Report InputMismatchException with original context information

Fixes #1922

runtime-testsuite/test/org/antlr/v4/test/runtime/descriptors/ParserErrorsDescriptors.java

@@ -618,4 +618,28 @@ public class ParserErrorsDescriptors {
 		public String grammar;
 
 	}
+
+	public static class ExtraneousInput extends BaseParserTestDescriptor {
+		public String input = "baa";
+		public String output = null;
+		public String errors = "line 1:0 mismatched input 'b' expecting {<EOF>, 'a'}\n";
+		public String startRule = "file";
+		public String grammarName = "T";
+
+		/**
+		 grammar T;
+
+		 member : 'a';
+		 body : member*;
+		 file : body EOF;
+		 B : 'b';
+		 */
+		@CommentHasStringValue
+		public String grammar;
+
+		@Override
+		public boolean ignore(String targetName) {
+			return !"Java".equals(targetName);
+		}
+	}
 }

runtime-testsuite/test/org/antlr/v4/test/runtime/descriptors/SemPredEvalParserDescriptors.java

@@ -283,11 +283,16 @@ public class SemPredEvalParserDescriptors {
 		public String input = "s\n\n\nx\n";
 		public String output = "(file_ (para (paraContent s) \\n \\n) (para (paraContent \\n x \\n)) <EOF>)\n";
 		/**
-		line 5:0 mismatched input '<EOF>' expecting '
+		line 5:0 mismatched input '<EOF>' expecting {'s', '
-		'
+		', 'x'}
 		 */
 		@CommentHasStringValue
 		public String errors;
+
+		@Override
+		public boolean ignore(String targetName) {
+			return !"Java".equals(targetName);
+		}
 	}
 
 	public static class PredFromAltTestedInLoopBack_2 extends PredFromAltTestedInLoopBack {

runtime/Java/src/org/antlr/v4/runtime/DefaultErrorStrategy.java

@@ -36,6 +36,21 @@ public class DefaultErrorStrategy implements ANTLRErrorStrategy {
 
 	protected IntervalSet lastErrorStates;
 
+	/**
+	 * This field is used to propagate information about the lookahead following
+	 * the previous match. Since prediction prefers completing the current rule
+	 * to error recovery efforts, error reporting may occur later than the
+	 * original point where it was discoverable. The original context is used to
+	 * compute the true expected sets as though the reporting occurred as early
+	 * as possible.
+	 */
+	protected ParserRuleContext nextTokensContext;
+
+	/**
+	 * @see #nextTokensContext
+	 */
+	protected int nextTokensState;
+
 	/**
 	 * {@inheritDoc}
 	 *
@@ -225,7 +240,20 @@ public class DefaultErrorStrategy implements ANTLRErrorStrategy {
 
         // try cheaper subset first; might get lucky. seems to shave a wee bit off
 		IntervalSet nextTokens = recognizer.getATN().nextTokens(s);
-		if (nextTokens.contains(Token.EPSILON) || nextTokens.contains(la)) {
+		if (nextTokens.contains(la)) {
+			// We are sure the token matches
+			nextTokensContext = null;
+			nextTokensState = ATNState.INVALID_STATE_NUMBER;
+			return;
+		}
+
+		if (nextTokens.contains(Token.EPSILON)) {
+			if (nextTokensContext == null) {
+				// It's possible the next token won't match; information tracked
+				// by sync is restricted for performance.
+				nextTokensContext = recognizer.getContext();
+				nextTokensState = recognizer.getState();
+			}
 			return;
 		}
 
@@ -450,7 +478,14 @@ public class DefaultErrorStrategy implements ANTLRErrorStrategy {
 		}
 
 		// even that didn't work; must throw the exception
-		throw new InputMismatchException(recognizer);
+		InputMismatchException e;
+		if (nextTokensContext == null) {
+			e = new InputMismatchException(recognizer);
+		} else {
+			e = new InputMismatchException(recognizer, nextTokensState, nextTokensContext);
+		}
+
+		throw e;
 	}
 
 	/**

runtime/Java/src/org/antlr/v4/runtime/InputMismatchException.java

@@ -13,4 +13,10 @@ public class InputMismatchException extends RecognitionException {
 		super(recognizer, recognizer.getInputStream(), recognizer._ctx);
 		this.setOffendingToken(recognizer.getCurrentToken());
 	}
+
+	public InputMismatchException(Parser recognizer, int state, ParserRuleContext ctx) {
+		super(recognizer, recognizer.getInputStream(), ctx);
+		this.setOffendingState(state);
+		this.setOffendingToken(recognizer.getCurrentToken());
+	}
 }