[llvm-bugs] [Bug 48365] Compiler is using wrong template argument when a child class calls a templated constructor of a template class.

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=48365

David Blaikie <dblaikie at gmail.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
                 CC|                            |dblaikie at gmail.com
             Status|NEW                         |RESOLVED

--- Comment #1 from David Blaikie <dblaikie at gmail.com> ---
Looks reasonable to me - the rest of the error message says:

"<source>:13:5: note: in instantiation of default argument for
'Base<Base<int>>' required here
    Base(const U& v) {
    ^~~~~~~~~~~~~~~~~~
<source>:10:7: note: while substituting deduced template arguments into
function template 'Base' [with U = Base<int>, $1 = (no value)]
class Base {
      ^
<source>:17:7: note: while declaring the implicit copy constructor for 'Child'
class Child : public Base<int> {
      ^"

So it's not while trying to evaluate "Child child;"'s call to the "Child()"
ctor, which does correctly use the Base<int>::Base<float> ctor, but it's while
trying to declare Child's copy ctor - which, is the absence of any other ctor,
tries to use Base<int>::Base<Base<int>> to do the copy construction.

If you make the Base ctor not a valid ctor to use for copying (eg, by changing
its template parameter list to SFINAE out that option: "template <typename U,
typename = std::enable_if_t<!std::is_same_v<U, Base>>, typename = typename
CheckInt<U>::success>") then the build error goes away.
https://godbolt.org/z/6fGjhd

Similarly, if the ctor in question had an extra parameter - there would be no
ambiguity: https://godbolt.org/z/oqGxhq

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