Fix PR32831 (Try Again): 'this' capture while instantiating generic lambda call operator specialization

When computing the appropriate cv-qualifiers for the 'this' capture, we have to examine each enclosing lambda - but when using the FunctionScopeInfo stack we have to ensure that the lambda below (outer) is the decl-context of the closure-class of the current lambda.

https://bugs.llvm.org/show_bug.cgi?id=32831

This patch was initially committed here: https://reviews.llvm.org/rL301735
Then reverted here: https://reviews.llvm.org/rL301916

The issue with the original patch was a failure to check that the closure type has been created within the LambdaScopeInfo before querying its DeclContext - instead of just assuming it has (silly!).  A reduced example such as this highlights the problem:
  struct X {
     int data;
     auto foo() { return [] { return [] -> decltype(data) { return 0; }; }; }
  };

When 'data' within decltype(data) tries to determine the type of 'this', none of the LambdaScopeInfo's have their closure types created at that point.

 

llvm-svn: 301972

1 files changed, 39 insertions, 14 deletions

diff --git a/clang/lib/Sema/SemaExprCXX.cpp b/clang/lib/Sema/SemaExprCXX.cpp
index d65570fcef7..9b88cddbc96 100644
--- a/clang/lib/Sema/SemaExprCXX.cpp
+++ b/clang/lib/Sema/SemaExprCXX.cpp
@@ -901,17 +901,36 @@ static QualType adjustCVQualifiersForCXXThisWithinLambda(
   // capturing lamdbda's call operator.
   //
 
-  // The issue is that we cannot rely entirely on the FunctionScopeInfo stack
-  // since ScopeInfos are pushed on during parsing and treetransforming. But
-  // since a generic lambda's call operator can be instantiated anywhere (even
-  // end of the TU) we need to be able to examine its enclosing lambdas and so
-  // we use the DeclContext to get a hold of the closure-class and query it for
-  // capture information.  The reason we don't just resort to always using the
-  // DeclContext chain is that it is only mature for lambda expressions
-  // enclosing generic lambda's call operators that are being instantiated.
-
+  // Since the FunctionScopeInfo stack is representative of the lexical
+  // nesting of the lambda expressions during initial parsing (and is the best
+  // place for querying information about captures about lambdas that are
+  // partially processed) and perhaps during instantiation of function templates
+  // that contain lambda expressions that need to be transformed BUT not
+  // necessarily during instantiation of a nested generic lambda's function call
+  // operator (which might even be instantiated at the end of the TU) - at which
+  // time the DeclContext tree is mature enough to query capture information
+  // reliably - we use a two pronged approach to walk through all the lexically
+  // enclosing lambda expressions:
+  //
+  //  1) Climb down the FunctionScopeInfo stack as long as each item represents
+  //  a Lambda (i.e. LambdaScopeInfo) AND each LSI's 'closure-type' is lexically
+  //  enclosed by the call-operator of the LSI below it on the stack (while
+  //  tracking the enclosing DC for step 2 if needed).  Note the topmost LSI on
+  //  the stack represents the innermost lambda.
+  //
+  //  2) If we run out of enclosing LSI's, check if the enclosing DeclContext
+  //  represents a lambda's call operator.  If it does, we must be instantiating
+  //  a generic lambda's call operator (represented by the Current LSI, and
+  //  should be the only scenario where an inconsistency between the LSI and the
+  //  DeclContext should occur), so climb out the DeclContexts if they
+  //  represent lambdas, while querying the corresponding closure types
+  //  regarding capture information.
+
+  // 1) Climb down the function scope info stack.
   for (int I = FunctionScopes.size();
-       I-- && isa<LambdaScopeInfo>(FunctionScopes[I]);
+       I-- && isa<LambdaScopeInfo>(FunctionScopes[I]) &&
+       (!CurLSI || !CurLSI->Lambda || CurLSI->Lambda->getDeclContext() ==
+                       cast<LambdaScopeInfo>(FunctionScopes[I])->CallOperator);
        CurDC = getLambdaAwareParentOfDeclContext(CurDC)) {
     CurLSI = cast<LambdaScopeInfo>(FunctionScopes[I]);
 
@@ -927,11 +946,17 @@ static QualType adjustCVQualifiersForCXXThisWithinLambda(
       return ASTCtx.getPointerType(ClassType);
     }
   }
-  // We've run out of ScopeInfos but check if CurDC is a lambda (which can
-  // happen during instantiation of generic lambdas)
+
+  // 2) We've run out of ScopeInfos but check if CurDC is a lambda (which can
+  // happen during instantiation of its nested generic lambda call operator)
   if (isLambdaCallOperator(CurDC)) {
-    assert(CurLSI);
-    assert(isGenericLambdaCallOperatorSpecialization(CurLSI->CallOperator));
+    assert(CurLSI && "While computing 'this' capture-type for a generic "
+                     "lambda, we must have a corresponding LambdaScopeInfo");
+    assert(isGenericLambdaCallOperatorSpecialization(CurLSI->CallOperator) &&
+           "While computing 'this' capture-type for a generic lambda, when we "
+           "run out of enclosing LSI's, yet the enclosing DC is a "
+           "lambda-call-operator we must be (i.e. Current LSI) in a generic "
+           "lambda call oeprator");
     assert(CurDC == getLambdaAwareParentOfDeclContext(CurLSI->CallOperator));
 
     auto IsThisCaptured =