[llvm-bugs] [Bug 42879] New: Candidate template ignored: invalid explicitly-specified argument for template parameter

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42879

            Bug ID: 42879
           Summary: Candidate template ignored: invalid
                    explicitly-specified argument for template parameter
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: m at mikaelsimonsson.com
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

The following code fails to compile with Clang but compiles with GCC (since
7.1). Clang wants a value for the defaulted template parameter Capacity.


#include <cstddef>

template <typename T, size_t Capacity = 10>
class smallvector {
  public:
    smallvector(const T*, size_t);
};

template <typename T>
class span {
  public:
    span(const T*, size_t);
};

class str {
  public:
    str(const char* data, size_t size) : data_{data}, size_{size} {}

    template <template <typename> typename Container>
    Container<const char> to_container() {
        return {data_, size_};
    }

  private:
    const char* data_;
    size_t size_;
};

void to_span(str& s) {
    s.to_container<span>(); // This works.
}

void to_smallvector(str& s) {
    s.to_container<smallvector>(); // This doesn't work.
}


<source>:34:7: error: no matching member function for call to 'to_container'

    s.to_container<smallvector>();

    ~~^~~~~~~~~~~~~~~~~~~~~~~~~

<source>:20:27: note: candidate template ignored: invalid explicitly-specified
argument for template parameter 'Container'

    Container<const char> to_container() {

                          ^

1 error generated.

Tested with clang version 10.0.0 (trunk 367740) and -std=c++17.

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