IMO making the grammar unambiguous is much better.

Change https://golang.org/cl/113720 mentions this issue: x/tools/cmd/goyacc: add %expect command

Addressing this in the change mentioned above.

The fact that Bison has this feature does not mean goyacc needs to add it. Like @cznic said, the better solution is to write a grammar with no shift-reduce conflicts.

If you write a C program and the compiler surfaces 10 warnings about your code, the right response is to fix the warnings, not write #pragma expect 10. What happens when the 11th warning pops up? The compiler prints 11 warnings and you're left to figure out which are the 10 you don't care about. Fixing the warnings, however you do that, makes them go away for good. A yacc grammar is a program too. If yacc prints warnings about your program and you don't want to hear about them anymore, the right solution is to fix the problems yacc has identified.

The old Go 1.4 yacc grammar for Go itself shows how to fix these warnings with precedence markers. See https://go.googlesource.com/go/+/go1.4/src/cmd/gc/go.y#104.