[PATCH] Fix PR17877

[Faisal Vali]

Hi rsmith, doug.gregor,

A quick fix to the bug introduced by generic-lambda-capturing that broke libc++.
See http://lists.cs.uiuc.edu/pipermail/cfe-dev/2013-November/033369.html for discussion on cfe-dev.

This fix explicitly checks whether we are within the declcontext of a lambda's call operator - which is what I had intended to be true (and assumed would be true if getCurLambda returns a valid pointer) before checking whether we should check to see if the lambda can capture.

A deeper fix (that addresses why getCurLambda() returns a valid pointer when perhaps it shouldn't?) - as proposed by Richard Smith in http://llvm.org/bugs/show_bug.cgi?id=17877 - has been suggested as a FIXME.

Will try and implement that at a later date - figured it is prolly important to not leave libc++ broken for too long (sorry about that).




http://llvm-reviews.chandlerc.com/D2144

Files:
  lib/Sema/SemaExprCXX.cpp

Index: lib/Sema/SemaExprCXX.cpp
===================================================================
--- lib/Sema/SemaExprCXX.cpp
+++ lib/Sema/SemaExprCXX.cpp
@@ -5988,9 +5988,24 @@
   //   full-expression +n + ({ 0; }); ends, but it's too late for us to see that 
   //   we need to capture n after all.
 
-  LambdaScopeInfo *const CurrentLSI = getCurLambda(); 
-  if (CurrentLSI && CurrentLSI->hasPotentialCaptures() && 
-      !FullExpr.isInvalid())
+  LambdaScopeInfo *const CurrentLSI = getCurLambda();
+  // FIXME: PR 17877 showed that CurrentLSI can exist even when the 
+  // CurContext is not a lambda call operator. Refer to that Bug Report
+  // for an example of the code that might cause this asynchrony.  
+  // By checking whether we are in the context of a lambda's call operator
+  // we can fix the bug (we only need to check whether we need to capture
+  // if we are within a lambda's body); but per the comments in that 
+  // PR, a proper fix would entail :
+  //   "Alternative suggestion:
+  //   - Add to Sema an integer holding the smallest (outermost) scope 
+  //     index that we are *lexically* within, and save/restore/set to 
+  //     FunctionScopes.size() in InstantiatingTemplate's 
+  //     constructor/destructor.
+  //  - Teach the handful of places that iterate over FunctionScopes to 
+  //    stop at the outermost enclosing lexical scope." 
+  const bool IsInLambdaDeclContext = isLambdaCallOperator(CurContext); 
+  if (IsInLambdaDeclContext && CurrentLSI && 
+      CurrentLSI->hasPotentialCaptures() && !FullExpr.isInvalid())
     CheckLambdaCaptures(FE, CurrentLSI, *this);
   return MaybeCreateExprWithCleanups(FullExpr);
 }
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