https://llvm.org/bugs/show_bug.cgi?id=31094
Bug ID: 31094
Summary: template parameter not deducible in partial
specialization of template inside template
Product: clang
Version: 3.9
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: C++
Assignee: unassignedclangb...@nondot.org
Reporter: akond...@kcg.com
CC: dgre...@apple.com, llvm-bugs@lists.llvm.org
Classification: Unclassified
The code below worked on clang 3.6, but warns on 3.8 and 3.9:
template <typename T>
struct outer
{
template <typename U>
struct inner
{
};
};
template <typename T>
struct is_inner_for
{
template <typename Whatever>
struct predicate
{
static constexpr bool value = false;
};
template <typename U>
struct predicate<typename outer<T>::template inner<U>>
{
static constexpr bool value = true;
};
};
static_assert(
is_inner_for<int>::template predicate<
outer<int>::inner<double>
>::value,
"Yay!"
);
Here is the warning:
<source>:23:12: warning: class template partial specialization contains a
template parameter that cannot be deduced; this partial specialization will
never be used
struct predicate<typename outer<T>::template inner<U>>
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:22:24: note: non-deducible template parameter 'U'
template <typename U>
^
1 warning generated.
Even though clang warns about the partial specialization not being used, the
code compiles fine and that specialization is in fact used.
GCC 6 bug that resulted in the same type of error:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70141
Godbolt output: https://godbolt.org/g/pqJcQu
Stack overflow question:
http://stackoverflow.com/questions/35875829/template-parameters-not-deducible-in-partial-specialization-in-gcc6-for-a-case
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