Fixes #31608.
@ -339,6 +339,10 @@ def configure_msvc(env, vcvars_msvc_config):
## Compile/link flags
if env["optimize"] in ["debug", "none"]:
# Always use dynamic runtime, static debug CRT breaks thread_local.
env.AppendUnique(CCFLAGS=["/MDd"])
else:
if env["use_static_cpp"]:
env.AppendUnique(CCFLAGS=["/MT"])