http://d.puremagic.com/issues/show_bug.cgi?id=2532

           Summary: '=' does not give a boolean result
           Product: D
           Version: 2.022
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla@digitalmars.com
        ReportedBy: adolf.mathias@googlemail.com


Using a boolean assignment in an if condition e.g. as follows gives IMHO an unjustified error message. When a new variable is declared in the if() condition, everything is OK:

// tst.d
void main()
{ bool b;

  if(b=false) { b = true; }
  if(bool c=false) { c = true; }
}

$ dmd tst
tst.d(4): Error: '=' does not give a boolean result


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------- Comment #1 from jarrett.billingsley@gmail.com  2008-12-21 11:47 -------
= does not give a boolean result for good reason: it's to avoid the common mistake in C/C++ where:

if(x = 5) { ... }

is used when:

if(x == 5) { ... }

was meant.


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2korden@gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |INVALID




------- Comment #2 from 2korden@gmail.com  2008-12-21 11:49 -------
It works as designed. It is designed to avoid mistakes.

"if (var = expr) {" is disallowed on purpose to avoid mistakes where you
inteded to write "if (var == expr) {":

int a = ...;
if (a = 42) { // oops, what a typo! I ment if (a == 42) here...

Bool is no different from int in this expression.

However, "if (type var == expression) {" is not a valid construct, so there is
no chance to make a mistake and thus "if (type var = expression) {" is allowed.

There is no 'special cases' for bools.


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------- Comment #3 from ddparnell@bigpond.com  2008-12-21 15:35 -------
The text of the message is not true if the variable being assigned to is ALREADY a bool. I know that the syntax form 'if (a = b)' is to avoided, but to say that, IN THIS CASE, the result is not a boolean is plainly wrong.

bool a;
bool b;
b = (a = true); // A boolean expression (a = true) is used to assign to 'b'.

As we know that (a = b) is a boolean expression, it seems wrong to say that it is a boolean EXCEPT in an 'if' construct. We do this to no other data type.

So maybe the wording of the message can be changed to reflect what is actually
being disallowed. Something like ...
"The syntax form 'if (a = b)' is not allowed".
That takes the argument away from the datatype issue and makes it a syntax
issue instead.


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------- Comment #4 from shro8822@vandals.uidaho.edu  2008-12-21 17:46 -------

Might "expression is not a valid condition" be better?


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http://d.puremagic.com/issues/show_bug.cgi?id=2532


2korden@gmail.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
             Status|RESOLVED                    |REOPENED
         Resolution|INVALID                     |




------- Comment #5 from 2korden@gmail.com  2008-12-22 04:35 -------
Reopening as an enhancement.


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http://d.puremagic.com/issues/show_bug.cgi?id=2532


gide@nwawudu.com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic




------- Comment #6 from gide@nwawudu.com  2008-12-22 04:40 -------
Added keyword diagnostic.


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http://d.puremagic.com/issues/show_bug.cgi?id=2532





------- Comment #7 from maxmo@pochta.ru  2008-12-22 05:55 -------
Hmm... After all D has switched to strict booleans?


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yebblies <yebblies@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |schveiguy@yahoo.com


--- Comment #8 from yebblies <yebblies@gmail.com> 2011-06-13 09:01:42 PDT ---
*** Issue 4163 has been marked as a duplicate of this issue. ***

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yebblies <yebblies@gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |patch
                 CC|                            |yebblies@gmail.com


--- Comment #9 from yebblies <yebblies@gmail.com> 2011-06-13 13:52:46 PDT ---
https://github.com/D-Programming-Language/dmd/pull/122

A better message for all?

Error: assignment cannot be used as a condition

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