[Bug c++/101435] New: Bad error with missing typename keyword

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101435

            Bug ID: 101435
           Summary: Bad error with missing typename keyword
           Product: gcc
           Version: 11.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tobi at gcc dot gnu.org
  Target Milestone: ---

Consider (https://godbolt.org/z/hxnv779Tr):
===================================

template<typename U, int dim>
class X {
    public:
    U v;

    using Scalar = U;
    static constexpr auto Dim = dim;

    explicit X(U x) : v(x) {}

    template<typename T>
    X<T, dim> cast()
    {
        return X<T, dim>(T{v});
    }
};

void f(int err) {
    auto propertyHelper = [err]<typename  M>(M&& fallback) -> M {
        using FloatM = X<float, M::Dim>;
        FloatM v{0};
        return err == 0 ? M{ v.cast<typename M::Scalar>() } : fallback;
    };
}
================================
This gives the fairly confusing error messages:
+++++++++++++++++++++++++++++++
<source>: In lambda function:
<source>:23:28: error: expected primary-expression before '{' token
   23 |         return err == 0 ? M{ v.cast<typename M::Scalar>() } : fallback;
      |                            ^
<source>:23:28: error: expected ':' before '{' token
   23 |         return err == 0 ? M{ v.cast<typename M::Scalar>() } : fallback;
      |                            ^
      |                            :
<source>:23:28: error: expected primary-expression before '{' token
<source>:23:28: error: expected ';' before '{' token
   23 |         return err == 0 ? M{ v.cast<typename M::Scalar>() } : fallback;
      |                            ^
      |                            ;
<source>:23:55: error: expected '(' before '>' token
   23 |         return err == 0 ? M{ v.cast<typename M::Scalar>() } : fallback;
      |                                                       ^
      |                                                       (
<source>:23:57: error: expected primary-expression before ')' token
   23 |         return err == 0 ? M{ v.cast<typename M::Scalar>() } : fallback;
      |                                                         ^
<source>:23:58: error: expected ';' before '}' token
   23 |         return err == 0 ? M{ v.cast<typename M::Scalar>() } : fallback;
      |                                                          ^~
      |                                                          ;
<source>:23:61: error: expected primary-expression before ':' token
   23 |         return err == 0 ? M{ v.cast<typename M::Scalar>() } : fallback;
      |                                                             ^
Compiler returned: 1
+++++++++++++++++++++++++++++++

The actual cause of the error is the missing typename keyword before M (clang
points this out correctly, MSVC of course accepts the code without error
message).  This is similar to #63392 but in that case we at least get a hint as
to what the actual problem is.