rule stat shows:

[12:17:17] warning(200): L.g:19:4: Decision can match input such as "<EPSILON>" using multiple alternatives: 1, 2
As a result, alternative(s) 2 were disabled for that input

works for k=3.

grammar L;

classDef
: 'class' ID '{' method+ '}'
;

method
: type ID '(' ')' body
;

type: 'int' | 'float' | 'void' ;

body: '{' stat+ '}' ;

stat
options {
k=4;
}
: assignment ';'
| call ';'
;

call: id '(' expr (',' expr)* ')'
;

assignment
: id '=' expr
;

expr: INT
| id
;

id : ID ('.' ID)* ;

INT : '0'..'9'+ ;

ID : ('a'..'z'|'A'..'Z')+ ;

WS : (' '|'\t'|'\r'|'\n')+ {$channel=HIDDEN;} ;