org.eclipse.xtext.conversion.impl.AbstractDeclarativeValueConverterService  - Only terminal rules are supported by lexer based converters but got INT which is an instance of ParserRule
java.lang.IllegalArgumentException: Only terminal rules are supported by lexer based converters but got INT which is an instance of ParserRule

// don't convert to EInt:
terminal MYINT: '-'? ('0'..'9')+;
// restore original semantics:
INT returns ecore::EInt : MYINT;

	public static class MyTerminalConverters extends DefaultTerminalConverters {
		@ValueConverter(rule = "MYINT")
		public IValueConverter<Integer> getMYINTConverter() {
			return new DualINTValueConverter();
		}
		@ValueConverter(rule = "INT")
		public IValueConverter<Integer> getINTConverter() {
			return new DualINTValueConverter();
		}
	}
	// an INT converter that copes with rules "MYINT" as well as "INT": at the lexical level we cannot distinguish!
	static class DualINTValueConverter extends INTValueConverter {
		@Override
		protected void assertTokens(Integer value, TokenSource tokenSource, String escapedString) {
			// copied from AbstractLexerBasedConverter
			if (tokenSource == null)
				return;
			Token token = tokenSource.nextToken();
			if (!escapedString.equals(token.getText())) {
				throw createTokenContentMismatchException(value, escapedString, token);
			}
			// modified: accept for MYINT and INT:
			String tokenRuleName = getRuleName(token);
			if (!"INT".equals(tokenRuleName) && !"MYINT".equals(tokenRuleName)) {
				throw createTokenTypeMismatchException(value, escapedString, token);
			}
			// --
			Integer reparsedValue = toValue(token.getText(), null);
			if (value != reparsedValue && !value.equals(reparsedValue)) {
				throw createTokenContentMismatchException(value, escapedString, token);
			}
		}
	}