[Issue 10487] New: Bad error message with assignment of const tuple

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=10487

           Summary: Bad error message with assignment of const tuple
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            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 2013-06-27 04:54:28 PDT ---
I tag this as enhancement request, but it's borderline with being a bug report.

This is a common programming mistake in D; using "auto" y becomes "const int"
and not just an int, so later you can't assign to it the result of bar():


int bar() {
    return 0;
}
void spam(in int x) {
    auto y = x;
    // ...
    y = bar();
}
void main() {}


The error message is quite clear and easy to understand:

test.d(7): Error: cannot modify const expression y



But if I replace the type int with a Tuple:

import std.typecons: Tuple;
alias Foo = Tuple!int;
Foo bar() {
    return Foo();
}
void spam(in Foo x) {
    auto y = x;
    // ...
    y = bar();
}
void main() {}


DMD 2.064alpha gives me:

test.d(9): Error: template std.typecons.Tuple!int.Tuple.opAssign does not match
any function template declaration. Candidates are:
...\dmd2\src\phobos\std\typecons.d(504):       
std.typecons.Tuple!int.Tuple.opAssign(R)(auto ref R rhs) if
(areCompatibleTuples!(typeof(this), R, "="))
test.d(9): Error: template std.typecons.Tuple!int.Tuple.opAssign(R)(auto ref R
rhs) if (areCompatibleTuples!(typeof(this), R, "=")) cannot deduce template
function from argument types !()(Tuple!int)


The error messages are too much long, and they don't even talk about const
tuples.

I think those error messages should be improved.

I expect future built-in D tuples to give an error message like the int case:

test.d(7): Error: cannot modify const expression y

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