[Bug c++/49117] New: 4.5 -> 4.6: user-unfriendly change in "invalid conversion" error message

["zeratul976 at hotmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49117

           Summary: 4.5 -> 4.6: user-unfriendly change in "invalid
                    conversion" error message
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: zeratul976@hotmail.com


Consider the following fragment of invalid code:

struct A
{
    struct B {};

    int g();

    B f() { return g(); }
};

GCC 4.5's error message is "error: conversion from 'int' to non-scalar type
'A::B' requested".

GCC 4.6's error message is "error: could not convert 'A::g()' to 'A::B'".

The 4.6 error message no longer mentions the actual type, int, that 
cannot be converted to the declared return type, insteading mentioning 
the expression yielding that type, A::g().

I think 4.5's error message is more useful because you can see from it 
exactly what conversion (int to A::B) is failing. With 4.6's error message, 
to find out why the conversion is failing you now have to look up the 
return value of A::g() in your code.

The difference becomes more pronounced with more complex examples.
For example, for the code in PR 49003,

GCC 4.5: "error: conversion from 'vector::const_iterator' to non-scalar type
'vector::iterator' requested"

GCC 4.6: "error: could not convert '((const
block*)this)->block::v.vector::begin()' to 'vector::iterator'"

Quite clearly, the 4.5 error message is better.

Having said that, I think the phrasing of the 4.6 error is better.

I think for 4.7, the error message should keep the 4.6 phrasing, but go back to
the 4.5 way of mentioning the type being converted from.

For example:

error: could not convert 'int' to 'A::B'".

or:

error: could not convert 'vector::const_iterator' to 'vector::iterator'