1. d4 d5 2. c4 c6 3. Nf3 Nf6 4. Nc3 dc4 5. a4 e6

In discussions of the Slav (Classical variation) during one of his 'opening clinics', GM Jan Gustafsson says he thinks rather than playing 5... Bf5, black has done pretty well after instead playing 5....e6. I've found no Slav books that discuss this. Any ideas? Is this simply a transposition to the Semi-Slav?

After 6e3 c5 it will transpose to a line of the Queen's gambit accepted, which for example Botvinnik played a lot for white, with difference pawn on a7, instead of a6. (No practical differences there).  Otherwise 6e4 Bb4!? maybe, goes into sharper lines see Alekhine v Euwe games.

Graham Burgess has analysis on the Slav with 5...e6 in his book An Idiot-Proof Chess Opening Repertoire - he gets to the position via a Queen's Gambit Accepted - 1.d4 d5  2.c4 dc

3.Nf3 c6  4.a4 Nf6  5.Nc3  e6 - pages 47-52

Thanks both!