[Diagnostics] Don't suggest unwrap with default value if type is not materializable

lib/Sema/CSDiagnostics.cpp

@@ -549,8 +549,59 @@ bool MemberAccessOnOptionalBaseFailure::diagnoseAsError() {
                                            resultIsOptional, SourceRange());
 }
 
+Optional<AnyFunctionType::Param>
+MissingOptionalUnwrapFailure::getOperatorParameterFor(Expr *expr) const {
+  auto *parentExpr = findParentExpr(expr);
+  if (!parentExpr)
+    return None;
+
+  auto getArgIdx = [](TupleExpr *tuple, Expr *argExpr) -> unsigned {
+    for (unsigned i = 0, n = tuple->getNumElements(); i != n; ++i) {
+      if (tuple->getElement(i) == argExpr)
+        return i;
+    }
+    llvm_unreachable("argument is not in enclosing tuple?!");
+  };
+
+  auto *tupleExpr = dyn_cast<TupleExpr>(parentExpr);
+  if (!(tupleExpr && tupleExpr->isImplicit()))
+    return None;
+
+  parentExpr = findParentExpr(tupleExpr);
+  if (!(parentExpr && isa<ApplyExpr>(parentExpr)))
+    return None;
+
+  auto &cs = getConstraintSystem();
+  auto *fnExpr = cast<ApplyExpr>(parentExpr)->getFn();
+  if (auto overload =
+          getOverloadChoiceIfAvailable(cs.getConstraintLocator(fnExpr))) {
+    if (auto *decl = overload->choice.getDecl()) {
+      if (!decl->isOperator())
+        return None;
+
+      auto *fnType = overload->openedType->castTo<FunctionType>();
+      return fnType->getParams()[getArgIdx(tupleExpr, expr)];
+    }
+  }
+
+  return None;
+}
+
 void MissingOptionalUnwrapFailure::offerDefaultValueUnwrapFixIt(
     DeclContext *DC, Expr *expr) const {
+  auto *anchor = getAnchor();
+
+  // If anchor is an explicit address-of, or expression which produces
+  // an l-value (e.g. first argument of `+=` operator), let's not
+  // suggest default value here because that would produce r-value type.
+  if (isa<InOutExpr>(anchor))
+    return;
+
+  if (auto param = getOperatorParameterFor(anchor)) {
+    if (param->isInOut())
+      return;
+  }
+
   auto diag = emitDiagnostic(expr->getLoc(), diag::unwrap_with_default_value);
 
   auto &TC = getTypeChecker();