The GNU C compiler apparently still permits this, and the VC++ 2005 compiler permits it too, though it issues a Level-4 warning if it’s C code, and a Level-2 warning if it’s C++ code.
S. Maxwell. Late Breaking Papers at the Genetic Programming 1996
Conference Stanford University July 28-31, 1996, page 125--128. Stanford University, CA, USA, Stanford Bookstore, (28--31 July 1996)
D. Howard. Late Breaking Papers at the Genetic Programming 1998
Conference, University of Wisconsin, Madison, Wisconsin, USA, Stanford University Bookstore, (22-25 July 1998)
J. Wrenn, S. Krishnamurthi, and K. Fisler. Proceedings of the 2018 ACM Conference on International Computing Education Research, page 51--59. New York, NY, USA, ACM, (2018)
D. Goldberg, and U. O'Reilly. Proceedings of the First European Workshop on Genetic
Programming, volume 1391 of LNCS, page 16--36. Paris, Springer-Verlag, (14-15 April 1998)
D. Lawrie, C. Morrell, H. Feild, and D. Binkley. Proceedings of the 14th Conference on Program Comprehension (ICPC '06), page 3--12. IEEE Computer Society, (2006)
J. Daida, H. Li, R. Tang, and A. Hilss. Genetic and Evolutionary Computation -- GECCO-2003, volume 2724 of LNCS, page 1665--1677. Chicago, Springer-Verlag, (12-16 July 2003)
A. Dittmar, A. Garc\'ıa Frey, and S. Dupuy-Chessa. Proceedings of the 4th ACM SIGCHI Symposium on Engineering Interactive Computing Systems, page 189--194. New York, NY, USA, ACM, (2012)
J. Fernandez, K. Farry, and J. Cheatham. Genetic Programming 1996: Proceedings of the First
Annual Conference, page 63--71. Stanford University, CA, USA, MIT Press, (28--31 July 1996)
T. Soule. Proceedings of the Genetic and Evolutionary
Computation Conference, 1, page 916--922. Orlando, Florida, USA, Morgan Kaufmann, (13-17 July 1999)
J. Daida, A. Hilss, D. Ward, and S. Long. Genetic and Evolutionary Computation -- GECCO-2003, volume 2724 of LNCS, page 1652--1664. Chicago, Springer-Verlag, (12-16 July 2003)
R. Crawford-Marks, L. Spector, and J. Klein. Late Breaking Papers at the 2004 Genetic and
Evolutionary Computation Conference, Seattle, Washington, USA, (26 July 2004)
J. Valdes, and A. Barton. International Joint Conference on Neural Networks,
IJCNN'06, page 4862--4869. Sheraton Vancouver Wall Centre Hotel, Vancouver, BC,
Canada, IEEE, (16-21 July 2006)
J. Valdes, A. Barton, and R. Orchard. 2007 IEEE Congress on Evolutionary Computation, page 4191--4198. Singapore, IEEE Computational Intelligence Society, IEEE Press, (25-28 September 2007)
T. Fernandez. Genetic and Evolutionary Computation -- GECCO-2004,
Part II, volume 3103 of Lecture Notes in Computer Science, page 471--482. Seattle, WA, USA, Springer-Verlag, (26-30 June 2004)
S. Qiao, C. Tang, J. Peng, H. Fan, and Y. Xiang. Proceedings of Intelligence and Security Informatics:
International Workshop, WISI 2006, volume 3917 of Lecture Notes in Computer Science, page 133--138. Singapore, Springer, (April 2006)
S. Ross, J. Daida, C. Doan, T. Bersano-Begey, and J. McClain. Genetic Programming 1996: Proceedings of the First
Annual Conference, page 191--199. Stanford University, CA, USA, MIT Press, (28--31 July 1996)
W. Ashlock. 2006 IEEE World Congress on Computational
Intelligence, 2006 IEEE Congress on Evolutionary
Computation, page 1023--1030. Vancouver, (16-21 July 2006)
E. Lukschandl, P. Nordin, and M. Nordahl. Late Breaking Papers at the 2000 Genetic and
Evolutionary Computation Conference, page 236--239. Las Vegas, Nevada, USA, (8 July 2000)
J. Duffy, and J. Engle-Warnick. Evolutionary Computation in Economics and Finance, volume 100 of Studies in Fuzziness and Soft Computing, chapter 4, Physica Verlag, (2002 2002)
J. Duffy, and J. Engle-Warnick. Fifth International Conference: Computing in Economics
and Finance, page 150. Boston College, MA, USA, (24-26 June 1999)Book of Abstracts.
L. Vanneschi, S. Gustafson, and G. Mauri. Proceedings of the 9th European Conference on Genetic
Programming, volume 3905 of Lecture Notes in Computer Science, page 238--249. Budapest, Hungary, Springer, (10 - 12 April 2006)
A. Howell, R. McGrann, R. Eckert, H. Sayama, and E. Way. Late breaking paper at Genetic and Evolutionary
Computation Conference (GECCO'2006), Seattle, WA, USA, (8-12 July 2006)
A. Carter, and C. Hundhausen. Proceedings of the 2017 ACM SIGCSE Technical Symposium on Computer Science Education, page 105--110. New York, NY, USA, ACM, (2017)
J. Koza, F. Bennett, and D. Andre. Evolutionary Programming VII: Proceedings of the
Seventh Annual Conference on Evolutionary Programming, volume 1447 of LNCS, Mission Valley Marriott, San Diego, California, USA, Springer-Verlag, (25-27 March 1998)
B. Edmonds. AISB'05: Proceedings of the Joint Symposium on
Socially Inspired Computing (Engineering with Social
Metaphors), page 127--134. University of Hertfordshire, Hatfield, UK, (12-15 April 2005)SSAISB 2005 Convention.
P. Ngan, M. Wong, K. Leung, and J. Cheng. Genetic Programming 1998: Proceedings of the Third
Annual Conference, page 254--259. University of Wisconsin, Madison, Wisconsin, USA, Morgan Kaufmann, (22-25 July 1998)
Y. Azaria, and M. Sipper. Proceedings of the 8th European Conference on Genetic
Programming, volume 3447 of Lecture Notes in Computer Science, page 132--142. Lausanne, Switzerland, Springer, (30 March - 1 April 2005)
H. Fernlund, and A. Gonzalez. Late Breaking Papers at the 2004 Genetic and
Evolutionary Computation Conference, Seattle, Washington, USA, (26 July 2004)
H. Fernlund, and A. Gonzalez. Genetic and Evolutionary Computation -- GECCO-2004,
Part II, volume 3103 of Lecture Notes in Computer Science, page 704--705. Seattle, WA, USA, Springer-Verlag, (26-30 June 2004)
N. Svangard, P. Nordin, and S. Lloyd. Proceedings of the 2003 Congress on Evolutionary
Computation CEC2003, page 1014--1017. Canberra, IEEE Press, (8-12 December 2003)
A. Neff, J. Haefner, and R. Dueser. Late Breaking Papers at the 1997 Genetic Programming
Conference, Stanford University, CA, USA, Stanford Bookstore, (13--16 July 1997)
E. Kirshenbaum, and H. Suermondt. Genetic and Evolutionary Computation -- GECCO-2004,
Part II, volume 3103 of Lecture Notes in Computer Science, page 543--556. Seattle, WA, USA, Springer-Verlag, (26-30 June 2004)
S. Chen, and C. Yeh. Proceedings of the 4th JAFEE International Conference
on Investments and Derivatives (JIC'97), page 288--306. Aoyoma Gakuin University, Tokyo, Japan, (July 1997)
S. Chen, and C. Yeh. Genetic Programming 1997: Proceedings of the Second
Annual Conference, page 58--63. Stanford University, CA, USA, Morgan Kaufmann, (13-16 July 1997)