Fixes #5994 - When DeferParse is cancelled finish parsing of functions using already known offsets as some of the context information

used during original scanning may now be unavailable.

Author: atulkatti

lib/Parser/Parse.cpp

@@ -11442,14 +11442,25 @@ ULONG Parser::GetDeferralThreshold(bool isProfileLoaded)
 
 void Parser::FinishDeferredFunction(ParseNodeBlock * pnodeScopeList)
 {
-    uint saveNextBlockId = m_nextBlockId;
+    ParseContext parseContext;
+    this->CaptureContext(&parseContext);
+
     m_nextBlockId = pnodeScopeList->blockId + 1;
 
     FinishFunctionsInScope(pnodeScopeList,
-        [this](ParseNodeFnc * pnodeFnc)
+        [this, &parseContext](ParseNodeFnc * pnodeFnc)
     {
         Assert(pnodeFnc->nop == knopFncDecl);
 
+        // We need to scan this function based on the already known limits of the function declaration as some of
+        // the state such as fAllowIn may not be available at this point. Some of this state depends on the context
+        // of the function declaration. For example, a function declaration may be inside a for..in statement's var
+        // declaration. It may not be appropriate/possible to try and save all such context information. Functions
+        // that actually get deferred achieve this by going through the ParseSourceWithOffset code path.
+        this->GetScanner()->Clear();
+        this->GetScanner()->SetText(parseContext.pszSrc, pnodeFnc->cbMin /*+ this->m_scan.m_cMinTokMultiUnits*/, pnodeFnc->LengthInBytes(), pnodeFnc->ichMin, parseContext.isUtf8, parseContext.grfscr, pnodeFnc->lineNumber);
+        this->GetScanner()->Scan();
+
         // Non-simple params (such as default) require a good amount of logic to put vars on appropriate scopes. ParseFncDecl handles it
         // properly (both on defer and non-defer case). This is to avoid write duplicated logic here as well. Function with non-simple-param
         // will remain deferred until they are called.
@@ -11601,7 +11612,7 @@ void Parser::FinishDeferredFunction(ParseNodeBlock * pnodeScopeList)
         }
     });
 
-    m_nextBlockId = saveNextBlockId;
+    this->RestoreContext(&parseContext);
 }
 
 void Parser::InitPids()

test/es6/bug_issue_5994.baseline

@@ -1511,4 +1511,11 @@
     <compile-flags>-args summary -endargs</compile-flags>
   </default>
 </test>
+<test>
+  <default>
+    <files>bug_issue_5994.js</files>
+    <compile-flags>-MuteHostErrorMsg</compile-flags>
+    <baseline>bug_issue_5994.baseline</baseline>
+  </default>
+</test>
 </regress-exe>