[Issue 6553] New: Better const(char[]) !-> string error message

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=6553

           Summary: Better const(char[]) !-> string error message
           Product: D
           Version: D2
          Platform: x86
        OS/Version: Windows
            Status: NEW
          Keywords: diagnostic
          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 2011-08-24 20:50:22 PDT ---
This is related to bug 5081 , see there for more context.


I think the behaviours shown by this little program are correct, but I think
that error message needs to be improved (completed with more details), because
in some cases (like foo1) that's an acceptable operation (dmd 2.055 head):


string foo1() pure {
    const(char[]) s2 = ['a'];
    return s2;
}
string foo2() pure {
    return ['a'];
}
string foo3(immutable char[] s) pure {
    return s;
}
string foo4(in char[] s1, immutable char[] s2) pure {
    return s2;
}
string foo5(in char[] s) pure {
    return s; // Error: cannot implicitly convert expression (s) of type
const(char[]) to string
}
void main() {
    immutable r1 = foo1();             // OK
    immutable r2 = foo2();             // OK
    immutable r3 = foo3(['a']);        // OK
    immutable r4 = foo4(['a'], ['b']); // OK
    immutable r5 = foo5(['a']);        // Error
}


Good and complete error messages are needed to help the programmer understand
her mistake and build a correct model of D semantics in her head.

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