A simple grammar ('g.g4'):

grammar g;
tree : 'X';

causes the following error

java -jar antlr-4.7-complete.jar g.g4 
error(50): g.g4::: syntax error: mismatched character ':' expecting 'g'
error(50): g.g4:2:7: syntax error: ''X'' came as a complete surprise to me

Removing the space between 'tree' and ':' or changing 'tree' to other rule name helps.

I've found that 'tree' was a keyword in antlr v3. But the reference book of v4 doesn't list it any more.

If 'tree' is a reserved keyword, the error message is confusing.