https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99799
Bug ID: 99799
Summary: Explicit instantiation function template with auto
deduced return type fails if soft instantiation
occurred
Product: gcc
Version: 10.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: bugzilla at halpernwightsoftware dot com
Target Milestone: ---
Source code (4 lines, no preprocessor directives):
template <typename T> auto f(T v) { return v; }
extern template auto f(int);
int i = f(0);
template auto f(int);
Compiler command line: g++ -std=c++14 -Wall -c autoreturnbug.cpp
Error produced:
autoreturnbug.cpp:4:15: error: template-id 'f<>' for 'auto f(int)' does not
match any template declaration
4 | template auto f(int);
| ^
autoreturnbug.cpp:1:28: note: candidate is: 'template<class T> auto f(T)'
1 | template <typename T> auto f(T v) { return v; }
| ^
However, if line 3 (int i = f(0)) is removed, the code compiles successfully.
In the C++14 standard, section [dcl.spec.auto], paragraph 15, it says:
An explicit instantiation declaration (14.7.2) does not cause the instantiation
of an entity declared using a placeholder type, but it also does not prevent
that entity from being instantiated as needed to determine its
type. [ Example:
template <typename T> auto f(T t) { return t; }
extern template auto f(int); // does not instantiate f<int>
int (*p)(int) = f; // instantiates f<int> to determine its return type, but an
explicit
// instantiation definition is still required somewhere in the program
— end example ]
I take that to mean that the four lines above should compile.
