[LLVMbugs] [Bug 18874] New: constexpr function ill-formed but error message incorrect

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=18874

            Bug ID: 18874
           Summary: constexpr function ill-formed but error message
                    incorrect
           Product: clang
           Version: trunk
          Hardware: Other
                OS: other
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++1y
          Assignee: unassignedclangbugs at nondot.org
          Reporter: yaghmour.shafik at gmail.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

In the following program:

#include <cstddef>

template <typename T, size_t s>
struct test
{  
     T x[s] ;
} ;

template <typename T>
constexpr T func() {
    test<T,3> test1 ;

  test1.x[0] = 10 ;
  return test1.x[0];
}

constexpr int func2() {
    test<int,3> test1 ;

  test1.x[0] = 10 ;
  return test1.x[0];
}


int main()
{
    constexpr int x = func<int>();
}

func and func2 should be ill-formed for the same reason that variable test1 is
not initialized nor does it have a constexpr constructor but in func the error
message is:

  main.cpp:13:11: note: assignment to object outside its lifetime is not
allowed in a constant expression
    test1.x[0] = 10 ;
               ^

which does not make any sense since test1 should be trivial and it's lifetime
should begin with it's declaration and extend over the whole function.

I would expect an error similar to the one we see for func2 which is as
follows:

  main.cpp:20:14: note: non-constexpr constructor 'test' cannot be used in a
constant expression
      test<int,3> test1 ;
                  ^

I am building using the following command line:

  clang++ -std=c++1y -O0 -Wall -Wextra -Wconversion -pedantic 

You can see this on Coliru here:

  http://coliru.stacked-crooked.com/a/16ed43ed478fbc55

This was originally brought up in this StackOverflow question:

 
http://stackoverflow.com/questions/21806154/c1y-c14-assignment-to-object-outside-its-lifetime-is-not-allowed-in-a-const

This does not seem like a major bug but the error message sure is confusing.

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