7
#19 0x00005572048d1875 Carbon::Interpreter::RunAllSteps(std::__1::unique_ptr<Carbon::Action, std::__1::default_delete<Carbon::Action> >) /proc/self/cwd/explorer/interpreter/interpreter.cpp:0:5
#20 0x00005572048d2680 ~unique_ptr /usr/lib/llvm-13/bin/../include/c++/v1/__memory/unique_ptr.h:272:19
#21 0x00005572048d2680 Carbon::InterpProgram(Carbon::AST const&, Carbon::Arena*, std::__1::optional<llvm::raw_ostream*>) /proc/self/cwd/explorer/interpreter/interpreter.cpp:1697:3
#22 0x000055720479a6e8 Carbon::ExecProgram(Carbon::Arena*, Carbon::AST, std::__1::optional<llvm::raw_ostream*>) /proc/self/cwd/explorer/interpreter/exec_program.cpp:56:1
#23 0x000055720478c857 ~AST /proc/self/cwd/./explorer/ast/ast.h:17:8
#24 0x000055720478c857 Carbon::Main(llvm::StringRef, int, char**) /proc/self/cwd/explorer/main.cpp:78:3
#25 0x000055720478b85c index /usr/lib/llvm-13/bin/../include/c++/v1/variant:788:12
#26 0x000055720478b85c index /usr/lib/llvm-13/bin/../include/c++/v1/variant:1438:59
#27 0x000055720478b85c __holds_alternative<1UL, Carbon::Error, Carbon::Success> /usr/lib/llvm-13/bin/../include/c++/v1/variant:1464:14
#28 0x000055720478b85c holds_alternative<Carbon::Success, Carbon::Error, Carbon::Success> /usr/lib/llvm-13/bin/../include/c++/v1/variant:1470:10
#29 0x000055720478b85c ok /proc/self/cwd/./common/error.h:62:36
#30 0x000055720478b85c Carbon::ExplorerMain(llvm::StringRef, int, char**) /proc/self/cwd/explorer/main.cpp:91:69
#31 0x000055720478acd1 main /proc/self/cwd/explorer/main_bin.cpp:0:10
#32 0x00007f15355d17fd __libc_start_main ./csu/../csu/libc-start.c:332:16
#33 0x00005572046d91fa _start (bazel_bin/explorer/explorer+0x49f1fa)
zsh: IOT instruction  bazel run //explorer explorer/testdata/string/associated_constant.carbon