c++: Add -Wexceptions warning option [PR97675]

This PR asks that we add a warning option for an existing (very old)
warning, so that it can be disabled selectively.  clang++ uses
-Wexceptions for this, so I added this new option rather than using
e.g. -Wnoexcept.

gcc/c-family/ChangeLog:

	PR c++/97675
	* c.opt (Wexceptions): New option.

gcc/cp/ChangeLog:

	PR c++/97675
	* except.c (check_handlers_1): Use OPT_Wexceptions for the
	warning.  Use inform for the second part of the warning.

gcc/ChangeLog:

	PR c++/97675
	* doc/invoke.texi: Document -Wexceptions.

gcc/testsuite/ChangeLog:

	PR c++/97675
	* g++.old-deja/g++.eh/catch10.C: Adjust dg-warning.
	* g++.dg/warn/Wexceptions1.C: New test.
	* g++.dg/warn/Wexceptions2.C: New test.

gcc/c-family/c.opt

@@ -579,6 +579,10 @@ Werror-implicit-function-declaration
 C ObjC RejectNegative Warning Alias(Werror=, implicit-function-declaration)
 This switch is deprecated; use -Werror=implicit-function-declaration instead.
 
+Wexceptions
+C++ ObjC++ Var(warn_exceptions) Init(1)
+Warn when an exception handler is shadowed by another handler.
+
 Wextra
 C ObjC C++ ObjC++ Warning
 ; in common.opt

gcc/cp/except.c

@@ -975,11 +975,10 @@ check_handlers_1 (tree master, tree_stmt_iterator i)
       tree handler = tsi_stmt (i);
       if (TREE_TYPE (handler) && can_convert_eh (type, TREE_TYPE (handler)))
 	{
-	  warning_at (EXPR_LOCATION (handler), 0,
-		      "exception of type %qT will be caught",
-		      TREE_TYPE (handler));
-	  warning_at (EXPR_LOCATION (master), 0,
-		      "   by earlier handler for %qT", type);
+	  if (warning_at (EXPR_LOCATION (handler), OPT_Wexceptions,
+			  "exception of type %qT will be caught by earlier "
+			  "handler", TREE_TYPE (handler)))
+	    inform (EXPR_LOCATION (master), "for type %qT", type);
 	  break;
 	}
     }

gcc/doc/invoke.texi

@@ -240,7 +240,7 @@ in the following sections.
 -Wctor-dtor-privacy  -Wno-delete-incomplete @gol
 -Wdelete-non-virtual-dtor  -Wdeprecated-copy  -Wdeprecated-copy-dtor @gol
 -Wno-deprecated-enum-enum-conversion -Wno-deprecated-enum-float-conversion @gol
--Weffc++  -Wextra-semi  -Wno-inaccessible-base @gol
+-Weffc++  -Wno-exceptions -Wextra-semi  -Wno-inaccessible-base @gol
 -Wno-inherited-variadic-ctor  -Wno-init-list-lifetime @gol
 -Wno-invalid-offsetof  -Wno-literal-suffix  -Wmismatched-tags @gol
 -Wmultiple-inheritance  -Wnamespaces  -Wnarrowing @gol
@@ -3739,6 +3739,12 @@ When selecting this option, be aware that the standard library
 headers do not obey all of these guidelines; use @samp{grep -v}
 to filter out those warnings.
 
+@item -Wno-exceptions @r{(C++ and Objective-C++ only)}
+@opindex Wexceptions
+@opindex Wno-exceptions
+Disable the warning about the case when an exception handler is shadowed by
+another handler, which can point out a wrong ordering of exception handlers.
+
 @item -Wstrict-null-sentinel @r{(C++ and Objective-C++ only)}
 @opindex Wstrict-null-sentinel
 @opindex Wno-strict-null-sentinel

gcc/testsuite/g++.dg/warn/Wexceptions1.C

@@ -0,0 +1,9 @@
+// PR c++/97675
+
+struct Base { };
+struct Child : Base { };
+int main() {
+    try { throw Child(); }
+    catch (Base const&) { }
+    catch (Child const&) { } // { dg-warning "exception of type .Child. will be caught by earlier handler" }
+}

gcc/testsuite/g++.dg/warn/Wexceptions2.C

@@ -0,0 +1,10 @@
+// PR c++/97675
+// { dg-additional-options -Wno-exceptions }
+
+struct Base { };
+struct Child : Base { };
+int main() {
+    try { throw Child(); }
+    catch (Base const&) { }
+    catch (Child const&) { } // { dg-bogus "exception of type .Child. will be caught by earlier handler" }
+}

gcc/testsuite/g++.old-deja/g++.eh/catch10.C

@@ -13,8 +13,8 @@ void g()
   catch (A*) { }
 
   try { f(); }
-  catch (A*) { }		// { dg-warning "" } A* before B*
-  catch (B*) { }		// { dg-warning "" } A* before B*
+  catch (A*) { }		// { dg-message "for type" } A* before B*
+  catch (B*) { }		// { dg-warning "will be caught" } A* before B*
 
   try { f(); }
   catch (A*) { }