[Bug c++/94038] [8/9/10 Regression] Compiling with -Wall causes function template body to get needlessly instantiated

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94038

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|Compiling with -Wall causes |[8/9/10 Regression]
                   |function template body to   |Compiling with -Wall causes
                   |get needlessly instantiated |function template body to
                   |                            |get needlessly instantiated
      Known to work|                            |5.5.0
      Known to fail|                            |10.0, 6.4.0, 7.4.0, 8.3.0,
                   |                            |9.3.0
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot 
gnu.org
   Target Milestone|---                         |10.0
   Last reconfirmed|                            |2020-05-04
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |ASSIGNED

--- Comment #5 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 11.

However, it looks like GCC 5.2-5.5 successfully compile the added testcase
g++.dg/warn/pr94038.C (with -O and -Wall), buh GCC 5.1, 6.1 and later do not,
so I am leaving this PR open and marking it as a 8/9/10 regression. 

Here's the exact testcase that exhibits a regression relative to gcc 5.5:

$ cat testcase.C
template<typename T>
constexpr int
foo()
{
  static_assert(T(1) == 0, "");
  return 0;
}

template<typename T>
constexpr int
bar()
{
  return foo<T>();
}

constexpr int
baz(int a)
{
  return a;
}

static_assert(decltype(baz(bar<int>())){} == 0, "");

$ g++-10 -std=c++11 -O -Wall testcase.C
testcase.C: In instantiation of ‘constexpr int foo() [with T = int]’:
testcase.C:17:16:   required from ‘constexpr int bar() [with T = int]’
testcase.C:26:36:   required from here
testcase.C:9:22: error: static assertion failed
    9 |   static_assert(T(1) == 0, "");
      |                 ~~~~~^~~~