[Sema] Fix a crash on invalid where invalid defaulted function is called

[erik.pilkington]

Summary

Previously, clang would crash on the test case below, because it misinterprets the operator=(bool) call as a move assignment operator.

[erik.pilkington]

Ping!

[manmanren]

LGTM.

Manman

lib/Sema/SemaExpr.cpp

       MarkVTableUsed(Loc, Destructor->getParent());
   } else if (CXXMethodDecl *MethodDecl = dyn_cast<CXXMethodDecl>(Func)) {
     if (MethodDecl->isOverloadedOperator() &&
         MethodDecl->getOverloadedOperator() == OO_Equal) {
       MethodDecl = cast<CXXMethodDecl>(MethodDecl->getFirstDecl());
       if (MethodDecl->isDefaulted() && !MethodDecl->isDeleted()) {
         if (MethodDecl->isCopyAssignmentOperator())
           DefineImplicitCopyAssignment(Loc, MethodDecl);
-        else
+        else if (MethodDecl->isMoveAssignmentOperator())
           DefineImplicitMoveAssignment(Loc, MethodDecl);
       }
     } else if (isa<CXXConversionDecl>(MethodDecl) &&
                MethodDecl->getParent()->isLambda()) {
       CXXConversionDecl *Conversion =
           cast<CXXConversionDecl>(MethodDecl->getFirstDecl());
       if (Conversion->isLambdaToBlockPointerConversion())
         DefineImplicitLambdaToBlockPointerConversion(Loc, Conversion);

test/SemaCXX/cxx0x-defaulted-functions.cpp

   };
   template<typename T> struct B {
     B() noexcept(false) = default; // expected-error {{does not match the calculated one}}
     ~B() noexcept(false) = default; // expected-error {{does not match the calculated one}}
   };
   A<int> a;
   B<int> b; // expected-note {{here}}
 }
+
+namespace PR27941 {
+struct ExplicitBool {
+  ExplicitBool &operator=(bool) = default; // expected-error{{only special member functions may be defaulted}}
+  int member;
+};
+
+int fn() {
+  ExplicitBool t;
+  t = true;
+}
+}