Re: lazy evaluation of logical operators in enum definition

[Basile B. via Digitalmars-d]

On Monday, 16 April 2018 at 05:57:01 UTC, Shachar Shemesh wrote:
Consider the following program:
struct S1 {
    enum member = 3;
}

struct S2 {
    enum member = 2;
}

struct S3 {
}

enum prop(T) = __traits(hasMember, T, "member") && T.member==3;

pragma(msg, prop!S1);
pragma(msg, prop!S2);
pragma(msg, prop!S3);

When compiled, it produces:
true
false
test.d(12): Error: no property member for type S3
test.d(16): Error: template instance `test.prop!(S3)` error 
instantiating
test.d(16):        while evaluating pragma(msg, prop!(S3))

If I change the definition of "prop" to:
template prop(T) {
    static if( __traits(hasMember, T, "member") && T.member==3 )
        enum prop = true;
    else
        enum prop = false;
}

then everything compiles as expected.

It seems that the && evaluation does not stop when the first 
false is found.

Hello, i've encountered a similar issue recently (see 
https://issues.dlang.org/show_bug.cgi?id=18115#c13). There are 
explanations in the last comments.