Igor Grichanov wrote:
Dolichopus discifer Stannius, 1831: Isis (Oken) 1831: 57 // syn. of Dolichopus nigricornis Meigen, 1824 (Loew, 1869: Beschr.eur. Dipt. 1; Becker, 1917: N.Acta Acad.leop., Halle 102: 148-149); rest. Collin, 1940: Ent.monthly Mag. 76 [= (4)1]: 263 ** Type locality: Germany. Palaearctic: Norway, Finland, Sweden, Denmark, Ireland, England, Germany, Austria, Netherlands, Belgium, France, Italy, Switzerland, Czech, Slovakia, Hungary, Romania, Poland, Bulgaria, Russia, Kazakhstan; Nearctic: Canada, USA *
=Dolichopus nigricornis Becker, 1917: N.Acta Acad.leop., Halle, 102: 148; Parent, 1925: Enc.ent., Ser.B, II, Dipt. 2: 55, 56, et auctt. (nec Meigen, 1824) ** Type locality: not given. * Dolichopus discifer Stannius, 1831.

This is a citation from Collin (1940):
nigricornis Mg. (1824) and discifer Stann. (1831).
This synonymy was accepted by Loew in 1869 after Haliday had reported it as a result of an examination of Meigen's supposed type in Paris. Meigen however attributed the species to Megerle, from whom he had seen an Austrian female only, and this female be considered the type of the species. If the specimen mentioned by Loew in 1859 as present in the Vienna Museum is Megerle's female, the name nigricornis Mg. must be used for Hercostomus gracilis Stann. In the meantime the suggested synonymy of discifer Stann. with nigricornis Mg. should not be accepted.

According to Meuffels (1999):
Dolichopus nigricornis Meigen, 1824: Syst.Beschr. 4: 82 // syn. of Hercostomus gracilis (Stannius, 1831) (Loew, 1859: Progr.Realsch.Meseritz 1859: 6) * ?Hercostomus (unrecognized) (Collin, 1940: Ent.monthly Mag. 76 [= ser.4, vol. 1]: 263).

As I know, nobody more discussed these names. So, I vote for discifer.