Introduce null_block_symbol

... in the spirit of null_ptid, null_frame_id, etc.

Fixes two instances of:

  /root/binutils-gdb/gdb/cp-namespace.c: In function 'block_symbol cp_lookup_nested_symbol(type*, const char*, const block*, domain_enum)':
  /root/binutils-gdb/gdb/cp-namespace.c:1010: warning: jump to case label
  /root/binutils-gdb/gdb/cp-namespace.c:1008: error:   crosses initialization of 'block_symbol <anonymous>'

Compiler info:

  Reading specs from /usr/lib/gcc-lib/amd64-unknown-openbsd5.8/4.2.1/specs
  Target: amd64-unknown-openbsd5.8
  Configured with: OpenBSD/amd64 system compiler
  Thread model: posix
  gcc version 4.2.1 20070719

gdb/ChangeLog:
2015-11-17  Pedro Alves  <palves@redhat.com>

	* cp-namespace.c (cp_lookup_bare_symbol)
	(cp_search_static_and_baseclasses, cp_lookup_symbol_via_imports)
	(cp_lookup_symbol_via_all_imports, cp_lookup_nested_symbol_1)
	(cp_lookup_nested_symbol): Use null_block_symbol.
	* d-namespace.c (d_lookup_symbol, d_lookup_nested_symbol)
	(d_lookup_symbol_imports, d_lookup_symbol_module): Use
	null_block_symbol.
	* symtab.c (null_block_symbol): New global.
	* symtab.h (null_block_symbol): Declare.

gdb/ChangeLog

@@ -1,3 +1,15 @@
+2015-11-17  Pedro Alves  <palves@redhat.com>
+
+	* cp-namespace.c (cp_lookup_bare_symbol)
+	(cp_search_static_and_baseclasses, cp_lookup_symbol_via_imports)
+	(cp_lookup_symbol_via_all_imports, cp_lookup_nested_symbol_1)
+	(cp_lookup_nested_symbol): Use null_block_symbol.
+	* d-namespace.c (d_lookup_symbol, d_lookup_nested_symbol)
+	(d_lookup_symbol_imports, d_lookup_symbol_module): Use
+	null_block_symbol.
+	* symtab.c (null_block_symbol): New global.
+	* symtab.h (null_block_symbol): Declare.
+
 2015-11-17  Pedro Alves  <palves@redhat.com>
 
 	* common/common-exceptions.h (GDB_XCPT_SJMP, GDB_XCPT_TRY)

gdb/cp-namespace.c

@@ -208,14 +208,14 @@ cp_lookup_bare_symbol (const struct language_defn *langdef,
 
       lang_this = lookup_language_this (language_def (language_cplus), block);
       if (lang_this.symbol == NULL)
-	return (struct block_symbol) {NULL, NULL};
+	return null_block_symbol;
 
       type = check_typedef (TYPE_TARGET_TYPE (SYMBOL_TYPE (lang_this.symbol)));
       /* If TYPE_NAME is NULL, abandon trying to find this symbol.
 	 This can happen for lambda functions compiled with clang++,
 	 which outputs no name for the container class.  */
       if (TYPE_NAME (type) == NULL)
-	return (struct block_symbol) {NULL, NULL};
+	return null_block_symbol;
 
       /* Look for symbol NAME in this class.  */
       sym = cp_lookup_nested_symbol (type, name, block, domain);
@@ -273,7 +273,7 @@ cp_search_static_and_baseclasses (const char *name,
   if (klass_sym.symbol == NULL)
     {
       do_cleanups (cleanup);
-      return (struct block_symbol) {NULL, NULL};
+      return null_block_symbol;
     }
   klass_type = SYMBOL_TYPE (klass_sym.symbol);
 
@@ -492,7 +492,7 @@ cp_lookup_symbol_via_imports (const char *scope,
 	}
     }
 
-  return (struct block_symbol) {NULL, NULL};
+  return null_block_symbol;
 }
 
 /* Helper function that searches an array of symbols for one named NAME.  */
@@ -640,7 +640,7 @@ cp_lookup_symbol_via_all_imports (const char *scope, const char *name,
       block = BLOCK_SUPERBLOCK (block);
     }
 
-  return (struct block_symbol) {NULL, NULL};
+  return null_block_symbol;
 }
 
 /* Searches for NAME in the current namespace, and by applying
@@ -928,7 +928,7 @@ cp_lookup_nested_symbol_1 (struct type *container_type,
 	return sym;
     }
 
-  return (struct block_symbol) {NULL, NULL};
+  return null_block_symbol;
 }
 
 /* Look up a symbol named NESTED_NAME that is nested inside the C++
@@ -1005,7 +1005,7 @@ cp_lookup_nested_symbol (struct type *parent_type,
 			      "cp_lookup_nested_symbol (...) = NULL"
 			      " (func/method)\n");
 	}
-      return (struct block_symbol) {NULL, NULL};
+      return null_block_symbol;
 
     default:
       internal_error (__FILE__, __LINE__,

gdb/d-namespace.c

@@ -132,7 +132,7 @@ d_lookup_symbol (const struct language_defn *langdef,
 	  if (lang_this.symbol == NULL)
 	    {
 	      do_cleanups (cleanup);
-	      return (struct block_symbol) {NULL, NULL};
+	      return null_block_symbol;
 	    }
 
 	  type = check_typedef (TYPE_TARGET_TYPE (SYMBOL_TYPE (lang_this.symbol)));
@@ -159,7 +159,7 @@ d_lookup_symbol (const struct language_defn *langdef,
       if (class_sym.symbol == NULL)
 	{
 	  do_cleanups (cleanup);
-	  return (struct block_symbol) {NULL, NULL};
+	  return null_block_symbol;
 	}
 
       /* Look for a symbol named NESTED in this class.  */
@@ -370,7 +370,7 @@ d_lookup_nested_symbol (struct type *parent_type,
 
     case TYPE_CODE_FUNC:
     case TYPE_CODE_METHOD:
-      return (struct block_symbol) {NULL, NULL};
+      return null_block_symbol;
 
     default:
       gdb_assert_not_reached ("called with non-aggregate type.");
@@ -530,7 +530,7 @@ d_lookup_symbol_imports (const char *scope, const char *name,
 	}
     }
 
-  return (struct block_symbol) {NULL, NULL};
+  return null_block_symbol;
 }
 
 /* Searches for NAME in the current module, and by applying relevant
@@ -562,7 +562,7 @@ d_lookup_symbol_module (const char *scope, const char *name,
       block = BLOCK_SUPERBLOCK (block);
     }
 
-  return (struct block_symbol) {NULL, NULL};
+  return null_block_symbol;
 }
 
 /* The D-specific version of name lookup for static and global names

gdb/symtab.c

@@ -85,6 +85,9 @@ static struct block_symbol
   lookup_symbol_in_objfile (struct objfile *objfile, int block_index,
 			    const char *name, const domain_enum domain);
 
+/* See symtab.h.  */
+const struct block_symbol null_block_symbol = { NULL, NULL };
+
 extern initialize_file_ftype _initialize_symtab;
 
 /* Program space key for finding name and language of "main".  */

gdb/symtab.h

@@ -832,6 +832,10 @@ struct block_symbol
 
 extern const struct symbol_impl *symbol_impls;
 
+/* For convenience.  All fields are NULL.  This means "there is no
+   symbol".  */
+extern const struct block_symbol null_block_symbol;
+
 /* Note: There is no accessor macro for symbol.owner because it is
    "private".  */