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<p class="MsoNormal">Hello Clang Dev,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Recently I looked into the clang bug 12053,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">template <typename T> auto foo(T t) -> decltype(foo(t)) {}<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><a href="http://llvm.org/bugs/show_bug.cgi?id=12053">http://llvm.org/bugs/show_bug.cgi?id=12053</a><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">which crashes clang (trunk) (and gcc 4.6 too).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">As far as I know, Clang does realize that there is no candidate
<o:p></o:p></p>
<p class="MsoNormal">available while resolving “decltype(foo(t))”, however BuildRecoveryCallExpr
<o:p></o:p></p>
<p class="MsoNormal">will find template foo (in DiagnoseTwoPhaseLookup) <o:p></o:p></p>
<p class="MsoNormal">and try to instantiate it again. This leads to the crash.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I am wondering if the following is the right way to fix this. The basic idea is:
<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Before starting instantiating a function template, we check if the same function<o:p></o:p></p>
<p class="MsoNormal">template instantiation *with the same template arguments* is already in-progress.
<o:p></o:p></p>
<p class="MsoNormal">If yes, then clang is not making any progress and should lead an infinite loop.<o:p></o:p></p>
<p class="MsoNormal">We treat it as an SFINAE.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The attached patch will fix the clang crashing on the above test and other similar tests like<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">template <typename T> auto foo(T t) -> decltype(bar(t)) {}<o:p></o:p></p>
<p class="MsoNormal">template <typename T> auto bar(T t) -> decltype(foo(t)) {}<o:p></o:p></p>
<p class="MsoNormal">int x = foo(0);<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">This is not a final patch since this change will affect two tests (only these two)
<o:p></o:p></p>
<p class="MsoNormal">instantiation-depth-subst.cpp and instantiation-depth-subst-2.cpp.<o:p></o:p></p>
<p class="MsoNormal"><o:p></o:p></p>
<p class="MsoNormal">Any thoughts?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks!<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Wei Pan<o:p></o:p></p>
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