[Bug c++/86246] [8/9 Regression] Template dispatching error inside a template function

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86246

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
      Known to work|                            |7.3.0
            Version|8.0.1                       |8.1.0
           Keywords|                            |rejects-valid
   Last reconfirmed|                            |2018-06-20
                 CC|                            |nathan at gcc dot gnu.org
     Ever confirmed|0                           |1
            Summary|Template dispatching error  |[8/9 Regression] Template
                   |inside a template function  |dispatching error inside a
                   |                            |template function
      Known to fail|                            |8.1.0, 9.0

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Tianqi Chen from comment #0)
> There is also evidence that it
> still exists in gcc-8.1

We're really not interested in bugs reported against pre-release 8.0.1 builds
now that there's an actual 8.1 release. It's very easy to check the 8.1 release
using an online compiler such as https://wandbox.org


It does fail with 8.1 though.

Reduced:


namespace std {
  template<typename T> struct is_class {
    static constexpr bool value = true;
  };
  template<> struct is_class<double> {
    static constexpr bool value = false;
  };
}

class MyClass {
 public:
  operator double() const {
    return 1;
  }
  template<typename T>
  operator T() const {
    static_assert(std::is_class<T>::value, "problem");
    return T();
  }
};

template<typename T>
void SetValue(const MyClass& obj, T* value) {
  //  always dispatches to operator T even if T is double
  *value = obj.operator T();
}

int main() {
  MyClass obj;
  // works fine                                                                 
  obj.operator double();
  double x;
  // error, when operator T is called in SetValue                               
  SetValue(obj, &x);
}


This compiled OK until r255605 when it started to ICE:

86246.cc: In instantiation of ‘void SetValue(const MyClass&, T*) [with T =
double]’:
86246.cc:34:19:   required from here
86246.cc:25:25: internal compiler error: in tsubst_baselink, at cp/pt.c:14471
   *value = obj.operator T();
            ~~~~~~~~~~~~~^
0xa0090e tsubst_baselink
        ../../gcc/cp/pt.c:14471
0xa14c27 tsubst_copy_and_build(tree_node*, tree_node*, int, tree_node*, bool,
bool)
        ../../gcc/cp/pt.c:17980
0xa1270a tsubst_copy_and_build(tree_node*, tree_node*, int, tree_node*, bool,
bool)
        ../../gcc/cp/pt.c:17592
0xa11b30 tsubst_copy_and_build(tree_node*, tree_node*, int, tree_node*, bool,
bool)
        ../../gcc/cp/pt.c:17433
0xa0e93c tsubst_expr(tree_node*, tree_node*, int, tree_node*, bool)
        ../../gcc/cp/pt.c:16767
0xa08d23 tsubst_expr(tree_node*, tree_node*, int, tree_node*, bool)
        ../../gcc/cp/pt.c:16008
0xa0aadb tsubst_expr(tree_node*, tree_node*, int, tree_node*, bool)
        ../../gcc/cp/pt.c:16251
0xa2be19 instantiate_decl(tree_node*, bool, bool)
        ../../gcc/cp/pt.c:23302
0xa2c7ca instantiate_pending_templates(int)
        ../../gcc/cp/pt.c:23416
0x8cfdc2 c_parse_final_cleanups()
        ../../gcc/cp/decl2.c:4666
0xb3f0b8 c_common_parse_file()
        ../../gcc/c-family/c-opts.c:1149
Please submit a full bug report,
with preprocessed source if appropriate.
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.

Then at r256986 the ICE was fixed but it started calling the wrong function:

86246.cc: In instantiation of ‘MyClass::operator T() const [with T = double]’:
86246.cc:25:10:   required from ‘void SetValue(const MyClass&, T*) [with T =
double]’
86246.cc:34:19:   required from here
86246.cc:17:19: error: static assertion failed: problem
     static_assert(std::is_class<T>::value, "problem");
                   ^~~