Revert GN declare_args() change.

tools/gn/functions.cc

Index: tools/gn/functions.cc
diff --git a/tools/gn/functions.cc b/tools/gn/functions.cc
index 41be19b5206c875f842283aeda5bfb15de00f766..e01a25c977f7a300ae1048f994c2c6d0b8a33851 100644
--- a/tools/gn/functions.cc
+++ b/tools/gn/functions.cc
@@ -45,38 +45,8 @@ bool VerifyNoBlockForFunctionCall(const FunctionCallNode* function,
   return false;
 }
 
-// This key is set as a scope property on the scope of a declare_args() block,
-// in order to prevent reading a variable defined earlier in the same call
-// (see `gn help declare_args` for more).
-const void *kInDeclareArgsKey = nullptr;
-
 }  // namespace
 
-
-bool EnsureNotReadingFromSameDeclareArgs(const ParseNode* node,
-                                         const Scope* cur_scope,
-                                         const Scope* val_scope,
-                                         Err* err) {
-  // If the value didn't come from a scope at all, we're safe.
-  if (!val_scope)
-    return true;
-
-  const Scope* val_args_scope = nullptr;
-  val_scope->GetProperty(&kInDeclareArgsKey, &val_args_scope);
-
-  const Scope* cur_args_scope = nullptr;
-  cur_scope->GetProperty(&kInDeclareArgsKey, &cur_args_scope);
-  if (!val_args_scope || !cur_args_scope || (val_args_scope != cur_args_scope))
-    return true;
-
-  *err = Err(node,
-      "Reading a variable defined in the same declare_args() call.\n"
-      "\n"
-      "If you need to set the value of one arg based on another, put\n"
-      "them in two separate declare_args() calls, one after the other.\n");
-  return false;
-}
-
 bool EnsureNotProcessingImport(const ParseNode* node,
                                const Scope* scope,
                                Err* err) {
@@ -387,9 +357,8 @@ const char kDeclareArgs_Help[] =
 
   The precise behavior of declare args is:
 
-   1. The declare_args() block executes. Any variable defined in the enclosing
-      scope is available for reading, but any variable defined earlier in
-      the current scope is not (since the overrides haven't been applied yet).
+   1. The declare_arg block executes. Any variables in the enclosing scope are
+      available for reading.
 
    2. At the end of executing the block, any variables set within that scope
       are saved globally as build arguments, with their current values being
@@ -408,10 +377,12 @@ const char kDeclareArgs_Help[] =
       like [], "", or -1, and after the declare_args block, call exec_script if
       the value is unset by the user.
 
-    - Because you cannot read the value of a variable defined in the same
-      block, if you need to make the default value of one arg depend
-      on the possibly-overridden value of another, write two separate
-      declare_args() blocks:
+    - Any code inside of the declare_args block will see the default values of
+      previous variables defined in the block rather than the user-overridden
+      value. This can be surprising because you will be used to seeing the
+      overridden value. If you need to make the default value of one arg
+      dependent on the possibly-overridden value of another, write two separate
+      declare_args blocks:
 
         declare_args() {
           enable_foo = true
@@ -444,7 +415,6 @@ Value RunDeclareArgs(Scope* scope,
     return Value();
 
   Scope block_scope(scope);
-  block_scope.SetProperty(&kInDeclareArgsKey, &block_scope);
   block->Execute(&block_scope, err);
   if (err->has_error())
     return Value();