https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116403
Bug ID: 116403
Summary: [modules] Deduction guides not reachable across
transitive non-exported imports
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Keywords: rejects-valid
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: nshead at gcc dot gnu.org
Blocks: 103524
Target Milestone: ---
Consider:
// a.cpp
export module A;
export template <typename T> struct S { S(int); };
S(int) -> S<double>;
// b.cpp
export module B;
import A;
export using ::S;
// c.cpp
import B;
int main() {
S s(123); // should be S<double>
}
This should compile successfully, as deduction guides are always visible if
reachable, and reachability doesn't care that we didn't 'export import A' from
module B.
Instead we get:
$ g++ -fmodules-ts -S [abc].cpp
c.cpp: In function ‘int main()’:
c.cpp:4:10: error: class template argument deduction failed:
4 | S s(123);
| ^
c.cpp:4:10: error: no matching function for call to ‘S@A(int)’
In module A, imported at b.cpp:2,
of module B, imported at c.cpp:1:
a.cpp:2:41: note: candidate: ‘template<class T> S@A(int) -> S@A<T>’
2 | export template <typename T> struct S { S(int); };
| ^
a.cpp:2:41: note: template argument deduction/substitution failed:
c.cpp:4:10: note: couldn’t deduce template parameter ‘T’
4 | S s(123);
| ^
In module A, imported at b.cpp:2,
of module B, imported at c.cpp:1:
a.cpp:2:37: note: candidate: ‘template<class T> S@A(S@A<T>) -> S@A<T>’
2 | export template <typename T> struct S { S(int); };
| ^
a.cpp:2:37: note: template argument deduction/substitution failed:
c.cpp:4:10: note: mismatched types ‘S@A<T>’ and ‘int’
4 | S s(123);
| ^
It looks like my original 'hack' solution to just always consider deduction
guides exported is insufficient, and we'll need a more general name lookup
solution.
Referenced Bugs:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103524
[Bug 103524] [meta-bug] modules issue
