https://bugs.llvm.org/show_bug.cgi?id=51973
Bug ID: 51973
Summary: Default non-type template parameters with immediately
invoked lambdas fail with deduction guides
Product: clang
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: C++
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected],
[email protected]
With the following code, clang claims the defaulted non-type template parameter
is not a constant expression, despite it being so (with GCC handling it fine as
well):
#include <concepts>
template<
typename T,
auto V = []() {
if constexpr (std::same_as<T, int>) {
return 1;
} else {
return 2;
}
}()
>
struct Test {
constexpr Test(T) { }
};
template<typename T>
Test(T) -> Test<T>;
void test() {
Test<int, 1>(1);
}
Godbolt link: https://godbolt.org/z/jT3zPx9fx
As you can see, even if you specify the template parameters when constructing,
it still complains, and if you duplicate the immediately invoked lambda in the
deduction guide and remove the explicated template arguments for the
constructor, clang actually says that it can't compile that expression yet.
However, if you specify the template arguments, it consumes everything fine.
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