[Issue 11410] New: Support equality operator chaining

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=11410

           Summary: Support equality operator chaining
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2013-11-01 07:35:12 PDT ---
This is valid C code:


int main() {
    int x = 3;
    int y = 4;
    int z = 5;
    int b = x < y < z;
    return 0;
}


Bug GCC tells us that code doesn't work as expected by a Python programmer:

test.c:5:15: warning: comparisons like 'X<=Y<=Z' do not have their mathematical
meaning [-Wparentheses]
     int b = x < y < z;


So D disallows that code:

test.d(5): Error: semicolon expected, not '<'
test.d(5): Error: found '<' instead of statement

- - - - - - - - - - - - - -

GCC accepts this code:


int main() {
    int x = 4;
    int y = 4;
    int z = 4;
    int b = x == y == z;
    return 0;
}


With a warning:

test.c:5:15: warning: suggest parentheses around comparison in operand of '=='
[-Wparentheses]
     int b = x == y == z;


While DMD refuses that code:

test.d(5): Error: semicolon expected, not '=='
test.d(5): Error: found '==' instead of statement


>From my experience with D code I know there several situations where the
chained equality is handy:

if (n == foo(n) == bar(n) && ...

It avoids code like:

aux = foo(n);
if (n == aux && aux == bar(n) && ...

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