Bug#538647: g++-4.3: g++ examines second value to "?" always if it is a template

[Philip Ashmore <contact@philipashmore.com>]

Package: g++-4.3
Version: 4.3.2-1.1
Severity: important


Let an example do the talking

    template< unsigned int result, unsigned int wanted, unsigned int guess >
    struct log2_calculator;

    // Specialisation that ends recursion.
    template< unsigned int result, unsigned int wanted >
    struct log2_calculator< result, wanted, 0>
    {
        enum { value = 0 }; // Invalid.
    };

    template< unsigned int result, unsigned int wanted, unsigned int guess >
    struct log2_calculator
    {
        typedef log2_calculator< result+1, wanted, guess << 1 > next_value_type;
/*HERE*/enum { value = (wanted <= guess) ? result : next_value_type::value };
    };

The intention of this code is to work out log2(a) at compile time, lacking
some other mechanism.

This allows...

    enum { is3 = log2_calculator< 0,  8, 1 >::value };
    enum { is4 = log2_calculator< 0, 16, 1 >::value };
    enum { is6 = log2_calculator< 0, 64, 1 >::value };

....something that's more obviously useful inside templates.

    enum { isX = log2_calculator< 0, sizeof(N)*8, 1 >::value };

Anyway, the problem becomes obvious when you omit the specialisation from the
above - g++ reaches it's template nesting limit because it's instantiating
the template that's the second value to "?" even though the conditional is
true.
I've marked the line above with /*HERE*/.

-- System Information:
Debian Release: 5.0.2
  APT prefers stable
  APT policy: (500, 'stable')
Architecture: amd64 (x86_64)

Kernel: Linux 2.6.26-2-amd64 (SMP w/2 CPU cores)
Locale: LANG=en_IE.UTF-8, LC_CTYPE=en_IE.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/bash

Versions of packages g++-4.3 depends on:
ii  gcc-4.3                   4.3.2-1.1      The GNU C compiler
ii  gcc-4.3-base              4.3.2-1.1      The GNU Compiler Collection (base 
ii  libc6                     2.7-18         GNU C Library: Shared libraries
ii  libgmp3c2                 2:4.2.2+dfsg-3 Multiprecision arithmetic library
ii  libmpfr1ldbl              2.3.1.dfsg.1-2 multiple precision floating-point 
ii  libstdc++6-4.3-dev        4.3.2-1.1      The GNU Standard C++ Library v3 (d

g++-4.3 recommends no packages.

Versions of packages g++-4.3 suggests:
ii  g++-4.3-multilib             4.3.2-1.1   The GNU C++ compiler (multilib fil
ii  gcc-4.3-doc                  4.3.2.nf1-1 documentation for the GNU compiler
pn  libstdc++6-4.3-dbg           <none>      (no description available)

-- no debconf information