`--threads=0` leads to an assertion violation

[eric-wieser]

Prerequisites

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Description

As of Lean 4.9.0, --threads=0 gives an assertion violation.

Steps to Reproduce

1. Run lean --threads=0 <(echo "example : True := by simp")

Expected behavior: Either silent, or complains that threads=0 is illegal

Actual behavior:

LEAN ASSERTION VIOLATION
File: /home/runner/work/lean4/lean4/src/runtime/object.cpp
Line: 1053
g_task_manager
'unreachable' code was reached

from

lean4/src/runtime/object.cpp

Lines 1052 to 1053 in daa2218

	extern "C" LEAN_EXPORT obj_res lean_io_promise_new(obj_arg) {
	lean_always_assert(g_task_manager);

Versions

v4.9.0 onwards

Additional Information

[Additional information, configuration or data that might be necessary to reproduce the issue]

Impact

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