Use C++20 (#2659)

include/mbgl/style/expression/expression.hpp

@@ -224,7 +224,7 @@ public:
     virtual void eachChild(const std::function<void(const Expression&)>&) const = 0;
 
     virtual bool operator==(const Expression&) const = 0;
-    bool operator!=(const Expression& rhs) const noexcept { return !operator==(rhs); }
+    bool operator!=(const Expression&) const = default;
 
     Kind getKind() const noexcept { return kind; };
     const type::Type& getType() const noexcept { return type; };

include/mbgl/style/expression/format_section_override.hpp

@@ -35,7 +35,7 @@ public:
         defaultValue.match([&fn](const style::PropertyExpression<T>& e) { fn(e.getExpression()); }, [](const T&) {});
     }
 
-    bool operator==(const Expression& e) const final {
+    bool operator==(const Expression& e) const final override {
         if (e.getKind() == Kind::FormatSectionOverride) {
             const auto* other = static_cast<const FormatSectionOverride*>(&e);
 
@@ -60,6 +60,8 @@ public:
         return false;
     }
 
+    bool operator==(const FormatSectionOverride& other) const { return *this == static_cast<const Expression&>(other); }
+
     std::vector<std::optional<Value>> possibleOutputs() const final { return {std::nullopt}; }
 
     std::string getOperator() const final { return "format-section-override"; }