gc: bug274

R=ken2
CC=golang-dev
https://golang.org/cl/1742044

src/cmd/gc/go.h

@@ -914,6 +914,8 @@ char*	lexname(int lex);
 void	mkpackage(char* pkgname);
 void	unimportfile(void);
 int32	yylex(void);
+extern	int	yylast;
+extern	int	yyprev;
 
 /*
  *	mparith1.c

src/cmd/gc/go.y

@@ -515,10 +515,33 @@ switch_body:
 	}
 
 caseblock:
-	case stmt_list
+	case
 	{
+		// If the last token read by the lexer was consumed
+		// as part of the case, clear it (parser has cleared yychar).
+		// If the last token read by the lexer was the lookahead
+		// leave it alone (parser has it cached in yychar).
+		// This is so that the stmt_list action doesn't look at
+		// the case tokens if the stmt_list is empty.
+		yylast = yychar;
+	}
+	stmt_list
+	{
+		int last;
+
+		// This is the only place in the language where a statement
+		// list is not allowed to drop the final semicolon, because
+		// it's the only place where a statement list is not followed 
+		// by a closing brace.  Handle the error for pedantry.
+
+		// Find the final token of the statement list.
+		// yylast is lookahead; yyprev is last of stmt_list
+		last = yyprev;
+
+		if(last > 0 && last != ';' && yychar != '}')
+			yyerror("missing statement after label");
 		$$ = $1;
-		$$->nbody = $2;
+		$$->nbody = $3;
 	}
 
 caseblock_list:

src/cmd/gc/lex.c

@@ -14,6 +14,8 @@
 
 extern int yychar;
 int windows;
+int yyprev;
+int yylast;
 
 static void	lexinit(void);
 static void	lexfini(void);
@@ -1140,6 +1142,10 @@ yylex(void)
 		curio.nlsemi = 0;
 		break;
 	}
+
+	// Track last two tokens returned by yylex.
+	yyprev = yylast;
+	yylast = lx;
 	return lx;
 }