c++: Diagnose [basic.scope.block]/2 violations even in compound-stmt of function-try-block [PR52953]

As the following testcase shows, while check_local_shadow diagnoses most of
the [basic.scope.block]/2 violations, it doesn't diagnose when parameter's
name is redeclared inside of the compound-stmt of a function-try-block.

There is in that case an extra scope (sk_try with parent artificial
sk_block with for FUNCTION_NEEDS_BODY_BLOCK another sk_block and only then
sk_function_param).

The in_function_try_handler case doesn't work correctly
void
foo (int x)
try {
}
catch (int)
{
  try {
  } catch (int x)
  {
  }
  try {
  } catch (int)
  {
    int x;
  }
}
(which is valid) is rejected, because
                || (TREE_CODE (old) == PARM_DECL
                    && (current_binding_level->kind == sk_catch
                        || current_binding_level->level_chain->kind == sk_catch)
                    && in_function_try_handler))
is true but nothing verified that for the first case
current_binding_level->level_chain->kind == sk_function_params
(with perhaps artificial scopes in between and in the latter case
with one extra level in between).

The patch also changes behavior where for catch handlers of function-try-block
the diagnostics will have the shadows function parameter wording as pedwarn
rather than the old redeclaration permerror.

2023-09-05  Jakub Jelinek  <jakub@redhat.com>

	PR c++/52953
	* name-lookup.h (struct cp_binding_level): Add artificial bit-field.
	Formatting fixes.
	* name-lookup.cc (check_local_shadow): Skip artificial bindings when
	checking if parameter scope is parent scope.  Don't special case
	FUNCTION_NEEDS_BODY_BLOCK.  Diagnose the in_function_try_handler
	cases in the b->kind == sk_function_parms test and verify no
	non-artificial intervening scopes.  Add missing auto_diagnostic_group.
	* decl.cc (begin_function_body): Set
	current_binding_level->artificial.
	* semantics.cc (begin_function_try_block): Likewise.

	* g++.dg/diagnostic/redeclaration-1.C: Expect different diagnostic
	wording.
	* g++.dg/diagnostic/redeclaration-3.C: New test.
	* g++.dg/parse/pr31952-1.C: Expect different diagnostic wording.
	* g++.dg/parse/pr31952-3.C: Likewise.

4 files changed, 22 insertions, 17 deletions

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 89e8b85..255c402 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -18002,6 +18002,7 @@ begin_function_body (void)
     keep_next_level (true);
 
   tree stmt = begin_compound_stmt (BCS_FN_BODY);
+  current_binding_level->artificial = 1;
 
   if (processing_template_decl)
     /* Do nothing now.  */;
diff --git a/gcc/cp/name-lookup.cc b/gcc/cp/name-lookup.cc
index 2d74756..a2bcd4f 100644
--- a/gcc/cp/name-lookup.cc
+++ b/gcc/cp/name-lookup.cc
@@ -3146,14 +3146,20 @@ check_local_shadow (tree decl)
 	     them there.  */
 	  cp_binding_level *b = current_binding_level->level_chain;
 
-	  if (FUNCTION_NEEDS_BODY_BLOCK (current_function_decl))
-	    /* Skip the ctor/dtor cleanup level.  */
+	  if (in_function_try_handler && b->kind == sk_catch)
+	    b = b->level_chain;
+
+	  /* Skip artificially added scopes which aren't present
+	     in the C++ standard, e.g. for function-try-block or
+	     ctor/dtor cleanups.  */
+	  while (b->artificial)
 	    b = b->level_chain;
 
 	  /* [basic.scope.param] A parameter name shall not be redeclared
 	     in the outermost block of the function definition.  */
 	  if (b->kind == sk_function_parms)
 	    {
+	      auto_diagnostic_group d;
 	      error_at (DECL_SOURCE_LOCATION (decl),
 			"declaration of %q#D shadows a parameter", decl);
 	      inform (DECL_SOURCE_LOCATION (old),
@@ -3194,17 +3200,10 @@ check_local_shadow (tree decl)
 	 shall not be redeclared in the outermost block of the handler.
 	 3.3.3/2:  A parameter name shall not be redeclared (...) in
 	 the outermost block of any handler associated with a
-	 function-try-block.
-	 3.4.1/15: The function parameter names shall not be redeclared
-	 in the exception-declaration nor in the outermost block of a
-	 handler for the function-try-block.  */
-      else if ((TREE_CODE (old) == VAR_DECL
-		&& old_scope == current_binding_level->level_chain
-		&& old_scope->kind == sk_catch)
-	       || (TREE_CODE (old) == PARM_DECL
-		   && (current_binding_level->kind == sk_catch
-		       || current_binding_level->level_chain->kind == sk_catch)
-		   && in_function_try_handler))
+	 function-try-block.  */
+      else if (TREE_CODE (old) == VAR_DECL
+	       && old_scope == current_binding_level->level_chain
+	       && old_scope->kind == sk_catch)
 	{
 	  auto_diagnostic_group d;
 	  if (permerror (DECL_SOURCE_LOCATION (decl),
diff --git a/gcc/cp/name-lookup.h b/gcc/cp/name-lookup.h
index 613745b..14e7915 100644
--- a/gcc/cp/name-lookup.h
+++ b/gcc/cp/name-lookup.h
@@ -292,11 +292,11 @@ struct GTY(()) cp_binding_level {
       only valid if KIND == SK_TEMPLATE_PARMS.  */
   BOOL_BITFIELD explicit_spec_p : 1;
 
-  /* true means make a BLOCK for this level regardless of all else.  */
+  /* True means make a BLOCK for this level regardless of all else.  */
   unsigned keep : 1;
 
   /* Nonzero if this level can safely have additional
-      cleanup-needing variables added to it.  */
+     cleanup-needing variables added to it.  */
   unsigned more_cleanups_ok : 1;
   unsigned have_cleanups : 1;
 
@@ -308,9 +308,13 @@ struct GTY(()) cp_binding_level {
   unsigned defining_class_p : 1;
 
   /* True for SK_FUNCTION_PARMS of a requires-expression.  */
-  unsigned requires_expression: 1;
+  unsigned requires_expression : 1;
 
-  /* 22 bits left to fill a 32-bit word.  */
+  /* True for artificial blocks which should be ignored when finding
+     parent scope.  */
+  unsigned artificial : 1;
+
+  /* 21 bits left to fill a 32-bit word.  */
 };
 
 /* The binding level currently in effect.  */
diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index 5b539ce..0f7f4e8 100644
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -1624,6 +1624,7 @@ begin_function_try_block (tree *compound_stmt)
   /* This outer scope does not exist in the C++ standard, but we need
      a place to put __FUNCTION__ and similar variables.  */
   *compound_stmt = begin_compound_stmt (0);
+  current_binding_level->artificial = 1;
   r = begin_try_block ();
   FN_TRY_BLOCK_P (r) = 1;
   return r;