diff --git a/flang/documentation/C++17.md b/flang/documentation/C++17.md
index cedf6923cc75..7c26c9e29fba 100644
--- a/flang/documentation/C++17.md
+++ b/flang/documentation/C++17.md
@@ -124,6 +124,8 @@ with lists of types.
 
 This simple little type is used wherever a value might or might not be
 present.
+It is especially useful for function results and
+rvalue reference arguments.
 It corresponds directly to the optional elements in the productions
 of the Fortran grammar.
 It is also used as a wrapper around a parse tree node type to define the
diff --git a/flang/documentation/C++style.md b/flang/documentation/C++style.md
index 58bfd1f1cd3e..b6304efb3181 100644
--- a/flang/documentation/C++style.md
+++ b/flang/documentation/C++style.md
@@ -147,6 +147,13 @@ We have an `ENUM_CLASS` macro that helps capture the names of constants.
 explicitly, it should contains either a `default:;` at its end or a
 `default:` label that obviously crashes; we have a `CRASH_NO_CASE` macro
 for such situations.
+1. When using `std::optional` values, avoid unprotected access to their content.
+This is usually by means of `x.has_value()` guarding execution of `*x`.
+This is implicit when they are function results assigned to local variables
+in `if`/`while` predicates.
+When no presence test is obviously protecting a `*x` reference to the
+contents, and it is assumed that the contents are present, validate that
+assumption by using `x.value()` instead.
 #### Classes
 1. Define POD structures with `struct`.
 1. Don't use `this->` in (non-static) member functions, unless forced to
diff --git a/flang/lib/evaluate/intrinsics.cc b/flang/lib/evaluate/intrinsics.cc
index 4e689ed13aca..751329e483ea 100644
--- a/flang/lib/evaluate/intrinsics.cc
+++ b/flang/lib/evaluate/intrinsics.cc
@@ -908,7 +908,7 @@ std::optional<SpecificCall> IntrinsicInterface::Match(
       if (sameArg == nullptr) {
         sameArg = arg;
       }
-      argOk = *type == sameArg->GetType();
+      argOk = type.value() == sameArg->GetType();
       break;
     case KindCode::effectiveKind:
       common::die("INTERNAL: KindCode::effectiveKind appears on argument '%s' "
@@ -1017,7 +1017,7 @@ std::optional<SpecificCall> IntrinsicInterface::Match(
     if (call.isSubroutineCall) {
       return std::nullopt;
     }
-    resultType = DynamicType{*result.categorySet.LeastElement(), 0};
+    resultType = DynamicType{result.categorySet.LeastElement().value(), 0};
     switch (result.kindCode) {
     case KindCode::defaultIntegerKind:
       CHECK(result.categorySet == IntType);
@@ -1124,11 +1124,7 @@ std::optional<SpecificCall> IntrinsicInterface::Match(
     break;
   case Rank::shaped:
     CHECK(shapeArg != nullptr);
-    {
-      std::optional<int> shapeLen{shapeArg->VectorSize()};
-      CHECK(shapeLen.has_value());
-      resultRank = *shapeLen;
-    }
+    resultRank = shapeArg->VectorSize().value();
     break;
   case Rank::elementalOrBOZ:
   case Rank::shape:
diff --git a/flang/lib/evaluate/real.h b/flang/lib/evaluate/real.h
index 3d9b7cd0b0bf..82c26078e30f 100644
--- a/flang/lib/evaluate/real.h
+++ b/flang/lib/evaluate/real.h
@@ -51,13 +51,6 @@ public:
   static constexpr int maxExponent{(1 << exponentBits) - 1};
   static constexpr int exponentBias{maxExponent / 2};
 
-  // Decimal precision of a binary floating-point representation is
-  // actually the same as the base-5 precision, as factors of two
-  // can be accommodated by the binary exponent.
-  // log(2)/log(5) = 0.430+ in any base.
-  // Calculate "precision*0.43" with integer arithmetic so as to be constexpr.
-  static constexpr int decimalDigits{(precision * 43) / 100};
-
   template<typename W, int P, bool I> friend class Real;
 
   constexpr Real() {}  // +0.0
diff --git a/flang/lib/evaluate/tools.h b/flang/lib/evaluate/tools.h
index 94798e9117a7..68cbb7af90a2 100644
--- a/flang/lib/evaluate/tools.h
+++ b/flang/lib/evaluate/tools.h
@@ -295,8 +295,9 @@ template<TypeCategory TOCAT, typename VALUE> struct ConvertToKindHelper {
 
 template<TypeCategory TOCAT, typename VALUE>
 Expr<SomeKind<TOCAT>> ConvertToKind(int kind, VALUE &&x) {
-  return *common::SearchDynamicTypes(
-      ConvertToKindHelper<TOCAT, VALUE>{kind, std::move(x)});
+  return common::SearchDynamicTypes(
+      ConvertToKindHelper<TOCAT, VALUE>{kind, std::move(x)})
+      .value();
 }
 
 // Given a type category CAT, SameKindExprs<CAT, N> is a variant that
diff --git a/flang/lib/parser/basic-parsers.h b/flang/lib/parser/basic-parsers.h
index c4891fc6636b..6a1167eceb8f 100644
--- a/flang/lib/parser/basic-parsers.h
+++ b/flang/lib/parser/basic-parsers.h
@@ -488,7 +488,7 @@ public:
       resultType result;
       result.emplace_back(std::move(*first));
       if (state.GetLocation() > start) {
-        result.splice(result.end(), *many(parser_).Parse(state));
+        result.splice(result.end(), many(parser_).Parse(state).value());
       }
       return {std::move(result)};
     }
@@ -581,8 +581,7 @@ public:
   constexpr DefaultedParser(const PA &p) : parser_{p} {}
   std::optional<resultType> Parse(ParseState &state) const {
     std::optional<std::optional<resultType>> ax{maybe(parser_).Parse(state)};
-    CHECK(ax.has_value());  // maybe() always succeeds
-    if (ax.value().has_value()) {
+    if (ax.value().has_value()) {  // maybe() always succeeds
       return std::move(*ax);
     }
     return {resultType{}};
diff --git a/flang/lib/parser/characters.h b/flang/lib/parser/characters.h
index 362ccc43e9d6..a54df93113f2 100644
--- a/flang/lib/parser/characters.h
+++ b/flang/lib/parser/characters.h
@@ -148,7 +148,7 @@ void EmitQuotedChar(char32_t ch, const NORMAL &emit, const INSERTED &insert,
     emit('\\');
   } else if (ch < ' ' || (ch >= 0x80 && ch <= 0xff)) {
     insert('\\');
-    if (std::optional escape{BackslashEscapeChar(ch)}) {
+    if (std::optional<char> escape{BackslashEscapeChar(ch)}) {
       emit(*escape);
     } else {
       // octal escape sequence
diff --git a/flang/lib/parser/parse-tree-visitor.h b/flang/lib/parser/parse-tree-visitor.h
index 664fbd92db58..8f759759203d 100644
--- a/flang/lib/parser/parse-tree-visitor.h
+++ b/flang/lib/parser/parse-tree-visitor.h
@@ -66,12 +66,12 @@ template<typename M> void Walk(format::IntrinsicTypeDataEditDesc &, M &);
 // Traversal of needed STL template classes (optional, list, tuple, variant)
 template<typename T, typename V>
 void Walk(const std::optional<T> &x, V &visitor) {
-  if (x) {
+  if (x.has_value()) {
     Walk(*x, visitor);
   }
 }
 template<typename T, typename M> void Walk(std::optional<T> &x, M &mutator) {
-  if (x) {
+  if (x.has_value()) {
     Walk(*x, mutator);
   }
 }
diff --git a/flang/lib/parser/token-parsers.h b/flang/lib/parser/token-parsers.h
index c7affcabb37b..c51287aefa5c 100644
--- a/flang/lib/parser/token-parsers.h
+++ b/flang/lib/parser/token-parsers.h
@@ -430,9 +430,8 @@ constexpr struct SkipDigitString {
 static std::optional<std::int64_t> SignedInteger(
     const std::optional<std::uint64_t> &x, Location at, bool negate,
     ParseState &state) {
-  std::optional<std::int64_t> result;
   if (!x.has_value()) {
-    return result;
+    return std::nullopt;
   }
   std::uint64_t limit{std::numeric_limits<std::int64_t>::max()};
   if (negate) {
@@ -442,8 +441,7 @@ static std::optional<std::int64_t> SignedInteger(
     state.Say(at, "overflow in signed decimal literal"_err_en_US);
   }
   std::int64_t value = *x;
-  result = negate ? -value : value;
-  return result;
+  return std::make_optional<std::int64_t>(negate ? -value : value);
 }
 
 struct SignedIntLiteralConstantWithoutKind {
@@ -580,9 +578,7 @@ struct HollerithLiteral {
       } else {
         // Multi-byte character
         while (bytes-- > 0) {
-          std::optional<const char *> byte{nextCh.Parse(state)};
-          CHECK(byte.has_value());
-          content += **byte;
+          content += *nextCh.Parse(state).value();
         }
       }
     }
diff --git a/flang/lib/semantics/expression.cc b/flang/lib/semantics/expression.cc
index ae12fdfb9f3d..9d068c4bccc2 100644
--- a/flang/lib/semantics/expression.cc
+++ b/flang/lib/semantics/expression.cc
@@ -818,8 +818,8 @@ MaybeExpr AnalyzeExpr(
             if (dynamicType->category == TypeCategory::Character) {
               return WrapperHelper<TypeCategory::Character, Designator,
                   Substring>(dynamicType->kind,
-                  Substring{
-                      std::move(*checked), std::move(first), std::move(last)});
+                  Substring{std::move(checked.value()), std::move(first),
+                      std::move(last)});
             }
           }
           context.Say("substring may apply only to CHARACTER"_err_en_US);
@@ -848,9 +848,8 @@ MaybeExpr AnalyzeExpr(ExpressionAnalysisContext &context,
         lower = Expr<SubscriptInteger>{1};
       }
       if (!upper.has_value()) {
-        std::optional<std::int64_t> size{ToInt64(length)};
-        CHECK(size.has_value());
-        upper = Expr<SubscriptInteger>{static_cast<std::int64_t>(*size)};
+        upper = Expr<SubscriptInteger>{
+            static_cast<std::int64_t>(ToInt64(length).value())};
       }
       return std::visit(
           [&](auto &&ckExpr) -> MaybeExpr {
@@ -861,8 +860,8 @@ MaybeExpr AnalyzeExpr(ExpressionAnalysisContext &context,
             staticData->set_alignment(Result::kind)
                 .set_itemBytes(Result::kind)
                 .Push(cp->value);
-            Substring substring{
-                std::move(staticData), std::move(*lower), std::move(*upper)};
+            Substring substring{std::move(staticData), std::move(lower.value()),
+                std::move(upper.value())};
             return AsGenericExpr(Expr<SomeCharacter>{
                 Expr<Result>{Designator<Result>{std::move(substring)}}});
           },