A torque converter in a manual transmission car?

[Matra et Alpine]

When gearboxes had 2 or 3 gears it may have had value.
But with 5 and 6 then the gearing is doing all the torque multiplication you need to get power at the wheels and as noted, the only advantage then is during gear change and good clutch will do a better job with less weight and losses.

[csl177]

isn't this the principle of the automatic clutch?

If it's what we think he's asking, yes, and of course it's been done in several ways.

Too complex to be worth it on a street car, and the same effect can be achieved with proper clutch control.

Not really that complex, but not worth the extra weight... and the market has always been rather limited for it. More people prefer autoboxes, they're easier to control electronically and provide greater profit for manufacturers.

When gearboxes had 2 or 3 gears it may have had value.
But with 5 and 6 then the gearing is doing all the torque multiplication you need to get power at the wheels and as noted, the only advantage then is during gear change and good clutch will do a better job with less weight and losses.

This, coupled to the aforementioned extra weight. IIRC at least 4 manufacturers produced models with some version of a torque converter managed clutch with 4 speeds: NSU, VW/Porsche, Renault, Hanomag... there were probably others.