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

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); // rust-lang#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 rust-lang#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 (rust-lang#2)
   and here rust-lang#2 isn't found in constexpr_fundef_table, because
   we're working on the BIND_EXPR copy where rust-lang#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): Use
	cp_get_fndecl_from_callee.
	* cvt.cc (cp_get_fndecl_from_callee): If there's a
	DECL_LOCAL_DECL_ALIAS, use it.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-redeclaration3.C: New test.
	* g++.dg/cpp0x/constexpr-redeclaration4.C: New test.

gcc/cp/constexpr.cc

@@ -702,16 +702,14 @@ build_constexpr_constructor_member_initializers (tree type, tree body)
 
 /* We have an expression tree T that represents a call, either CALL_EXPR
    or AGGR_INIT_EXPR.  If the call is lexically to a named function,
-   retrun the _DECL for that function.  */
+   return the _DECL for that function.  */
 
 static tree
 get_function_named_in_call (tree t)
 {
-  tree fun = cp_get_callee (t);
-  if (fun && TREE_CODE (fun) == ADDR_EXPR
-      && TREE_CODE (TREE_OPERAND (fun, 0)) == FUNCTION_DECL)
-    fun = TREE_OPERAND (fun, 0);
-  return fun;
+  tree callee = cp_get_callee (t);
+  tree fun = cp_get_fndecl_from_callee (callee, /*fold*/false);
+  return fun ? fun : callee;
 }
 
 /* Subroutine of check_constexpr_fundef.  BODY is the body of a function

gcc/cp/cvt.cc

@@ -1001,8 +1001,22 @@ cp_get_fndecl_from_callee (tree fn, bool fold /* = true */)
 {
   if (fn == NULL_TREE)
     return fn;
+
+  /* We evaluate constexpr functions on the original, pre-genericization
+     bodies.  So block-scope extern declarations have not been mapped to
+     declarations in outer scopes.  Use the namespace-scope declaration,
+     if any, so that retrieve_constexpr_fundef can find it (PR111132).  */
+  auto fn_or_local_alias = [] (tree f)
+    {
+      if (DECL_LOCAL_DECL_P (f))
+	if (tree alias = DECL_LOCAL_DECL_ALIAS (f))
+	  if (alias != error_mark_node)
+	    return alias;
+      return f;
+    };
+
   if (TREE_CODE (fn) == FUNCTION_DECL)
-    return fn;
+    return fn_or_local_alias (fn);
   tree type = TREE_TYPE (fn);
   if (type == NULL_TREE || !INDIRECT_TYPE_P (type))
     return NULL_TREE;
@@ -1013,7 +1027,7 @@ cp_get_fndecl_from_callee (tree fn, bool fold /* = true */)
       || TREE_CODE (fn) == FDESC_EXPR)
     fn = TREE_OPERAND (fn, 0);
   if (TREE_CODE (fn) == FUNCTION_DECL)
-    return fn;
+    return fn_or_local_alias (fn);
   return NULL_TREE;
 }
 

gcc/testsuite/g++.dg/cpp0x/constexpr-redeclaration3.C

@@ -0,0 +1,13 @@
+// PR c++/111132
+// { dg-do compile { target c++11 } }
+
+constexpr bool bar(void) {
+    return true;
+}
+
+constexpr bool foo() {
+    constexpr bool bar(void);
+    return bar();
+}
+
+static_assert(foo(), "");

gcc/testsuite/g++.dg/cpp0x/constexpr-redeclaration4.C

@@ -0,0 +1,14 @@
+// PR c++/111132
+// { dg-do compile { target c++11 } }
+
+constexpr bool bar(void) {
+    return true;
+}
+
+constexpr bool bar(void);
+
+constexpr bool foo() {
+    return bar();
+}
+
+static_assert(foo(), "");