[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, 18 insertions, 10 deletions

diff --git a/clang/lib/Sema/SemaLambda.cpp b/clang/lib/Sema/SemaLambda.cpp
index 1a62f0dfcba..0f5eb413526 100644
--- a/clang/lib/Sema/SemaLambda.cpp
+++ b/clang/lib/Sema/SemaLambda.cpp
@@ -924,7 +924,12 @@ void Sema::ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
     = Intro.Default == LCD_None? Intro.Range.getBegin() : Intro.DefaultLoc;
   for (auto C = Intro.Captures.begin(), E = Intro.Captures.end(); C != E;
        PrevCaptureLoc = C->Loc, ++C) {
-    if (C->Kind == LCK_This) {
+    if (C->Kind == LCK_This || C->Kind == LCK_StarThis) {
+      if (C->Kind == LCK_StarThis) 
+        Diag(C->Loc, !getLangOpts().CPlusPlus1z
+                             ? diag::ext_star_this_lambda_capture_cxx1z
+                             : diag::warn_cxx14_compat_star_this_lambda_capture);
+
       // C++11 [expr.prim.lambda]p8:
       //   An identifier or this shall not appear more than once in a 
       //   lambda-capture.
@@ -936,10 +941,12 @@ void Sema::ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
         continue;
       }
 
-      // C++11 [expr.prim.lambda]p8:
-      //   If a lambda-capture includes a capture-default that is =, the 
-      //   lambda-capture shall not contain this [...].
-      if (Intro.Default == LCD_ByCopy) {
+      // C++1z [expr.prim.lambda]p8:
+      //  If a lambda-capture includes a capture-default that is =, each
+      //  simple-capture of that lambda-capture shall be of the form "&
+      //  identifier" or "* this". [ Note: The form [&,this] is redundant but
+      //  accepted for compatibility with ISO C++14. --end note ]
+      if (Intro.Default == LCD_ByCopy && C->Kind != LCK_StarThis) {
         Diag(C->Loc, diag::err_this_capture_with_copy_default)
             << FixItHint::CreateRemoval(
                 SourceRange(getLocForEndOfToken(PrevCaptureLoc), C->Loc));
@@ -955,7 +962,9 @@ void Sema::ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
         continue;
       }
       
-      CheckCXXThisCapture(C->Loc, /*Explicit=*/true);
+      CheckCXXThisCapture(C->Loc, /*Explicit=*/true, /*BuildAndDiagnose*/ true,
+                          /*FunctionScopeIndexToStopAtPtr*/ nullptr,
+                          C->Kind == LCK_StarThis);
       continue;
     }
 
@@ -1529,10 +1538,9 @@ ExprResult Sema::BuildLambdaExpr(SourceLocation StartLoc, SourceLocation EndLoc,
       // Handle 'this' capture.
       if (From.isThisCapture()) {
         Captures.push_back(
-            LambdaCapture(From.getLocation(), IsImplicit, LCK_This));
-        CaptureInits.push_back(new (Context) CXXThisExpr(From.getLocation(),
-                                                         getCurrentThisType(),
-                                                         /*isImplicit=*/true));
+            LambdaCapture(From.getLocation(), IsImplicit,
+                          From.isCopyCapture() ? LCK_StarThis : LCK_This));
+        CaptureInits.push_back(From.getInitExpr());
         ArrayIndexStarts.push_back(ArrayIndexVars.size());
         continue;
       }