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.
