Re: [RFC] c++: Print function template parms when relevant (was: [PATCH v4] c++: Add gnu::diagnose_as attribute)

[Matthias Kretz <m.kretz@gsi.de>]

On Friday, 19 November 2021 10:53:27 CET Matthias Kretz wrote:
> > >> Ah, you're trying to omit defaulted parms from the <list>?  I'm not
> > >> sure
> > >> that's necessary, leaving them out of the [with ...] list should be
> > >> sufficient.
> > >
> > > I was thinking about all the std::allocator defaults in the standard
> > > library. I don't want to see them. E.g. vector<int>::clear() on const
> > > object:
> > > 
> > > error: passing 'const std::vector<int>' as 'this' argument discards
> > > qualifiers [...]/stl_vector.h:1498:7: note:   in call to 'void
> > > std::vector<_Tp, _Alloc>::clear() [with _Tp = int; _Alloc =
> > > std::allocator<int>]'
> > > 
> > > With my patch the last line becomes
> > > [...]/stl_vector.h:1498:7: note:   in call to 'void
> > > std::vector<_Tp>::clear() [with _Tp = int]'
> > > 
> > > 
> > > Another case I didn't consider before:
> > > 
> > > template <class T, class U = int> struct A {
> > > 
> > > [[deprecated]] void f(U);
> > > 
> > > };
> > > 
> > > A<float> a; a.f(1);
> > > 
> > > With my patch it prints 'void A<T>::f(U) [with T = float]', with your
> > > suggestion 'void A<T, U>::f(U) [with T = float]'. Both are missing
> > > important information in the substitution list, IMHO. Would 'void A<T, U
> > > = int>::f(U) [with T = float]' be an improvement? Or should
> > > find_typenames (in cp/error.c) find defaulted template parms and add
> > > them
> > > to its list? IIUC find_typenames would find all template parms and
> > > couldn't know whether they're defaulted.
> > 
> > That sounds good: omit defaulted parms only if they don't appear in the
> > signature (other than as another default template argument).
> 
> Let me check whether I have the right idea:
> 
> I could extend find_typenames (which walks the complete) tree to look for
> TEMPLATE_TYPE_PARM (and the 3 others I don't recall right now). But since
> that walks the *complete* tree, it'll simply find all parms with no
> indication whether they appear in the signature. Ideas:
> 
> 1. Count occurrences: with 2 occurrences, one of them must be a use in the
> signature.
> 
> 2. Walk only over TYPE_ARG_TYPES (TREE_TYPE (DECL_TEMPLATE_RESULT (fn))) to
> collect TEMPLATE_TYPE_PARMs.

I tried the latter:

@@ -1641,8 +1652,11 @@ dump_substitution (cxx_pretty_printer *pp,                              
       && !(flags & TFF_NO_TEMPLATE_BINDINGS))                                                 
     {
       vec<tree, va_gc> *typenames = t ? find_typenames (t) : NULL;                            
-      dump_template_bindings (pp, template_parms, template_args, typenames,                   
-			      flags);                                                          
+      tree fn_arguments = TYPE_ARG_TYPES (TREE_TYPE (DECL_TEMPLATE_RESULT 
(t)));              
+      tree used_template_parms = find_template_parameters (fn_arguments,                      
+							   template_parms);                    
+      dump_template_bindings (pp, template_parms, template_args,                              
+			      used_template_parms, typenames, flags);                          
     }
 }

Now in dump_template_bindings it skips all defaulted template_parms that are 
not in used_template_parms. Makes this test pass:

template <class T>                                                              
  struct id                                                                     
  { using type = T; };                                                          
                                                                                
template <class T0, class T1 = int>                                             
  struct A                                                                      
  {                                                                             
    template <class U0 = const T1&>                                             
      [[deprecated]] static void                                                
      f(typename id<U0>::type);                                                 
  };                                                                            
                                                                                
int main()                                                                      
{                                                                               
  A<int>::f(0); // { dg-warning "'static void A<T0>::f\\(typename 
id<U0>::type\\) .with U0 = const int&; T0 = int; typename id<U0>::type = const 
int&.'" }
}

-- 
──────────────────────────────────────────────────────────────────────────
 Dr. Matthias Kretz                           https://mattkretz.github.io
 GSI Helmholtz Centre for Heavy Ion Research               https://gsi.de
 stdₓ::simd
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