[llvm-bugs] [Bug 42406] New: Valid code fails with "reference to local variable declared in enclosing function"

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42406

            Bug ID: 42406
           Summary: Valid code fails with "reference to local variable
                    declared in enclosing function"
           Product: clang
           Version: 8.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: realnc at gmail.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 4.0.0 and newer. The last
version that can compile this code is 3.9.1 (tested on godbolt.org.)

  template <int>
  struct S {
      constexpr int val() const { return 0; }
  };

  template <typename = void>
  S<0> foo()
  {
      S<0> s;
      return S<s.val()>{};
  }

  int main()
  {
      foo();
  }

Build with -std=c++11 or higher.

Error message:

  error: reference to local variable 's' declared in enclosing function 'foo'
     return S<s.val()>{};
              ^
  note: in instantiation of function template specialization 'foo<void>'
  requested here
    foo();
    ^
  note: 's' declared here
    S<0> s;
         ^
  1 error generated.

When making foo() a non-template, the error disappears:

  S<0> foo()
  {
      S<0> s;
      return S<s.val()>{};
  }

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