[LLVMbugs] [Bug 24128] New: eagerly-instantiated entities whose templates are defined after the first point of instantiation don't get instantiated at all

[bugzilla-daemon at llvm.org]

https://llvm.org/bugs/show_bug.cgi?id=24128

            Bug ID: 24128
           Summary: eagerly-instantiated entities whose templates are
                    defined after the first point of instantiation don't
                    get instantiated at all
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: richard-llvm at metafoo.co.uk
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified

If a template specialization is referenced in a way that requires a definition,
but the definition of the template appears after the use, the instantiation is
never performed.

Examples through the ages:

// C++98:
template<typename T> struct X { static const int n; };
int k = X<int>::n;
template<typename T> const int X<T>::n = 5;

// C++11:
template<typename T> constexpr int f();
int k = f<int>();
template<typename T> constexpr int f() { return 5; }

// C++14:
template<typename T> extern const int n;
int k = n<int>;  
template<typename T> const int n = 5;

These are valid, and should cause the instantiation of the relevant template,
according to C++14 [temp]/6, which requires the template to "be defined in
every translation unit in which it is implicitly instantiated", but doesn't
require the definition to precede the use.

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