[Ada] Avoid unwanted warnings for statically-known-successful assertions

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

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The -gnatwc switch enables warnings for test condition outcomes that are
known at compile time. Such warnings are unlikely to be useful in the
case of an assertion expression (or a subexpression thereof), so do not
generate them in that case.

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

gcc/ada/

	* sem_warn.adb (Warn_On_Constant_Valid_Condition): Do not
	generate a warning if the expression in question is an assertion
	expression, or a subexpression thereof. But do call
	Test_Comparison so that it can generate warnings for the cases
	that it generates warnings for.
	* sem_prag.ads: Modify Assertion_Expression_Pragma constant so
	that the predicate Sem_Util.In_Assertion_Expression_Pragma
	returns True for the expression of a Compile_Time_Error pragma.

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diff --git a/gcc/ada/sem_prag.ads b/gcc/ada/sem_prag.ads
--- a/gcc/ada/sem_prag.ads
+++ b/gcc/ada/sem_prag.ads
@@ -135,6 +135,7 @@ package Sem_Prag is
       Pragma_Assert_And_Cut            => True,
       Pragma_Assume                    => True,
       Pragma_Check                     => True,
+      Pragma_Compile_Time_Error        => True,
       Pragma_Contract_Cases            => True,
       Pragma_Default_Initial_Condition => True,
       Pragma_Initial_Condition         => True,


diff --git a/gcc/ada/sem_warn.adb b/gcc/ada/sem_warn.adb
--- a/gcc/ada/sem_warn.adb
+++ b/gcc/ada/sem_warn.adb
@@ -3401,9 +3401,14 @@ package body Sem_Warn is
             False_Result => False_Result);
 
          --  Warn on a possible evaluation to False / True in the presence of
-         --  invalid values.
+         --  invalid values. But issue no warning for an assertion expression
+         --  (or a subexpression thereof); in particular, we don't want a
+         --  warning about an assertion that will always succeed.
 
-         if True_Result then
+         if In_Assertion_Expression_Pragma (Op) then
+            null;
+
+         elsif True_Result then
             Error_Msg_N
               ("condition can only be False if invalid values present?c?", Op);