https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98524
Bug ID: 98524
Summary: accepts-invalid instantiation of variable template
with different function pointer type from declaration
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: adr26__gcc at nunsway dot co.uk
Target Milestone: ---
Consider:
template<typename T> void (*TF)(int);
template void (*TF<int>)(int);
Per [temp.explicit]/6, the above are two declarations of the same entity:
The declaration in an explicit-instantiation and the declaration produced by
the corresponding substitution into the templated function, variable, or class
are two declarations of the same entity.
[Note 1: These declarations are required to have matching types as specified
in [basic.link], except as specified in [except.spec].
[Example 2:
template<typename T> T var = {};
template float var<float>; // OK, instantiated variable has type float
template int var<int[16]>[]; // OK, absence of major array bound is
permitted
template int *var<int>; // error: instantiated variable has type
int
template<typename T> auto av = T();
template int av<int>; // OK, variable with type int can be
redeclared with type auto
template<typename T> auto f() {}
template void f<int>(); // error: function with deduced return type
// redeclared with non-deduced return type
([dcl.spec.auto])
— end example]
— end note]
http://eel.is/c++draft/temp.explicit#6
and per [basic.link]/11.1, these two declarations of a entity which is a
variable shall declare the variable to be of the same (function pointer) type:
For any two declarations of an entity E :
(11.1) If one declares E to be a variable or function, the other shall
declare E as one of the same type.
http://eel.is/c++draft/basic.link#11.1
There isn't a "no diagnostic required" exception for this case in either
[temp.explicit] or [basic.link], and clang and ICC (EDG front end) both
correctly detect this error:
clang:
<source>:2:10: error: type 'void (*)(int)' of explicit instantiation of 'TF'
does not match expected type 'void (*)()'
template void (*TF<int>)(int);
^
<source>:1:29: note: variable template 'TF' declared here
template<typename T> void (*TF)();
^
ICC:
<source>(2): error: declaration is incompatible with "void (*TF)() [with
T=int]" (declared at line 1)
template void (*TF<int>)(int);
^
https://gcc.godbolt.org/z/qa6395
[NB: MSVC also incorrectly fails to detect this error, and I have reported this
in the context of a separate bug in its handling of variable templates:
https://developercommunity2.visualstudio.com/t/Incorrect-C3410-on-explicit-instantiatio/1301005
where it doesn't handle the correct code:
template<typename T> void (*TF)(int);
template void (*TF<int>)(int);
!]
g++ should issue a diagnostic for this, similarly to clang and ICC/EDG.
Thanks, Andrew R
