[llvm-bugs] [Bug 31771] New: Incorrect identification of Most Vexing Parse when expanding parameter pack and calling constructors

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=31771

            Bug ID: 31771
           Summary: Incorrect identification of Most Vexing Parse when
                    expanding parameter pack and calling constructors
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Frontend
          Assignee: unassignedclangbugs at nondot.org
          Reporter: victory at csail.mit.edu
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

In the C++11 program below, the indicated statement attempts to expand a
parameter pack to construct a Tuple object, used to initialize the variable
"s". Clang warns that this is a Vexing Parse and interprets the statement as a
function declaration, as well giving an error about mismatched parentheses. The
error indicates Clang realizes the statement cannot be parsed as a function
declaration. Since it isn't a function declaration, there is no ambiguity, and
Clang should reinterpret the statement as the definition and initialization of
variable "s" of type "S". I have verified that g++ has the desired behavior.

clang version 5.0.0 (trunk 293190)
Target: x86_64-unknown-linux-gnu
Thread model: posix

$ cat test.cpp
template <class ... Types>
struct Tuple {
  Tuple(Types ...args) { }
} ;

template <class ... Types>
void f(Types ...args) {
  struct S {
      S(Tuple<Types...> t) { }
  };

  S s(Tuple<Types...>(args...));  // This line causes clang error!
}

int main() {
  f<int,double>(1, 42.0);
  return 0;
}
$ clang++ -std=c++11 -O0 -S -emit-llvm test.cpp
test.cpp:12:27: error: expected ')'
  S s(Tuple<Types...>(args...));  // This line causes clang error!
                          ^
test.cpp:12:22: note: to match this '('
  S s(Tuple<Types...>(args...));  // This line causes clang error!
                     ^
test.cpp:12:6: warning: parentheses were disambiguated as a function
declaration [-Wvexing-parse]
  S s(Tuple<Types...>(args...));  // This line causes clang error!
     ^~~~~~~~~~~~~~~~~~~~~~~~~~
test.cpp:12:7: note: add a pair of parentheses to declare a variable
  S s(Tuple<Types...>(args...));  // This line causes clang error!
      ^
      (                       )
1 warning and 1 error generated.

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