[llvm-bugs] [Bug 31938] New: libc++ declares basic_string constructors with non-deducible types

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=31938

            Bug ID: 31938
           Summary: libc++ declares basic_string constructors with
                    non-deducible types
           Product: libc++
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: richard-llvm at metafoo.co.uk
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com
    Classification: Unclassified

The standard says:

template<class charT, class traits = char_traits<charT>,
         class Allocator = allocator<charT>>
class basic_string {
  using value_type = typename traits::char_type;
  // ...
  basic_string(const charT* s, const Allocator& a = Allocator());
};

libc++ actually chooses to declare the constructor as

  basic_string(const value_type* s, const Allocator& a = Allocator());

The implicit deduction guides from class template argument deduction make what
was previously an implementation detail visible:

std::basic_string s = "foo"; // error, can't deduce charT.

The constructor in question is in the libc++ DSO, but fortunately it looks like
fixing this will not result in an ABI break.

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