[clang][Interp] Start handling mutable record members

[tbaeder]

Ping

[shafik]

Two cases to consider: https://godbolt.org/z/ovofPExGK

namespace MutableFields {
  class Foo {
  public:
    constexpr Foo() : I(1) {}
    mutable int I; // ref-note {{declared here}}
  };


  constexpr int foo() {
    constexpr Foo F;  
    F.I = 12;
    return F.I;
  }
  static_assert(foo() == 12, "");
}

clang and gcc disagree on this one, I think clang is right not 100%.

and this one:

namespace MutableFields {
  class Foo {
  public:
    constexpr Foo() : I(1) {}
    mutable int I; // ref-note {{declared here}}
  };


  constexpr int foo() {
    constexpr Foo F;  // A
    F.I = 12;
    constexpr int x = F.I; // B
    return F.I;
  }
  static_assert(foo() == 12, "");
}

[tbaeder]

Ping

clang/lib/AST/Interp/Interp.cpp

bool CheckConst(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
   // The This pointer is writable in constructors and destructors,
   // even if isConst() returns true.
   if (const Function *Func = S.Current->getFunction();
       Func && (Func->isConstructor() || Func->isDestructor()) &&
       Ptr.block() == S.Current->getThis().block()) {
     return true;
   }
 
+  // Writing to mutable fields is fine even if the parent is
+  // declared const.
+  if (S.getLangOpts().CPlusPlus14) {
+    if (const auto *FD = dyn_cast<FieldDecl>(Ptr.getFieldDesc()->asDecl());
+        FD && FD->isMutable())
+      return true;
+  }
+
   const QualType Ty = Ptr.getType();
   const SourceInfo &Loc = S.Current->getSource(OpPC);
   S.FFDiag(Loc, diag::note_constexpr_modify_const_type) << Ty;
   return false;
 }
 
 bool CheckMutable(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {
   assert(Ptr.isLive() && "Pointer is not live");
   if (!Ptr.isMutable()) {
     return true;
   }
 
+  // In C++14 onwards, it is permitted to read a mutable member whose
+  // lifetime began within the evaluation.
+  if (S.getLangOpts().CPlusPlus14)
+    return true;
+
   const SourceInfo &Loc = S.Current->getSource(OpPC);
   const FieldDecl *Field = Ptr.getField();
   S.FFDiag(Loc, diag::note_constexpr_access_mutable, 1) << AK_Read << Field;
   S.Note(Field->getLocation(), diag::note_declared_at);
   return false;
 }
 
 bool CheckLoad(InterpState &S, CodePtr OpPC, const Pointer &Ptr) {

clang/test/AST/Interp/records.cpp

namespace VirtualFunctionPointers {
   static_assert(dynamicDispatch(s2) == 7);
 };
 
 
 
 };
 #endif
 
+namespace MutableFields {
+  class Foo {
+  public:
+    constexpr Foo() : I(1) {}
+    mutable int I; // ref-note {{declared here}}
+  };
+
+
+  constexpr int foo() {
+    const Foo F;
+    F.I = 12;
+    return F.I;
+  }
+  static_assert(foo() == 12, "");
+
+  /// FIXME: This should also be rejected in the new interpreter.
+  constexpr Foo GlobalF;
+  constexpr int foo2() { // ref-error {{never produces a constant expression}}
+    return GlobalF.I; // ref-note {{read of mutable member 'I' is not allowed in a constant expression}}
+  }
+}
+
 /// FIXME: The following tests are broken.
 ///   They are using CXXDefaultInitExprs which contain a CXXThisExpr. The This pointer
 ///   in those refers to the declaration we are currently initializing, *not* the
 ///   This pointer of the current stack frame. This is something we haven't
 ///   implemented in the new interpreter yet.
 namespace DeclRefs {
   struct A{ int m; const int &f = m; }; // expected-note {{implicit use of 'this'}}