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Reported by Frank Heckenbach. https://lists.gnu.org/archive/html/bug-bison/2019-11/msg00016.html The cast is needed when yytranslate_'s argument type is token_type, i.e., when api.token.constructor is defined. 373. types.at:138: testing lalr1.cc api.value.type=variant api.token.constructor ... ======== Testing with C++ standard flags: '' ../../tests/types.at:138: bison --color=no -fno-caret -o test.cc test.y ../../tests/types.at:138: $CXX $CXXFLAGS $CPPFLAGS $LDFLAGS -o test test.cc $LIBS stderr: test.cc:966:16: error: result of comparison of constant 257 with expression of type 'yy::parser::token_type' (aka 'yy::parser::token::yytokentype') is always true [-Werror,-Wtautological-constant-out-of-range-compare] else if (t <= user_token_number_max_) ~ ^ ~~~~~~~~~~~~~~~~~~~~~~ 1 error generated. It is because it is expected that when api.token.constructor is defined, only symbol constructors will be used, that yytranslate_ then takes a token_type. But it is wrong: we still allow literal characters in this case, as demonstrated by test 373 for instance. %define api.value.type variant %define api.token.constructor %token <std::pair<int, int>> '1' '2'; [...] static yy::parser::symbol_type yylex () { static char const input[] = "12"; int res = input[toknum++]; typedef yy::parser::symbol_type symbol; if (res) return symbol (res, std::make_pair (res - '0', res - '0' + 1)); else return symbol (res); } So let yytranslate_ always take an int, which makes the cast truly useless. * data/skeletons/c++.m4, data/skeletons/lalr1.cc (yytranslate_): here.