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digitalmars.D.bugs - [Issue 8978] New: Constructor is called instead of implicit cast when initializing from expression tuple

reply d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8978

           Summary: Constructor is called instead of implicit cast when
                    initializing from expression tuple
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: wrong-code
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody puremagic.com
        ReportedBy: verylonglogin.reg gmail.com


--- Comment #0 from Denis Shelomovskij <verylonglogin.reg gmail.com> 2012-11-08
16:26:34 MSK ---
The second `static assert` fails as `this(const S)` is called:
---
template expressionTuple(expressions...)
{ alias expressions expressionTuple; }

struct S
{
    int* p;
    this(const S) { }
}

void main()
{
    const S cs;

    static assert(!__traits(compiles, { S s = cs; })); // ok, cast is invalid

    alias expressionTuple!(cs) args;
    static assert(!__traits(compiles, { S s = args; })); // fails
}
---

Another example:
---
template expressionTuple(expressions...)
{ alias expressions expressionTuple; }

struct S
{
    int i;
    this(S) { assert(0); }
}

void main()
{
    alias expressionTuple!(S()) args;
    S s = args; // `assert(0);` is triggered
}
---


Workaround:
---
alias expressionTuple!(...) args;

static if(args.length == 1)
    S s = args[0];
else
    S s = args;
---

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parent d-bugmail puremagic.com writes:
http://d.puremagic.com/issues/show_bug.cgi?id=8978



--- Comment #1 from Denis Shelomovskij <verylonglogin.reg gmail.com> 2012-11-08
16:49:42 MSK ---
It also affects templates with tuple parameter:
---
template expressionTuple(expressions...)
{ alias expressions expressionTuple; }

struct S
{
    int* p;
    this(const S) { }
}

void main()
{
    const S cs;

    static assert(!__traits(compiles, { S s = cs; })); // ok, cast is invalid

    void f(Args...)(Args args)
    {
        static assert(!__traits(compiles, { S s = args; })); // fails
    }
    f(cs);
}
---

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