[Bug c++/84573] New: missing warning on an uninstantiated function template returning T with no return statement

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84573

            Bug ID: 84573
           Summary: missing warning on an uninstantiated function template
                    returning T with no return statement
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

G++ diagnoses a return statement with a value in a function template returning
void even when the template is never instantiated, but it doesn't issue a
warning when a function template declared to return the template argument T. 
Since the only valid specialization of such a template is one where T = void
it's likely that the missing return statement is a mistake that issuing a
warning for would be helpful.

$ cat z.C && gcc -Wall -Wextra -S z.C
template <class T>
void f () { return 1; }   // error: return-statement with a value

template <class T>
T g () { }   // missing warning

z.C: In function ‘void f()’:
z.C:2:20: error: return-statement with a value, in function returning ‘void’
[-fpermissive]
 void f () { return 1; }   // error: return-statement with a value
                    ^