Q51863: Failure to Flag ELSEIF THEN <Statement> as Syntax Error

Article: Q51863
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom buglist4.00 buglist4.00b buglist4.50 SR# S890922- | mspl13_basic
Last Modified: 20-SEP-1990

In the products listed below, both the compiler and editor fail to
trap an illegal block ELSEIF THEN <statement> syntax where a statement
improperly follows on the same line as the THEN keyword. Despite the
failure to trap this programming error, the code in the illegal syntax
executes successfully.

Microsoft has confirmed this to be a problem in the QB.EXE and BC.EXE
environments of Microsoft QuickBASIC versions 4.00, 4.00b, and 4.50
for MS-DOS; in the QB.EXE and BC.EXE environments of Microsoft BASIC
Compiler versions 6.00 and 6.00b (buglist6.00, buglist6.00b) for
MS-DOS and MS OS/2; and in the QBX.EXE and BC.EXE environments of
Microsoft BASIC Professional Development System (PDS) versions 7.00
and 7.10 for MS-DOS and MS OS/2 (buglist7.00, buglist7.10). Microsoft
is researching this problem and will post new information here as it
becomes available.

Code Example
------------

The following code violates the block IF...ELSEIF...END IF statement
syntax, but fails to generate a syntax error. The program successfully
prints both "Hello" and "Hello2", despite the untrapped syntax error:

   IF 0 THEN
   ELSEIF 1 THEN PRINT "Hello"
      PRINT "Hello2"
   END IF

(In comparison, this code correctly generates a "Syntax Error" on the
ELSEIF phrase in Microsoft QuickBASIC for Apple Macintosh systems.)

As an offshoot issue, when you single-step through the above program
in the QB.EXE or QBX.EXE editor environment with F8, the debugger
fails to highlight the PRINT "Hello" statement but correctly
highlights the PRINT "Hello2" statement. ("Hello" correctly prints
despite the PRINT "Hello" not being highlighted.) When you correct the
program by placing PRINT "Hello" on the next line, F8 in the debugger
correctly highlights that line.

The ELSEIF...THEN line is not supposed to allow any statement
following the THEN on that line. The following is the correct,
required syntax:

   IF 0 THEN
   ELSEIF 1 THEN
      PRINT "Hello"
      PRINT "Hello2"
   END IF

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