[clang][bytecode] Add special handling for union copy assign operators (llvm#125476)

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.cpp

@@ -4676,7 +4676,7 @@ bool Compiler<Emitter>::VisitCallExpr(const CallExpr *E) {
       OCE && OCE->isAssignmentOp()) {
     // Just like with regular assignments, we need to special-case assignment
     // operators here and evaluate the RHS (the second arg) before the LHS (the
-    // first arg. We fix this by using a Flip op later.
+    // first arg). We fix this by using a Flip op later.
     assert(Args.size() == 2);
     IsAssignmentOperatorCall = true;
     std::reverse(Args.begin(), Args.end());
@@ -5664,6 +5664,21 @@ bool Compiler<Emitter>::compileDestructor(const CXXDestructorDecl *Dtor) {
   return this->emitPopPtr(Dtor) && this->emitRetVoid(Dtor);
 }
 
+template <class Emitter>
+bool Compiler<Emitter>::compileUnionCopyAssignmentOperator(
+    const CXXMethodDecl *MD) {
+  if (!this->emitThis(MD))
+    return false;
+
+  auto PVD = MD->getParamDecl(0);
+  ParamOffset PO = this->Params[PVD]; // Must exist.
+
+  if (!this->emitGetParam(PT_Ptr, PO.Offset, MD))
+    return false;
+
+  return this->emitMemcpy(MD) && this->emitRet(PT_Ptr, MD);
+}
+
 template <class Emitter>
 bool Compiler<Emitter>::visitFunc(const FunctionDecl *F) {
   // Classify the return type.
@@ -5675,9 +5690,15 @@ bool Compiler<Emitter>::visitFunc(const FunctionDecl *F) {
     return this->compileDestructor(Dtor);
 
   // Emit custom code if this is a lambda static invoker.
-  if (const auto *MD = dyn_cast<CXXMethodDecl>(F);
-      MD && MD->isLambdaStaticInvoker())
-    return this->emitLambdaStaticInvokerBody(MD);
+  if (const auto *MD = dyn_cast<CXXMethodDecl>(F)) {
+    const RecordDecl *RD = MD->getParent();
+
+    if (RD->isUnion() && MD->isCopyAssignmentOperator())
+      return this->compileUnionCopyAssignmentOperator(MD);
+
+    if (MD->isLambdaStaticInvoker())
+      return this->emitLambdaStaticInvokerBody(MD);
+  }
 
   // Regular functions.
   if (const auto *Body = F->getBody())

clang/lib/AST/ByteCode/Compiler.h

@@ -383,6 +383,7 @@ class Compiler : public ConstStmtVisitor<Compiler<Emitter>, bool>,
   bool emitBuiltinBitCast(const CastExpr *E);
   bool compileConstructor(const CXXConstructorDecl *Ctor);
   bool compileDestructor(const CXXDestructorDecl *Dtor);
+  bool compileUnionCopyAssignmentOperator(const CXXMethodDecl *MD);
 
   bool checkLiteralType(const Expr *E);
 

clang/test/AST/ByteCode/unions.cpp

@@ -402,7 +402,6 @@ namespace UnionInBase {
   static_assert(return_uninit().a.x == 2);
 }
 
-/// FIXME: Our diagnostic here is a little off.
 namespace One {
   struct A { long x; };
 
@@ -421,4 +420,33 @@ namespace One {
                       // both-note {{constinit}}
 }
 
+namespace CopyAssign {
+  union A {
+    int a;
+    int b;
+  };
+
+  constexpr int f() {
+    A a{12};
+    A b{13};
+
+    b.b = 32;
+    b = a ;
+    return b.a;
+  }
+  static_assert(f()== 12);
+
+
+  constexpr int f2() {
+    A a{12};
+    A b{13};
+
+    b.b = 32;
+    b = a ;
+    return b.b; // both-note {{read of member 'b' of union with active member 'a'}}
+  }
+  static_assert(f2() == 12); // both-error {{not an integral constant expression}} \
+                             // both-note {{in call to}}
+
+}
 #endif