[Ada] Spurious ambiguity error on call returning an access type

If F is a function with a single defaulted parameter that returns an
access_to_array type, then F (I) may designate either the return type or
an indexing of the result of the call, after implicit dereferencing.  If
the component type C of the array type AR is accces AR this is ambiguous
in a context whose expected type is C. If F is parameterless the call is
not ambiguous.

2018-08-21  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_res.adb (Resolve_Call): Resolve correctly a parameterless
	call that returns an access type whose designated type is the
	component type of an array, when the function has no defaulted
	parameters.

gcc/testsuite/

	* gnat.dg/access5.adb, gnat.dg/access5.ads: New testcase.

From-SVN: r263726

gcc/ada/ChangeLog

+2018-08-21  Ed Schonberg  <schonberg@adacore.com>
+
+	* sem_res.adb (Resolve_Call): Resolve correctly a parameterless
+	call that returns an access type whose designated type is the
+	component type of an array, when the function has no defaulted
+	parameters.
+
 2018-08-21  Yannick Moy  <moy@adacore.com>
 
 	* doc/gnat_ugn/building_executable_programs_with_gnat.rst:

gcc/ada/sem_res.adb

@@ -6128,7 +6128,18 @@ package body Sem_Res is
             Ret_Type   : constant Entity_Id := Etype (Nam);
 
          begin
-            if Is_Access_Type (Ret_Type)
+            --  If this is a parameterless call there is no ambiguity
+            --  and the call has the type of the function.
+
+            if No (First_Actual (N)) then
+               Set_Etype (N, Etype (Nam));
+               if Present (First_Formal (Nam)) then
+                  Resolve_Actuals (N, Nam);
+               end if;
+               Build_Call_Marker (N);
+
+            elsif Is_Access_Type (Ret_Type)
+
               and then Ret_Type = Component_Type (Designated_Type (Ret_Type))
             then
                Error_Msg_N

gcc/testsuite/ChangeLog

+2018-08-21  Ed Schonberg  <schonberg@adacore.com>
+
+	* gnat.dg/access5.adb, gnat.dg/access5.ads: New testcase.
+
 2018-08-21  Eric Botcazou  <ebotcazou@adacore.com>
 
 	* gnat.dg/rep_clause7.adb: New testcase.

gcc/testsuite/gnat.dg/access5.adb

+--  { dg-do compile }
+
+package body Access5 is
+  procedure Dummy is null;
+end;

gcc/testsuite/gnat.dg/access5.ads

+package Access5 is
+  type Vec;
+  type Ptr is access all Vec;
+  type Vec is array (1..3) of Ptr;
+  function F return Ptr;
+  pragma Import (Ada, F);
+  Tail : Vec := (F, F, F);
+
+  procedure Dummy;
+end;