[modules] Make duplicate export error deterministic.

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

R=adamk@chromium.org
BUG=v8:5358,v8:1569

Review-Url: https://codereview.chromium.org/2331003002
Cr-Commit-Position: refs/heads/master@{#39424}

src/ast/modules.cc

@@ -136,17 +136,30 @@ void ModuleDescriptor::MakeIndirectExportsExplicit(Zone* zone) {
 
 const ModuleDescriptor::Entry* ModuleDescriptor::FindDuplicateExport(
     Zone* zone) const {
+  const ModuleDescriptor::Entry* candidate = nullptr;
   ZoneSet<const AstRawString*> 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;
+    DCHECK(entry->location.IsValid());
+    bool is_duplicate = !export_names.insert(entry->export_name).second;
+    if (is_duplicate &&
+        (candidate == nullptr ||
+         entry->location.beg_pos > candidate->location.beg_pos)) {
+      candidate = entry;
+    }
   }
   for (auto entry : special_exports_) {
     if (entry->export_name == nullptr) continue;  // Star export.
-    if (!export_names.insert(entry->export_name).second) return entry;
+    DCHECK(entry->location.IsValid());
+    bool is_duplicate = !export_names.insert(entry->export_name).second;
+    if (is_duplicate &&
+        (candidate == nullptr ||
+         entry->location.beg_pos > candidate->location.beg_pos)) {
+      candidate = entry;
+    }
   }
-  return nullptr;
+  return candidate;
 }
 
 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
+  // If there are multiple export entries with the same export name, return the
-  // of them.  Otherwise return nullptr.
+  // 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'
+*%(basename)s:10: SyntaxError: Duplicate export of 'b'
-export { a, 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
 ]