[llvm-bugs] [Bug 25271] New: Parameter default values of forward-declared template classes with template parameter packs don't PCH roundtrip

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=25271

            Bug ID: 25271
           Summary: Parameter default values of forward-declared template
                    classes with template parameter packs don't PCH
                    roundtrip
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: jix at jixco.de
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 15131
  --> https://llvm.org/bugs/attachment.cgi?id=15131&action=edit
tar of cpp file, header file and shell script to reproduce the bug

When having a forward declaration like this

    template<unsigned T=123, unsigned... U> class dummy;

in a header file that also contains the class definition, using dummy<> fails
with

    error: too few template arguments for class template 'dummy'

when using a precompiled header, even though it works fine otherwise (as it
should).

The bug applies to integer template parameters (as in the given example) as
well as to typename parameters.
The bug doesn't apply to class templates without forward declaration.
The bug also doesn't apply to class templates without a following parameter
pack.

A real world example that triggers this is using gsl::array_view<int> from:

https://github.com/Microsoft/GSL/blob/master/include/array_view.h

I tested with 3.7.0 and svn trunk, which both are affected.

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