Re: [PATCH] c++: constexpr error with fn redecl in local scope [PR111132]

[Marek Polacek <polacek@redhat.com>]

On Wed, Apr 03, 2024 at 01:14:46PM -0400, Jason Merrill wrote:
> On 4/2/24 13:52, Marek Polacek wrote:
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/13?
> > 
> > -- >8 --
> > We evaluate constexpr functions on the original, pre-genericization bodies.
> > That means that the function body we're evaluating will not have gone
> > through cp_genericize_r's "Map block scope extern declarations to visible
> > declarations with the same name and type in outer scopes if any".  Here:
> > 
> >    constexpr bool bar() { return true; } // #1
> >    constexpr bool foo() {
> >      constexpr bool bar(void); // #2
> >      return bar();
> >    }
> > 
> > it means that we:
> > 1) register_constexpr_fundef (#1)
> > 2) cp_genericize (#1)
> >     nothing interesting happens
> > 3) register_constexpr_fundef (foo)
> >     does copy_fn, so we have two copies of the BIND_EXPR
> > 4) cp_genericize (foo)
> >     this remaps #2 to #1, but only on one copy of the BIND_EXPR
> > 5) retrieve_constexpr_fundef (foo)
> >     we find it, no problem
> > 6) retrieve_constexpr_fundef (#2)
> >     and here #2 isn't found in constexpr_fundef_table, because
> >     we're working on the BIND_EXPR copy where #2 wasn't mapped to #1
> >     so we fail.  We've only registered #1.
> > 
> > It should work to use DECL_LOCAL_DECL_ALIAS (which used to be
> > extern_decl_map).  We evaluate constexpr functions on pre-cp_fold
> > bodies to avoid diagnostic problems, but the remapping I'm proposing
> > should not interfere with diagnostics.
> > 
> > This is not a problem for a global scope redeclaration; there we go
> > through duplicate_decls which keeps the DECL_UID:
> >    DECL_UID (olddecl) = olddecl_uid;
> > and DECL_UID is what constexpr_fundef_hasher::hash uses.
> > 
> > 	PR c++/111132
> > 
> > gcc/cp/ChangeLog:
> > 
> > 	* constexpr.cc (get_function_named_in_call): If there's
> > 	a DECL_LOCAL_DECL_ALIAS, use it.
> 
> Perhaps this function should use cp_get_fndecl_from_callee, and this change
> should be made there instead?

It doesn't seem that get_function_named_in_call can use cp_get_fndecl_from_callee,
(or be replaced with cp_get_callee_fndecl_nofold).  We can get e.g. a CALL_EXPR
whose CALL_EXPR_FN is a TEMPLATE_ID_EXPR, and get_function_named_in_call
returns the TEMPLATE_ID_EXPR whereas cp_get_fndecl_from_callee would return
null:

  if (TREE_CODE (fn) == FUNCTION_DECL)
    return fn;
  tree type = TREE_TYPE (fn);
  if (type == NULL_TREE || !INDIRECT_TYPE_P (type))
    return NULL_TREE;
    
Marek