[Ada] Spurious error on incomplete tagged formal parameter

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

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This patch fixes an issue whereby a check for competing controlling
formals led to a spurious dispatching error due to an incomplete type
being used within a subprogram specification.

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

2019-07-04  Justin Squirek  <squirek@adacore.com>

gcc/ada/

	* sem_disp.adb (Check_Controlling_Formals): Obtain the full view
	before type comparison.

gcc/testsuite/

	* gnat.dg/tagged2.adb, gnat.dg/tagged2.ads: New testcase.

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--- gcc/ada/sem_disp.adb
+++ gcc/ada/sem_disp.adb
@@ -210,6 +210,14 @@ package body Sem_Disp is
          Ctrl_Type := Check_Controlling_Type (Etype (Formal), Subp);
 
          if Present (Ctrl_Type) then
+            --  Obtain the full type in case we are looking at an incomplete
+            --  view.
+
+            if Ekind (Ctrl_Type) = E_Incomplete_Type
+              and then Present (Full_View (Ctrl_Type))
+            then
+               Ctrl_Type := Full_View (Ctrl_Type);
+            end if;
 
             --  When controlling type is concurrent and declared within a
             --  generic or inside an instance use corresponding record type.

--- /dev/null
new file mode 100644
+++ gcc/testsuite/gnat.dg/tagged2.adb
@@ -0,0 +1,9 @@
+--  { dg-do compile }
+
+package body Tagged2 is
+
+   procedure Get_Parent
+     (DeviceX : Device;
+      Parent  : out Device) is null;
+
+end Tagged2;

--- /dev/null
new file mode 100644
+++ gcc/testsuite/gnat.dg/tagged2.ads
@@ -0,0 +1,9 @@
+package Tagged2 is
+   type Device;
+
+   procedure Get_Parent
+     (DeviceX : Device;
+      Parent  : out Device);
+
+   type Device is tagged null record;
+end Tagged2;