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Fix build on gcc 4.8 

Fixes https://github.com/harfbuzz/harfbuzz/issues/1724
This commit is contained in:
Behdad Esfahbod 2019-06-07 12:41:09 -07:00
parent 3c240bd3dc
commit e4e518f33d
1 changed files with 27 additions and 2 deletions
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29
src/hb-algs.hh
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@ -167,14 +167,39 @@ template <unsigned Pos=1, typename Appl, typename V>
auto hb_partial (Appl&& a, V&& v) HB_AUTO_RETURN
(( hb_partial_t<Pos, Appl, V> (a, v) ))
/* The following hacky replacement version is to make Visual Stuiod build:. */ \
/* https://github.com/harfbuzz/harfbuzz/issues/1730 */ \
/* The following, HB_PARTIALIZE, macro uses a particular corner-case
* of C++11 that is not particularly well-supported by all compilers.
* What's happening is that it's using "this" in a trailing return-type
* via decltype(). Broken compilers deduce the type of "this" pointer
* in that context differently from what it resolves to in the body
* of the function.
*
* One probable cause of this is that at the time of trailing return
* type declaration, "this" points to an incomplete type, whereas in
* the function body the type is complete. That doesn't justify the
* error in any way, but is probably what's happening.
*
* In the case of MSVC, we get around this by using C++14 "decltype(auto)"
* which deduces the type from the actual return statement. For gcc 4.8
* we use "this+0" instead of "this" which produces an rvalue that seems
* to do be deduces as the same type with this particular compiler. Note
* that "this+0" is illegal in the trailing decltype position since at
* that point this points to incomplete type! But seems to do the trick.
*/
#ifdef _MSC_VER
/* https://github.com/harfbuzz/harfbuzz/issues/1730 */ \
#define HB_PARTIALIZE(Pos) \
template <typename _T> \
decltype(auto) operator () (_T&& _v) const \
{ return hb_partial<Pos> (this, hb_forward<_T> (_v)); } \
static_assert (true, "")
#elif defined(__GNUC__) && (__GNUC__ == 4) && (__GNUC_MINOR__ <= 8)
/* https://github.com/harfbuzz/harfbuzz/issues/1724 */
#define HB_PARTIALIZE(Pos) \
template <typename _T> \
auto operator () (_T&& _v) const HB_AUTO_RETURN \
(hb_partial<Pos> (this+0, hb_forward<_T> (_v))) \
static_assert (true, "")
#else
#define HB_PARTIALIZE(Pos) \
template <typename _T> \