[Bug c++/101232] New: Bad error message with stray semicolon in initializer

[Bug c++/101232] New: Bad error message with stray semicolon in initializer

[tobi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101232

            Bug ID: 101232
           Summary: Bad error message with stray semicolon in initializer
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tobi at gcc dot gnu.org
  Target Milestone: ---

This example (Link to compiler explorer: -> https://godbolt.org/z/rxsreYs8Y
<-):

=========================

struct X {
    int a;
    int b;
};

void f() {
    auto x = X{ 1, 2; };
}
========================

Gives the fairly misleading error message

<source>: In function 'void f()':
<source>:7:15: error: expected primary-expression before '{' token
    7 |     auto x = X{ 1, 2; };
      |               ^

The error message is of the same kind all the way from GCC 4.1.2 up to the
current trunk.

For comparison, clang gives the much more helpful
<source>:7:21: error: unexpected ';' before '}'
    auto x = X{ 1, 2; };
                    ^

[Bug c++/101232] Bad error message with stray semicolon in initializer

[tobi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101232

--- Comment #1 from Tobias Schlüter <tobi at gcc dot gnu.org> ---
BTW an equivalent C example gives the proper error both with C and C++
https://godbolt.org/z/sWc67eWT8
=====================
struct X {
    int a;
    int b;
};

void f() {
    struct X x = { 1, 2; };
}
=====================

<source>: In function 'void f()':
<source>:7:24: error: expected '}' before ';' token
    7 |     struct X x = { 1, 2; };
      |                  ~     ^
<source>: At global scope:
<source>:8:1: error: expected declaration before '}' token
    8 | }
      | ^

[Bug c++/101232] Bad error message with stray semicolon in initializer

[tobi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101232

--- Comment #2 from Tobias Schlüter <tobi at gcc dot gnu.org> ---
Here's another way to trigger this, inspired by my other PR101435
(https://godbolt.org/z/no6aEqvh3):
===============================
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.template cast<typename M::Scalar>(); } :
fallback;
    };
}
=======================
Which gives a comically wrong cascade of errors:
---------------------------
<source>: In lambda function:
<source>:23:28: error: expected primary-expression before '{' token
   23 |         return err == 0 ? M{ v.template cast<typename M::Scalar>(); } :
fallback;
      |                            ^
<source>:23:28: error: expected ':' before '{' token
   23 |         return err == 0 ? M{ v.template 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.template cast<typename M::Scalar>(); } :
fallback;
      |                            ^
      |                            ;
<source>:23:71: error: expected primary-expression before ':' token
   23 |         return err == 0 ? M{ v.template cast<typename M::Scalar>(); } :
fallback;
      |                                                                       ^
Compiler returned: 1
---------------------------------