[Cxx1z] Implement Lambda Capture of *this by Value as [=,*this] (P0018R3)

Implement lambda capture of *this by copy.
For e.g.:
struct A {

  int d = 10;
  auto foo() { return [*this] (auto a) mutable { d+=a; return d; }; }

};

auto L = A{}.foo(); // A{}'s lifetime is gone.

// Below is still ok, because *this was captured by value.
assert(L(10) == 20);
assert(L(100) == 120);

If the capture was implicit, or [this] (i.e. *this was captured by reference), this code would be otherwise undefined.

Implementation Strategy:
  - amend the parser to accept *this in the lambda introducer
  - add a new king of capture LCK_StarThis
  - teach Sema::CheckCXXThisCapture to handle by copy captures of the
    enclosing object (i.e. *this)
  - when CheckCXXThisCapture does capture by copy, the corresponding 
    initializer expression for the closure's data member 
    direct-initializes it thus making a copy of '*this'.
  - in codegen, when assigning to CXXThisValue, if *this was captured by 
    copy, make sure it points to the corresponding field member, and
    not, unlike when captured by reference, what the field member points
    to.
  - mark feature as implemented in svn

Much gratitude to Richard Smith for his carefully illuminating reviews!   

llvm-svn: 263921

1 files changed, 10 insertions, 2 deletions

diff --git a/clang/lib/Parse/ParseExprCXX.cpp b/clang/lib/Parse/ParseExprCXX.cpp
index 558d2371f6c..8e5f35ad65c 100644
--- a/clang/lib/Parse/ParseExprCXX.cpp
+++ b/clang/lib/Parse/ParseExprCXX.cpp
@@ -846,8 +846,16 @@ Optional<unsigned> Parser::ParseLambdaIntroducer(LambdaIntroducer &Intro,
     IdentifierInfo *Id = nullptr;
     SourceLocation EllipsisLoc;
     ExprResult Init;
-    
-    if (Tok.is(tok::kw_this)) {
+
+    if (Tok.is(tok::star)) {
+      Loc = ConsumeToken(); 
+      if (Tok.is(tok::kw_this)) {
+        ConsumeToken();     
+        Kind = LCK_StarThis;      
+      } else {
+        return DiagResult(diag::err_expected_star_this_capture);
+      }
+    } else if (Tok.is(tok::kw_this)) {
       Kind = LCK_This;
       Loc = ConsumeToken();
     } else {