[Issue 3955] New: Very poor error message: accidentally assigning to string literal in template

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=3955

           Summary: Very poor error message: accidentally assigning to
                    string literal in template
           Product: D
           Version: 1.056
          Platform: All
        OS/Version: All
            Status: NEW
          Keywords: diagnostic
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: matti.niemenmaa+dbugzilla at iki.fi


--- Comment #0 from Matti Niemenmaa <matti.niemenmaa+dbugzilla at iki.fi> 2010-03-13 08:34:35 PST ---
For the following code:


template T(char[] s) {
    static if (s[0] = 'f') // error here; line 2
        const T = "";
    else
        const T = "" ~ T!(s);
}

const char[] A = T!("foo"); // line 8
const char[] B = "" ~ A;


DMD 1.056 emits:

$ dmd -c arst.d
arst.d(8): Error: cannot implicitly convert expression (__error ~ __error) of
type int to char[]

Which is about as obtuse as possible:

- __error looks like something that should only show up when debugging the
compiler. It's certainly not to be found anywhere in the source.
- There is no ~ on line 8: the message points only to the use of T, not the
actual problem which is within T.
- DMD's usual habit of using "int" when it hits any kind of error.

Removing the apparently unrelated constant B from line 9, we get the following
message that actually indicates the problem. This is what I'd expect to get
regardless of B:

$ dmd -c arst.d
arst.d(2): Error: string literals are immutable
arst.d(2): Error: expression 'f' = 'f' is not constant or does not evaluate to
a bool

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