c++: Small tweak to can_convert_eh [PR81660]

While messing with check_handlers_1, I spotted this bug report which
complains that we don't warn about the case when we have two duplicated
handlers of type int.  can_convert_eh implements [except.handle] and
that says: A handler is a match for an exception object of type E if
 - The handler is of type cv T or cv T& and E and T are the same type
   (ignoring the top-level cv-qualifiers), or [...]

but we don't implement this bullet properly for non-class types.  The
fix therefore seems pretty obvious.  Also change the return type to
bool when we're only returning yes/no.

gcc/cp/ChangeLog:

	PR c++/81660
	* except.c (can_convert_eh): Change the return type to bool.  If
	the type TO and FROM are the same, return true.

gcc/testsuite/ChangeLog:

	PR c++/81660
	* g++.dg/warn/Wexceptions3.C: New test.
	* g++.dg/eh/pr42859.C: Add dg-warning.
	* g++.dg/torture/pr81659.C: Likewise.

gcc/cp/except.c

@@ -41,7 +41,6 @@ static tree do_allocate_exception (tree);
 static tree wrap_cleanups_r (tree *, int *, void *);
 static int complete_ptr_ref_or_void_ptr_p (tree, tree);
 static bool is_admissible_throw_operand_or_catch_parameter (tree, bool);
-static int can_convert_eh (tree, tree);
 
 /* Sets up all the global eh stuff that needs to be initialized at the
    start of compilation.  */
@@ -932,31 +931,34 @@ nothrow_libfn_p (const_tree fn)
 /* Returns nonzero if an exception of type FROM will be caught by a
    handler for type TO, as per [except.handle].  */
 
-static int
+static bool
 can_convert_eh (tree to, tree from)
 {
   to = non_reference (to);
   from = non_reference (from);
 
+  if (same_type_ignoring_top_level_qualifiers_p (to, from))
+    return true;
+
   if (TYPE_PTR_P (to) && TYPE_PTR_P (from))
     {
       to = TREE_TYPE (to);
       from = TREE_TYPE (from);
 
       if (! at_least_as_qualified_p (to, from))
-	return 0;
+	return false;
 
       if (VOID_TYPE_P (to))
-	return 1;
+	return true;
 
       /* Else fall through.  */
     }
 
   if (CLASS_TYPE_P (to) && CLASS_TYPE_P (from)
       && publicly_uniquely_derived_p (to, from))
-    return 1;
+    return true;
 
-  return 0;
+  return false;
 }
 
 /* Check whether any of the handlers in I are shadowed by another handler

gcc/testsuite/g++.dg/eh/pr42859.C

@@ -13,7 +13,7 @@ ptw32_terminate (void)
     catch (int)
     {
     }
-    catch (int)
+    catch (int) // { dg-warning "will be caught by earlier handler" }
     {
     }
   }

gcc/testsuite/g++.dg/torture/pr81659.C

@@ -12,7 +12,7 @@ a (int b)
   catch (int)
     {
     }
-  catch (int)
+  catch (int) // { dg-warning "will be caught by earlier handler" }
     {
     }
 }

gcc/testsuite/g++.dg/warn/Wexceptions3.C

@@ -0,0 +1,29 @@
+// PR c++/81660
+
+void bar (int);
+
+void
+fn (int b)
+{
+  if (b)
+    throw;
+  try
+    {
+      bar (3);
+    }
+  catch (int)
+    {
+    }
+  catch (int) // { dg-warning "will be caught by earlier handler" }
+    {
+    }
+  catch (const int) // { dg-warning "will be caught by earlier handler" }
+    {
+    }
+  catch (int &) // { dg-warning "will be caught by earlier handler" }
+    {
+    }
+  catch (const int &) // { dg-warning "will be caught by earlier handler" }
+    {
+    }
+}