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From: "sfranzen85 at hotmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/97214] New: ICE in lookup_template_class_1, at cp/pt.c:9896
Date: Sat, 26 Sep 2020 15:38:10 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97214

            Bug ID: 97214
           Summary: ICE in lookup_template_class_1, at cp/pt.c:9896
           Product: gcc
           Version: 10.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sfranzen85 at hotmail dot com
  Target Milestone: ---

The following snippet reproduces an error I encountered with lambdas:

struct Foo {
    // void operator()(int) {}

    template<typename T>
    void operator()(T, T)
    {
        auto bar =3D [this](auto&& v){ operator()(v); };
    }
};

int main ()
{
    Foo{}(0,1);
    return 0;
}

Full error:
../src/main.cpp: In instantiation of =E2=80=98void Foo::operator()(T, T) [w=
ith T =3D
int]=E2=80=99:
../src/main.cpp:13:14:   required from here
../src/main.cpp:7:38: internal compiler error: in lookup_template_class_1, =
at
cp/pt.c:9896
    7 |         auto bar =3D [this](auto&& v){ operator()(v); };
      |                                      ^~~~~~~~~~

Further observations:
* The error appears regardless of available operator()() overloads;
* It only appears if the function call is unqualified, e.g. (*this)(v) is f=
ine
if the overload exists.

A possibly related error is given using a version of Foo without a function
template:
struct Foo {
    void operator()(int) {}

    void operator()(int a, int)
    {
        auto bar =3D [this](auto&& v){ operator()(v); };
        bar(a);
    }
};

../src/main.cpp: In instantiation of =E2=80=98Foo::operator()(int,
int)::<lambda(auto:1&&)> [with auto:1 =3D int&]=E2=80=99:
../src/main.cpp:8:14:   required from here
../src/main.cpp:7:48: error: use of =E2=80=98Foo::operator()(int,
int)::<lambda(auto:1&&)> [with auto:1 =3D int&]=E2=80=99 before deduction o=
f =E2=80=98auto=E2=80=99
    7 |         auto bar =3D [this](auto&& v){ operator()(v); };
      |                                      ~~~~~~~~~~^~~

This error similarly only appears with the unqualified call, and also
disappears if the lambda has '-> void'.=