[Sema] Teach -Wcast-align to compute alignment of CXXThisExpr

[ahatanak]

Summary

This fixes https://bugs.llvm.org/show_bug.cgi?id=46605.

[rjmccall]

LGTM

clang/lib/Sema/SemaChecking.cpp

if (auto *VD = dyn_cast<VarDecl>(cast<DeclRefExpr>(E)->getDecl())) {
         return std::make_pair(Ctx.getDeclAlign(VD), CharUnits::Zero());
       if (VD->hasInit())
         return getBaseAlignmentAndOffsetFromLValue(VD->getInit(), Ctx);
     }
     break;
   }
   case Stmt::MemberExprClass: {
     auto *ME = cast<MemberExpr>(E);
-    if (ME->isArrow())
-      break;
     auto *FD = dyn_cast<FieldDecl>(ME->getMemberDecl());
     if (!FD || FD->getType()->isReferenceType())
       break;
-    auto P = getBaseAlignmentAndOffsetFromLValue(ME->getBase(), Ctx);
+    Optional<std::pair<CharUnits, CharUnits>> P;
+    if (ME->isArrow())
+      P = getBaseAlignmentAndOffsetFromPtr(ME->getBase(), Ctx);
+    else
+      P = getBaseAlignmentAndOffsetFromLValue(ME->getBase(), Ctx);
     if (!P)
       break;
     const ASTRecordLayout &Layout = Ctx.getASTRecordLayout(FD->getParent());
     uint64_t Offset = Layout.getFieldOffset(FD->getFieldIndex());
     return std::make_pair(P->first,
                           P->second + CharUnits::fromQuantity(Offset));
   }
   case Stmt::UnaryOperatorClass: {
case CK_DerivedToBase: {
       if (!P)
         break;
       return getDerivedToBaseAlignmentAndOffset(
           CE, From->getType()->getPointeeType(), P->first, P->second, Ctx);
     }
     }
     break;
   }
+  case Stmt::CXXThisExprClass: {
+    auto *RD = E->getType()->getPointeeType()->getAsCXXRecordDecl();
+    CharUnits Alignment = Ctx.getASTRecordLayout(RD).getNonVirtualAlignment();
+    return std::make_pair(Alignment, CharUnits::Zero());
+  }
   case Stmt::UnaryOperatorClass: {
     auto *UO = cast<UnaryOperator>(E);
     if (UO->getOpcode() == UO_AddrOf)
       return getBaseAlignmentAndOffsetFromLValue(UO->getSubExpr(), Ctx);
     break;
   }
   case Stmt::BinaryOperatorClass: {
     auto *BO = cast<BinaryOperator>(E);

clang/test/SemaCXX/warn-cast-align.cpp

void test1(void *P) {
 
   c = (int*) P;
   c = static_cast<int*>(P);
   c = reinterpret_cast<int*>(P);
   typedef int *IntPtr;
   c = IntPtr(P);
 }
 
+struct __attribute__((aligned(16))) AlignedS {
+  char m[16];
+};
+
 struct __attribute__((aligned(16))) A {
   char m0[16];
   char m1[16];
+  AlignedS *getAlignedS() {
+    return (AlignedS *)m1;
+  }
 };
 
 struct B0 {
   char m0[16];
 };
 
 struct B1 {
   char m0[16];

 struct __attribute__((aligned(16))) D3 : B4, D2 {
 };
 
 struct __attribute__((aligned(16))) D4 : virtual D2 {
 };
 
 struct D5 : virtual D0 {
   char m0[16];
+  AlignedS *get() {
+    return (AlignedS *)m0; // expected-warning {{cast from 'char *' to 'AlignedS *'}}
+  }
 };
 
 struct D6 : virtual D5 {
 };
 
 struct D7 : virtual D3 {
 };