explorer crashes when a function with auto return type has a __continuation/__run/__await as the only (or last) statement in function body

[pk19604014]

$ cat /tmp/crash3.carbon 
// Generated by proto_to_carbon.

package EmptyIdentifier impl;

fn EmptyIdentifier() -> auto 
{
   __await;
}

fn Main() -> i32 {
  return 0;
}

$ bazel run explorer:explorer  /tmp/crash3.carbon
INFO: Invocation ID: 90b23507-8256-4849-9f15-0058b33311b7
INFO: Analyzed target //explorer:explorer (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
Target //explorer:explorer up-to-date:
  bazel-bin/explorer/explorer
INFO: Elapsed time: 0.358s, Critical Path: 0.00s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action
INFO: Build completed successfully, 1 total action
/usr/local/google/home/pk19604014/.linuxbrew/Cellar/llvm/13.0.1_1/bin/../include/c++/v1/optional:911: _LIBCPP_ASSERT 'this->has_value()' failed. optional operator* called on a disengaged value
Please report issues to https://github.com/carbon-language/carbon-lang/issues and include the crash backtrace.
Stack dump:
0.	Program arguments: /usr/local/google/home/pk19604014/.cache/bazel/_bazel_pk19604014/0c0c6d5ac7dde4460fd40f446af91e37/execroot/carbon/bazel-out/k8-fastbuild/bin/explorer/explorer /tmp/crash3.carbon
 #0 0x00005626ebbdea7b backtrace /tmp/llvm-20220302-9880-1faggz0/llvm-project-13.0.1.src/compiler-rt/lib/asan/../sanitizer_common/sanitizer_common_interceptors.inc:4202:13
 #1 0x00005626ebef71ab llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) /proc/self/cwd/external/llvm-project/llvm/lib/Support/Unix/Signals.inc:565:13
 #2 0x00005626ebef744b PrintStackTraceSignalHandler(void*) /proc/self/cwd/external/llvm-project/llvm/lib/Support/Unix/Signals.inc:632:1
 #3 0x00005626ebef1aa8 llvm::sys::RunSignalHandlers() /proc/self/cwd/external/llvm-project/llvm/lib/Support/Signals.cpp:98:18
 #4 0x00005626ebef89fe SignalHandler(int) /proc/self/cwd/external/llvm-project/llvm/lib/Support/Unix/Signals.inc:407:1
 #5 0x00007f3d5f6f3200 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x12200)
 #6 0x00007f3d5f39c8a1 raise ./signal/../sysdeps/unix/sysv/linux/raise.c:50:1
 #7 0x00007f3d5f386546 abort ./stdlib/abort.c:81:7
 #8 0x00005626ec07200f (/usr/local/google/home/pk19604014/.cache/bazel/_bazel_pk19604014/0c0c6d5ac7dde4460fd40f446af91e37/execroot/carbon/bazel-out/k8-fastbuild/bin/explorer/explorer+0x90900f)
 #9 0x00005626ebc843f2 std::__1::optional<Carbon::Value const*>::operator*() const & /usr/local/google/home/pk19604014/.linuxbrew/Cellar/llvm/13.0.1_1/bin/../include/c++/v1/optional:911:9
#10 0x00005626ebcd3d1d Carbon::ReturnTerm::static_type() const /proc/self/cwd/./explorer/ast/return_term.h:64:54
#11 0x00005626ebcd7920 Carbon::TypeChecker::DeclareFunctionDeclaration(Carbon::FunctionDeclaration*, Carbon::ImplScope const&) /proc/self/cwd/explorer/interpreter/type_checker.cpp:1729:3
#12 0x00005626ebcde8c2 index /usr/local/google/home/pk19604014/.linuxbrew/Cellar/llvm/13.0.1_1/bin/../include/c++/v1/variant:788:12
#13 0x00005626ebcde8c2 index /usr/local/google/home/pk19604014/.linuxbrew/Cellar/llvm/13.0.1_1/bin/../include/c++/v1/variant:1438:59
#14 0x00005626ebcde8c2 __holds_alternative<1UL, Carbon::Error, Carbon::Success> /usr/local/google/home/pk19604014/.linuxbrew/Cellar/llvm/13.0.1_1/bin/../include/c++/v1/variant:1464:14
#15 0x00005626ebcde8c2 holds_alternative<Carbon::Success, Carbon::Error, Carbon::Success> /usr/local/google/home/pk19604014/.linuxbrew/Cellar/llvm/13.0.1_1/bin/../include/c++/v1/variant:1470:10
#16 0x00005626ebcde8c2 ok /proc/self/cwd/./common/error.h:61:36
#17 0x00005626ebcde8c2 Carbon::TypeChecker::DeclareDeclaration(Carbon::Declaration*, Carbon::ImplScope&) /proc/self/cwd/explorer/interpreter/type_checker.cpp:2160:7
#18 0x00005626ebceb10b index /usr/local/google/home/pk19604014/.linuxbrew/Cellar/llvm/13.0.1_1/bin/../include/c++/v1/variant:788:12
#19 0x00005626ebceb10b index /usr/local/google/home/pk19604014/.linuxbrew/Cellar/llvm/13.0.1_1/bin/../include/c++/v1/variant:1438:59
#20 0x00005626ebceb10b __holds_alternative<1UL, Carbon::Error, Carbon::Success> /usr/local/google/home/pk19604014/.linuxbrew/Cellar/llvm/13.0.1_1/bin/../include/c++/v1/variant:1464:14
#21 0x00005626ebceb10b holds_alternative<Carbon::Success, Carbon::Error, Carbon::Success> /usr/local/google/home/pk19604014/.linuxbrew/Cellar/llvm/13.0.1_1/bin/../include/c++/v1/variant:1470:10
#22 0x00005626ebceb10b ok /proc/self/cwd/./common/error.h:61:36
#23 0x00005626ebceb10b Carbon::TypeChecker::TypeCheck(Carbon::AST&) /proc/self/cwd/explorer/interpreter/type_checker.cpp:2078:5
#24 0x00005626ebc6645b ~__tree /usr/local/google/home/pk19604014/.linuxbrew/Cellar/llvm/13.0.1_1/bin/../include/c++/v1/__tree:1791:11
#25 0x00005626ebc6645b ~set /usr/local/google/home/pk19604014/.linuxbrew/Cellar/llvm/13.0.1_1/bin/../include/c++/v1/set:619:5
#26 0x00005626ebc6645b ~TypeChecker /proc/self/cwd/./explorer/interpreter/type_checker.h:21:7
#27 0x00005626ebc6645b Carbon::ExecProgram(Carbon::Arena*, Carbon::AST, std::__1::optional<llvm::raw_ostream*>) /proc/self/cwd/explorer/interpreter/exec_program.cpp:46:3
#28 0x00005626ebc58be0 ~AST /proc/self/cwd/./explorer/ast/ast.h:17:8
#29 0x00005626ebc58be0 Carbon::Main(llvm::StringRef, int, char**) /proc/self/cwd/explorer/main.cpp:78:3
#30 0x00005626ebc57bec index /usr/local/google/home/pk19604014/.linuxbrew/Cellar/llvm/13.0.1_1/bin/../include/c++/v1/variant:788:12
#31 0x00005626ebc57bec index /usr/local/google/home/pk19604014/.linuxbrew/Cellar/llvm/13.0.1_1/bin/../include/c++/v1/variant:1438:59
#32 0x00005626ebc57bec __holds_alternative<1UL, Carbon::Error, Carbon::Success> /usr/local/google/home/pk19604014/.linuxbrew/Cellar/llvm/13.0.1_1/bin/../include/c++/v1/variant:1464:14
#33 0x00005626ebc57bec holds_alternative<Carbon::Success, Carbon::Error, Carbon::Success> /usr/local/google/home/pk19604014/.linuxbrew/Cellar/llvm/13.0.1_1/bin/../include/c++/v1/variant:1470:10
#34 0x00005626ebc57bec ok /proc/self/cwd/./common/error.h:61:36
#35 0x00005626ebc57bec Carbon::ExplorerMain(llvm::StringRef, int, char**) /proc/self/cwd/explorer/main.cpp:88:69
#36 0x00005626ebc57061 main /proc/self/cwd/explorer/main_bin.cpp:0:10
#37 0x00007f3d5f3877fd __libc_start_main ./csu/../csu/libc-start.c:332:16
#38 0x00005626ebba5e5a _start (/usr/local/google/home/pk19604014/.cache/bazel/_bazel_pk19604014/0c0c6d5ac7dde4460fd40f446af91e37/execroot/carbon/bazel-out/k8-fastbuild/bin/explorer/explorer+0x43ce5a)