C++ PATCH to implement P1094R2, Nested inline namespaces

[Marek Polacek <polacek at redhat dot com>]

This patch implements another C++20 feature, nested inline namespaces.

This was fairly simple, one just has to be careful not to blithely consume the
inline keyword, making a non-valid program valid.  Another minor gotcha was
to handle the innermost 'inline' correctly.

Note that 

  inline namespace A::B { ... }

continues to be invalid.

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2018-11-19  Marek Polacek  <polacek@redhat.com>

	Implement P1094R2, Nested inline namespaces.
	* parser.c (cp_parser_namespace_definition): Parse the optional inline
	keyword in a nested-namespace-definition.  Adjust push_namespace call.
	Formatting fix.

	* g++.dg/cpp2a/nested-inline-ns1.C: New test.
	* g++.dg/cpp2a/nested-inline-ns2.C: New test.
	* g++.dg/cpp2a/nested-inline-ns3.C: New test.

diff --git gcc/cp/parser.c gcc/cp/parser.c
index 88fc426102b..8b8304acca7 100644
--- gcc/cp/parser.c
+++ gcc/cp/parser.c
@@ -18864,6 +18864,7 @@ cp_parser_namespace_definition (cp_parser* parser)
   cp_ensure_no_oacc_routine (parser);
 
   bool is_inline = cp_lexer_next_token_is_keyword (parser->lexer, RID_INLINE);
+  const bool topmost_inline_p = is_inline;
 
   if (is_inline)
     {
@@ -18882,6 +18883,17 @@ cp_parser_namespace_definition (cp_parser* parser)
     {
       identifier = NULL_TREE;
       
+      bool nested_inline_p = cp_lexer_next_token_is_keyword (parser->lexer,
+							     RID_INLINE);
+      if (nested_inline_p && nested_definition_count != 0)
+	{
+	  if (cxx_dialect < cxx2a)
+	    pedwarn (cp_lexer_peek_token (parser->lexer)->location,
+		     OPT_Wpedantic, "nested inline namespace definitions only "
+		     "available with -std=c++2a or -std=gnu++2a");
+	  cp_lexer_consume_token (parser->lexer);
+	}
+
       if (cp_lexer_next_token_is (parser->lexer, CPP_NAME))
 	{
 	  identifier = cp_parser_identifier (parser);
@@ -18896,7 +18908,12 @@ cp_parser_namespace_definition (cp_parser* parser)
 	}
 
       if (cp_lexer_next_token_is_not (parser->lexer, CPP_SCOPE))
-	break;
+	{
+	  /* Don't forget that the innermost namespace might have been
+	     marked as inline.  */
+	  is_inline |= nested_inline_p;
+	  break;
+	}
   
       if (!nested_definition_count && cxx_dialect < cxx17)
         pedwarn (input_location, OPT_Wpedantic,
@@ -18905,7 +18922,9 @@ cp_parser_namespace_definition (cp_parser* parser)
 
       /* Nested namespace names can create new namespaces (unlike
 	 other qualified-ids).  */
-      if (int count = identifier ? push_namespace (identifier) : 0)
+      if (int count = (identifier
+		       ? push_namespace (identifier, nested_inline_p)
+		       : 0))
 	nested_definition_count += count;
       else
 	cp_parser_error (parser, "nested namespace name required");
@@ -18918,7 +18937,7 @@ cp_parser_namespace_definition (cp_parser* parser)
   if (nested_definition_count && attribs)
     error_at (token->location,
 	      "a nested namespace definition cannot have attributes");
-  if (nested_definition_count && is_inline)
+  if (nested_definition_count && topmost_inline_p)
     error_at (token->location,
 	      "a nested namespace definition cannot be inline");
 
@@ -18927,7 +18946,7 @@ cp_parser_namespace_definition (cp_parser* parser)
 
   bool has_visibility = handle_namespace_attrs (current_namespace, attribs);
 
-  warning  (OPT_Wnamespaces, "namespace %qD entered", current_namespace);
+  warning (OPT_Wnamespaces, "namespace %qD entered", current_namespace);
 
   /* Look for the `{' to validate starting the namespace.  */
   matching_braces braces;
diff --git gcc/testsuite/g++.dg/cpp2a/nested-inline-ns1.C gcc/testsuite/g++.dg/cpp2a/nested-inline-ns1.C
new file mode 100644
index 00000000000..95b4d3378d1
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/nested-inline-ns1.C
@@ -0,0 +1,26 @@
+// P1094R2
+// { dg-do compile { target c++2a } }
+
+namespace A::inline B::C {
+  int i;
+}
+
+namespace D::E::inline F {
+  int j;
+}
+
+inline namespace X {
+  int x;
+}
+
+// Make sure the namespaces are marked inline.
+void
+g ()
+{
+  A::B::C::i++;
+  A::C::i++;
+  D::E::j++;
+  D::E::F::j++;
+  X::x++;
+  x++;
+}
diff --git gcc/testsuite/g++.dg/cpp2a/nested-inline-ns2.C gcc/testsuite/g++.dg/cpp2a/nested-inline-ns2.C
new file mode 100644
index 00000000000..9b5f2cab47b
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/nested-inline-ns2.C
@@ -0,0 +1,26 @@
+// P1094R2
+// { dg-do compile { target c++2a } }
+
+inline namespace A::B { // { dg-error "a nested namespace definition cannot be inline" }
+  int i;
+}
+
+namespace inline C::D { // { dg-error "expected|does not name a type" }
+  int i;
+}
+
+namespace E::F inline { // { dg-error "expected" }
+  int i;
+}
+
+namespace inline G { // { dg-error "expected|does not name a type" }
+  int i;
+}
+
+inline namespace inline H { // { dg-error "expected|does not name a type" }
+  int i;
+}
+
+inline namespace inline I::J { // { dg-error "expected|does not name a type" }
+  int i;
+}
diff --git gcc/testsuite/g++.dg/cpp2a/nested-inline-ns3.C gcc/testsuite/g++.dg/cpp2a/nested-inline-ns3.C
new file mode 100644
index 00000000000..8b81c1383db
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp2a/nested-inline-ns3.C
@@ -0,0 +1,11 @@
+// P1094R2
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wpedantic" }
+
+namespace A::inline B::C { // { dg-warning "nested inline namespace definitions only" "" { target c++17_down } }
+  int i;
+}
+
+namespace D::E::inline F { // { dg-warning "nested inline namespace definitions only" "" { target c++17_down } }
+  int i;
+}