https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115672
Bug ID: 115672
Summary: Incorrect template type parameter deduced in
non-deduced context if class template has template
template parameter
Product: gcc
Version: 14.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: s.murthy at outlook dot com
Target Milestone: ---
Created attachment 58524
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58524&action=edit
Repro source code with static_asserts such that the program compiles
This issue is related to Bug 115656 in that the two likely have the same or
largely the same underlying cause, but this bug is also distinct because it
exists even after circumventing the situation leading to 115656. I chose to
report this issue separately because IMHO the effect is rather serious due to
incorrect type deduction, but it is obviously up to the maintainers to merge
the two issues or to treat them separately.
Like 115656, this issue is also a regression: no issues in GCC 10.5; issues in
GCC 11.1 and later.
Setup: Class template A with a constrained type parameter and a constrained
template template parameter. A has ctors for both deducing and non-deducing
contexts.
When an A object is created in a non-deduced context (after circumventing the
ordering issue in 115656), the template type parameter is assigned the type
used in the non-deduced ctor call instead of assigning the default specified
for the template type parameter. The incorrect type then propagates to all
entities carrying that deduced type. E.g., a data member that should be
`unsigned` due to that being the default for the template type parameter is
instead made `long` because the non-deducing ctor call has long argument. This
**even though the template type parameter is constrained to be unsigned
integral.**
Repro follows. Remove the template template parameter from A and the issues
disappear. (Comparison at: https://sigcpp.godbolt.org/z/TG4vva8P5)
----------------------------------------------------------------------------
//stub for use in class template A
template<std::unsigned_integral U = unsigned> class dv;
template<std::unsigned_integral U = unsigned, template<std::unsigned_integral>
class V = dv>
struct A
{
U u_;
A(U u) : u_(u) {};
template<std::signed_integral S>
A(S s) : u_( s < 0 ? -s : s) {}
};
int main()
{
A a1(45ul); //use deducing-ctor first to circumvent the issue in 115656
static_assert(std::same_as<decltype(a1.u_), unsigned long>); //passes,
correct
A a2(45l); //U should be unsigned
static_assert(std::same_as<decltype(a2.u_), unsigned>); //fails in GCC
11.1+, incorrect
static_assert(std::same_as<decltype(a2.u_), long>); //passes in GCC
11.1+, incorrect
//a2.u_ = -1;
//std::cout << a2.u_ << '\n'; //-1 instead of UINT_MAX in GCC 11.1+
}
