[Ada] Crash on improper pragma Weak_External

[Pierre-Marie de Rodat <derodat@adacore.com>]

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This patch adds a guard on the use of pragma Weak_External. This pragma
affects link-time addresses of entities, and does not apply to types.
Previous to this patch the compiler would abort on a misuse of the
pragma.

Tested on x86_64-pc-linux-gnu, committed on trunk

2019-07-01  Ed Schonberg  <schonberg@adacore.com>

gcc/ada/

	* sem_prag.adb (Analyze_Pragma, case Weak_External): Pragma only
	applies to entities with run-time addresses, not to types.

gcc/testsuite/

	* gnat.dg/weak3.adb, gnat.dg/weak3.ads: New testcase.

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--- gcc/ada/sem_prag.adb
+++ gcc/ada/sem_prag.adb
@@ -25608,6 +25608,12 @@ package body Sem_Prag is
                Ent := Underlying_Type (Ent);
             end if;
 
+            --  The pragma applies to entities with addresses.
+
+            if Is_Type (Ent) then
+               Error_Pragma ("pragma applies to objects and subprograms");
+            end if;
+
             --  The only processing required is to link this item on to the
             --  list of rep items for the given entity. This is accomplished
             --  by the call to Rep_Item_Too_Late (when no error is detected

--- /dev/null
new file mode 100644
+++ gcc/testsuite/gnat.dg/weak3.adb
@@ -0,0 +1,11 @@
+--  { dg-do compile }
+
+package body Weak3 is
+
+   type T is new Integer;
+   pragma Weak_External (T); --  { dg-error "pragma applies to objects and subprograms" }
+   X : T;
+
+   procedure Foo is null;
+
+end Weak3;

--- /dev/null
new file mode 100644
+++ gcc/testsuite/gnat.dg/weak3.ads
@@ -0,0 +1,3 @@
+package Weak3 is
+   procedure Foo;
+end Weak3;