[Ada] Misleading warning on unresolvable package name

This patch fixes an issue whereby the compiler misidentified a package name
containing the name of a standard runtime package as said package - leading to
and improper error message prompting the user to "With" a package already in
scope.

2018-05-28  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* sem_ch8.adb (Find_Expanded_Name): Add extra guard to make sure the
	misresolved package name is not a case of mistaken identity.

gcc/testsuite/

	* gnat.dg/warn15-core-main.adb, gnat.dg/warn15-core.ads,
	gnat.dg/warn15-interfaces.ads, gnat.dg/warn15.ads: New testcase.

From-SVN: r260817

gcc/ada/ChangeLog

+2018-05-28  Justin Squirek  <squirek@adacore.com>
+
+	* sem_ch8.adb (Find_Expanded_Name): Add extra guard to make sure the
+	misresolved package name is not a case of mistaken identity.
+
 2018-05-28  Yannick Moy  <moy@adacore.com>
 
 	* sem_res.adb (Resolve_Range): Re-resolve the low bound of a range in

gcc/ada/sem_ch8.adb

@@ -6336,7 +6336,11 @@ package body Sem_Ch8 is
                --  If this is a selection from Ada, System or Interfaces, then
                --  we assume a missing with for the corresponding package.
 
-               if Is_Known_Unit (N) then
+               if Is_Known_Unit (N)
+                 and then not (Present (Entity (Prefix (N)))
+                                and then Scope (Entity (Prefix (N))) /=
+                                           Standard_Standard)
+               then
                   if not Error_Posted (N) then
                      Error_Msg_Node_2 := Selector;
                      Error_Msg_N -- CODEFIX

gcc/testsuite/ChangeLog

+2018-05-28  Justin Squirek  <squirek@adacore.com>
+
+	* gnat.dg/warn15-core-main.adb, gnat.dg/warn15-core.ads,
+	gnat.dg/warn15-interfaces.ads, gnat.dg/warn15.ads: New testcase.
+
 2018-05-27  Steven G. Kargl  <kargl@gcc.gnu.org>
 
 	* gfortran.dg/data_stmt_pointer.f90: new test.

gcc/testsuite/gnat.dg/warn15-core-main.adb

+--  { dg-do compile }
+
+with Interfaces.C;
+
+procedure Warn15.Core.Main is
+   use type Interfaces.C.unsigned;  --  { dg-error "\"C\" not declared in \"Interfaces\"" }
+begin
+   null;
+end Warn15.Core.Main;

gcc/testsuite/gnat.dg/warn15-core.ads

+with Warn15.Interfaces;
+
+package Warn15.Core is
+end Warn15.Core;

gcc/testsuite/gnat.dg/warn15-interfaces.ads

+package Warn15.Interfaces is
+end Warn15.Interfaces;
+

gcc/testsuite/gnat.dg/warn15.ads

+package Warn15 is
+end Warn15;