[llvm-bugs] [Bug 25856] New: Clang hangs in template instantiation after diagnosing C++11 narrowing in NTTP.

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=25856

            Bug ID: 25856
           Summary: Clang hangs in template instantiation after diagnosing
                    C++11 narrowing in NTTP.
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++11
          Assignee: unassignedclangbugs at nondot.org
          Reporter: eric at efcs.ca
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 15473
  --> https://llvm.org/bugs/attachment.cgi?id=15473&action=edit
reproducer.cpp

The attached reproducer causes clang to hang after in instantiates the `__make`
template with `-3`. Clang will emit a C++11 narrowing error before silently
hanging. My guess is that clang trys to instantiate the base class of
`__parity` with the converted value 4294967293.

metaprogramming often uses an integer NTTP to control the number of
instantiations. If clang instantiates a template with a narrowed NTTP it could
be in for a long ride.

I would be willing to work on a fix if somebody could offer direction on the
solution.

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