Reland of "[modules] Make duplicate export error deterministic."

In case of duplicate exports, always report the error for the very last
    one.

(Fixed a bug.)

BUG=v8:5358,v8:1569

Review-Url: https://codereview.chromium.org/2340953002
Cr-Commit-Position: refs/heads/master@{#39434}

src/ast/modules.cc

@@ -134,19 +134,40 @@ void ModuleDescriptor::MakeIndirectExportsExplicit(Zone* zone) {
   }
 }
 
+namespace {
+
+const ModuleDescriptor::Entry* BetterDuplicate(
+    const ModuleDescriptor::Entry* candidate,
+    ZoneMap<const AstRawString*, const ModuleDescriptor::Entry*>& export_names,
+    const ModuleDescriptor::Entry* current_duplicate) {
+  DCHECK_NOT_NULL(candidate->export_name);
+  DCHECK(candidate->location.IsValid());
+  auto insert_result =
+      export_names.insert(std::make_pair(candidate->export_name, candidate));
+  if (insert_result.second) return current_duplicate;
+  if (current_duplicate == nullptr) {
+    current_duplicate = insert_result.first->second;
+  }
+  return (candidate->location.beg_pos > current_duplicate->location.beg_pos)
+             ? candidate
+             : current_duplicate;
+}
+
+}  // namespace
+
 const ModuleDescriptor::Entry* ModuleDescriptor::FindDuplicateExport(
     Zone* zone) const {
-  ZoneSet<const AstRawString*> export_names(zone);
+  const ModuleDescriptor::Entry* duplicate = nullptr;
+  ZoneMap<const AstRawString*, const ModuleDescriptor::Entry*> export_names(
+      zone);
   for (const auto& it : regular_exports_) {
-    const Entry* entry = it.second;
-    DCHECK_NOT_NULL(entry->export_name);
-    if (!export_names.insert(entry->export_name).second) return entry;
+    duplicate = BetterDuplicate(it.second, export_names, duplicate);
   }
   for (auto entry : special_exports_) {
     if (entry->export_name == nullptr) continue;  // Star export.
-    if (!export_names.insert(entry->export_name).second) return entry;
+    duplicate = BetterDuplicate(entry, export_names, duplicate);
   }
-  return nullptr;
+  return duplicate;
 }
 
 bool ModuleDescriptor::Validate(ModuleScope* module_scope,

src/ast/modules.h

@@ -151,8 +151,8 @@ class ModuleDescriptor : public ZoneObject {
   ZoneMultimap<const AstRawString*, Entry*> regular_exports_;
   ZoneMap<const AstRawString*, const Entry*> regular_imports_;
 
-  // If there are multiple export entries with the same export name, return one
-  // of them.  Otherwise return nullptr.
+  // If there are multiple export entries with the same export name, return the
+  // last of them (in source order).  Otherwise return nullptr.
   const Entry* FindDuplicateExport(Zone* zone) const;
 
   // Find any implicitly indirect exports and make them explicit.

test/message/export-duplicate-as.js

@@ -4,6 +4,6 @@
 //
 // MODULE
 
-var a, b;
+var a, b, c;
 export { a as c };
-export { a, b as c };
+export { a, b as c, c, b };

test/message/export-duplicate-as.out

@@ -2,6 +2,6 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 *%(basename)s:9: SyntaxError: Duplicate export of 'c'
-export { a, b as c };
-                 ^
+export { a, b as c, c, b };
+                    ^
 SyntaxError: Duplicate export of 'c'

test/message/export-duplicate-default.js

@@ -5,4 +5,5 @@
 // MODULE
 
 export default function f() {};
+export default 42;
 export default class C {};

test/message/export-duplicate-default.out

@@ -1,7 +1,7 @@
 # Copyright 2015 the V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
-*%(basename)s:8: SyntaxError: Duplicate export of 'default'
+*%(basename)s:9: SyntaxError: Duplicate export of 'default'
 export default class C {};
        ^^^^^^^
 SyntaxError: Duplicate export of 'default'

test/message/export-duplicate.js

@@ -4,6 +4,7 @@
 //
 // MODULE
 
-var a, b;
+var a, b, c;
 export { a };
 export { a, b };
+export { b, c };

test/message/export-duplicate.out

@@ -1,7 +1,7 @@
 # Copyright 2015 the V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
-*%(basename)s:9: SyntaxError: Duplicate export of 'a'
-export { a, b };
+*%(basename)s:10: SyntaxError: Duplicate export of 'b'
+export { b, c };
          ^
-SyntaxError: Duplicate export of 'a'
+SyntaxError: Duplicate export of 'b'

test/message/message.status

@@ -31,7 +31,5 @@
   # escapes (we need to parse to distinguish octal escapes from valid
   # back-references).
   'strict-octal-regexp': [SKIP],
-  # See issue v8:5358
-  'export-duplicate-default': [SKIP],
 }],  # ALWAYS
 ]