diff --git a/include/upa/str_arg.h b/include/upa/str_arg.h
index 509918f..21c5067 100644
--- a/include/upa/str_arg.h
+++ b/include/upa/str_arg.h
@@ -18,6 +18,7 @@ inline void procfn(StrT&& str) {
 #ifndef UPA_STR_ARG_H
 #define UPA_STR_ARG_H
 
+#include "config.h"
 #include "url_utf.h"
 #include <cassert>
 #include <cstddef>
@@ -76,26 +77,26 @@ class str_arg {
         >;
 
     // constructors
-    str_arg(const str_arg&) noexcept = default;
+    constexpr str_arg(const str_arg&) noexcept = default;
 
-    str_arg(const CharT* s)
+    constexpr str_arg(const CharT* s)
         : first_(s)
         , last_(s + traits_type::length(s))
     {}
 
     template <typename SizeT, std::enable_if_t<is_size_type_v<SizeT>, int> = 0>
-    str_arg(const CharT* s, SizeT length)
+    constexpr str_arg(const CharT* s, SizeT length)
         : first_(s)
         , last_(s + length)
     { assert(length >= 0); }
 
-    str_arg(const CharT* first, const CharT* last)
+    constexpr str_arg(const CharT* first, const CharT* last)
         : first_(first)
         , last_(last)
     { assert(first <= last); }
 
     // destructor
-    ~str_arg() noexcept = default;
+    UPA_CONSTEXPR_20 ~str_arg() noexcept = default;
 
     // assignment is not used
     str_arg& operator=(const str_arg&) = delete;
@@ -176,7 +177,7 @@ constexpr bool convertible_to_string_view_v =
 template<typename CharT, class ArgT>
 struct str_arg_char_common {
     using type = CharT;
-    static str_arg<CharT> to_str_arg(ArgT str) {
+    static constexpr str_arg<CharT> to_str_arg(ArgT str) {
         return { str.data(), str.size() };
     }
 };
@@ -234,7 +235,7 @@ struct str_arg_char : detail::str_arg_char_default<StrT> {};
 template<class CharT>
 struct str_arg_char<CharT*, std::enable_if_t<is_char_type_v<remove_cvref_t<CharT>>>> {
     using type = remove_cvref_t<CharT>;
-    static str_arg<type> to_str_arg(const type* s) {
+    static constexpr str_arg<type> to_str_arg(const type* s) {
         return s;
     }
 };
@@ -243,7 +244,7 @@ struct str_arg_char<CharT*, std::enable_if_t<is_char_type_v<remove_cvref_t<CharT
 template<class CharT>
 struct str_arg_char<str_arg<CharT>> {
     using type = CharT;
-    static str_arg<type> to_str_arg(str_arg<type> s) {
+    static constexpr str_arg<type> to_str_arg(str_arg<type> s) {
         return s;
     }
 };
@@ -267,7 +268,7 @@ using enable_if_str_arg_t = std::enable_if_t<
 // String arguments helper function
 
 template <class StrT>
-inline auto make_str_arg(StrT&& str) -> str_arg<str_arg_char_t<StrT>> {
+constexpr auto make_str_arg(StrT&& str) -> str_arg<str_arg_char_t<StrT>> {
     return str_arg_char_s<StrT>::to_str_arg(std::forward<StrT>(str));
 }
 
@@ -304,7 +305,7 @@ inline std::string&& make_string(std::string&& str) {
 }
 
 template <class StrT, enable_if_str_arg_to_char8_t<StrT> = 0>
-inline string_view make_string(StrT&& str) {
+constexpr string_view make_string(StrT&& str) {
     const auto inp = make_str_arg(std::forward<StrT>(str));
     return { inp.data(), inp.length() };
 }
diff --git a/include/upa/url.h b/include/upa/url.h
index 55bb3c2..88d08c9 100644
--- a/include/upa/url.h
+++ b/include/upa/url.h
@@ -907,7 +907,7 @@ class url_parser {
 // part start
 extern const uint8_t kPartStart[url::PART_COUNT];
 
-inline int port_from_str(const char* first, const char* last) noexcept {
+constexpr int port_from_str(const char* first, const char* last) noexcept {
     int port = 0;
     for (auto it = first; it != last; ++it) {
         port = port * 10 + (*it - '0');
@@ -930,7 +930,7 @@ constexpr bool is_removable_char(CharT ch) noexcept {
 }
 
 template <typename CharT>
-inline void do_trim(const CharT*& first, const CharT*& last) noexcept {
+constexpr void do_trim(const CharT*& first, const CharT*& last) noexcept {
     // remove leading C0 controls and space
     while (first < last && is_trim_char(*first))
         ++first;
@@ -965,7 +965,7 @@ inline void do_remove_whitespace(const CharT*& first, const CharT*& last, simple
 // reverse find
 
 template<class InputIt, class T>
-inline InputIt find_last(InputIt first, InputIt last, const T& value) {
+constexpr InputIt find_last(InputIt first, InputIt last, const T& value) {
     for (auto it = last; it > first;) {
         --it;
         if (*it == value) return it;
@@ -1023,7 +1023,7 @@ constexpr bool is_normalized_windows_drive(CharT c1, CharT c2) noexcept {
 
 // https://url.spec.whatwg.org/#start-with-a-windows-drive-letter
 template <typename CharT>
-inline bool starts_with_windows_drive(const CharT* pointer, const CharT* last) noexcept {
+constexpr bool starts_with_windows_drive(const CharT* pointer, const CharT* last) noexcept {
     const auto length = last - pointer;
     return
         (length == 2 || (length > 2 && detail::is_special_authority_end_char(pointer[2]))) &&
@@ -1042,7 +1042,7 @@ inline bool starts_with_windows_drive(const CharT* pointer, const CharT* last) n
 
 // Check url's pathname has Windows drive, i.e. starts with "/C:/" or is "/C:"
 // see also: detail::starts_with_windows_drive
-inline bool pathname_has_windows_os_drive(string_view pathname) noexcept {
+constexpr bool pathname_has_windows_os_drive(string_view pathname) noexcept {
     return
         (pathname.length() == 3 || (pathname.length() > 3 && is_windows_slash(pathname[3]))) &&
         is_windows_slash(pathname[0]) &&
@@ -2319,11 +2319,11 @@ inline void url_parser::parse_path(url_serializer& urls, const CharT* first, con
     // path state; includes:
     // 1. [ (/,\) - 1, 2, 3, 4 - [ 1 (if first segment), 2 ] ]
     // 2. [ 1 ... 4 ]
-    static const auto escaped_dot = [](const CharT* const pointer) -> bool {
+    static constexpr auto escaped_dot = [](const CharT* const pointer) constexpr -> bool {
         // "%2e" or "%2E"
         return pointer[0] == '%' && pointer[1] == '2' && (pointer[2] | 0x20) == 'e';
     };
-    static const auto double_dot = [](const CharT* const pointer, const std::size_t len) -> bool {
+    static constexpr auto double_dot = [](const CharT* const pointer, const std::size_t len) constexpr -> bool {
         switch (len) {
         case 2: // ".."
             return pointer[0] == '.' && pointer[1] == '.';
@@ -2336,7 +2336,7 @@ inline void url_parser::parse_path(url_serializer& urls, const CharT* first, con
             return false;
         }
     };
-    static const auto single_dot = [](const CharT* const pointer, const std::size_t len) -> bool {
+    static constexpr auto single_dot = [](const CharT* const pointer, const std::size_t len) constexpr -> bool {
         switch (len) {
         case 1: return pointer[0] == '.';
         case 3: return escaped_dot(pointer); // "%2e"
@@ -3031,7 +3031,7 @@ inline const CharT* is_unc_path(const CharT* first, const CharT* last)
 /// @param[in] is_slash function to check char is slash (or backslash)
 /// @return true if path contains ".." segment
 template <typename CharT, typename IsSlash>
-inline bool has_dot_dot_segment(const CharT* first, const CharT* last, IsSlash is_slash) {
+constexpr bool has_dot_dot_segment(const CharT* first, const CharT* last, IsSlash is_slash) {
     if (last - first >= 2) {
         const auto* ptr = first;
         const auto* end = last - 1;
diff --git a/include/upa/url_percent_encode.h b/include/upa/url_percent_encode.h
index e8a4ca4..bc1baee 100644
--- a/include/upa/url_percent_encode.h
+++ b/include/upa/url_percent_encode.h
@@ -297,37 +297,37 @@ inline constexpr code_points_multiset code_points;
 // Check char is in predefined set
 
 template <typename CharT>
-inline bool is_char_in_set(CharT c, const code_point_set& cpset) {
+constexpr bool is_char_in_set(CharT c, const code_point_set& cpset) {
     return cpset[c];
 }
 
 template <typename CharT>
-inline bool is_ipv4_char(CharT c) {
+constexpr bool is_ipv4_char(CharT c) {
     return code_points.char_in_set(c, IPV4_CHAR_SET);
 }
 
 template <typename CharT>
-inline bool is_hex_char(CharT c) {
+constexpr bool is_hex_char(CharT c) {
     return code_points.char_in_set(c, HEX_DIGIT_SET);
 }
 
 template <typename CharT>
-inline bool is_scheme_char(CharT c) {
+constexpr bool is_scheme_char(CharT c) {
     return code_points.char_in_set(c, SCHEME_SET);
 }
 
 template <typename CharT>
-inline bool is_forbidden_domain_char(CharT c) {
+constexpr bool is_forbidden_domain_char(CharT c) {
     return code_points.char_in_set(c, DOMAIN_FORBIDDEN_SET);
 }
 
 template <typename CharT>
-inline bool is_forbidden_host_char(CharT c) {
+constexpr bool is_forbidden_host_char(CharT c) {
     return code_points.char_in_set(c, HOST_FORBIDDEN_SET);
 }
 
 template <typename CharT>
-inline bool is_ascii_domain_char(CharT c) {
+constexpr bool is_ascii_domain_char(CharT c) {
     return code_points.char_in_set(c, ASCII_DOMAIN_SET);
 }
 
diff --git a/include/upa/url_result.h b/include/upa/url_result.h
index f69fbc6..4502b25 100644
--- a/include/upa/url_result.h
+++ b/include/upa/url_result.h
@@ -123,11 +123,11 @@ struct result_value {
     T value{};
     R result{};
 
-    result_value(R res) noexcept
+    constexpr result_value(R res) noexcept
         : result(res) {}
-    result_value(R res, T val) noexcept
+    constexpr result_value(R res, T val) noexcept
         : value(val), result(res) {}
-    [[nodiscard]] operator R() const noexcept {
+    [[nodiscard]] constexpr operator R() const noexcept {
         return result;
     }
 };
diff --git a/include/upa/url_utf.h b/include/upa/url_utf.h
index dd01b96..c367271 100644
--- a/include/upa/url_utf.h
+++ b/include/upa/url_utf.h
@@ -41,8 +41,8 @@ class url_utf {
 protected:
     // low level
     static bool read_code_point(const char*& first, const char* last, uint32_t& code_point) noexcept;
-    static bool read_code_point(const char16_t*& first, const char16_t* last, uint32_t& code_point) noexcept;
-    static bool read_code_point(const char32_t*& first, const char32_t* last, uint32_t& code_point) noexcept;
+    static constexpr bool read_code_point(const char16_t*& first, const char16_t* last, uint32_t& code_point) noexcept;
+    static constexpr bool read_code_point(const char32_t*& first, const char32_t* last, uint32_t& code_point) noexcept;
 private:
     const static char kReplacementCharUtf8[];
     const static uint8_t k_U8_LEAD3_T1_BITS[16];
@@ -179,7 +179,7 @@ namespace detail {
     // Get a supplementary code point value (U+10000..U+10ffff)
     // from its lead and trail surrogates.
     // Based on U16_GET_SUPPLEMENTARY in utf16.h from ICU
-    inline uint32_t u16_get_supplementary(uint32_t lead, uint32_t trail) noexcept {
+    constexpr uint32_t u16_get_supplementary(uint32_t lead, uint32_t trail) noexcept {
         constexpr uint32_t u16_surrogate_offset = (0xd800 << 10UL) + 0xdc00 - 0x10000;
         return (lead << 10UL) + trail - u16_surrogate_offset;
     }
@@ -187,7 +187,7 @@ namespace detail {
 
 // Modified version of the U16_NEXT_OR_FFFD macro in utf16.h from ICU
 
-inline bool url_utf::read_code_point(const char16_t*& first, const char16_t* last, uint32_t& c) noexcept {
+constexpr bool url_utf::read_code_point(const char16_t*& first, const char16_t* last, uint32_t& c) noexcept {
     c = *first++;
     if (detail::u16_is_surrogate(c)) {
         if (detail::u16_is_surrogate_lead(c) && first != last && detail::u16_is_trail(*first)) {
@@ -201,7 +201,7 @@ inline bool url_utf::read_code_point(const char16_t*& first, const char16_t* las
     return true;
 }
 
-inline bool url_utf::read_code_point(const char32_t*& first, const char32_t*, uint32_t& c) noexcept {
+constexpr bool url_utf::read_code_point(const char32_t*& first, const char32_t*, uint32_t& c) noexcept {
     // no conversion
     c = *first++;
     // don't allow surogates (U+D800..U+DFFF) and too high values
diff --git a/test/wpt-url.cpp b/test/wpt-url.cpp
index fe055e5..e409b5b 100644
--- a/test/wpt-url.cpp
+++ b/test/wpt-url.cpp
@@ -221,7 +221,7 @@ void test_host_parser(DataDrivenTest& ddt, const parsed_obj& obj)
     // Test file format (toascii.json):
     // https://github.com/web-platform-tests/wpt/tree/master/url#toasciijson
     // https://github.com/web-platform-tests/wpt/pull/5976
-    static const auto make_url = [](const std::string& host)->std::string {
+    static constexpr auto make_url = [](const std::string& host)->std::string {
         std::string str_url("http://");
         str_url += host;
         str_url += "/x";
@@ -287,14 +287,14 @@ void test_idna_v2(DataDrivenTest& ddt, const parsed_obj& obj)
     // Test file format (toascii.json):
     // https://github.com/web-platform-tests/wpt/tree/master/url#toasciijson
     // https://github.com/web-platform-tests/wpt/pull/5976
-    static const auto make_url = [](const std::string& host) -> std::string {
+    static constexpr auto make_url = [](const std::string& host) -> std::string {
         std::string str_url("http://");
         str_url += host;
         str_url += "/x";
         return str_url;
     };
 
-    static const auto encodeHostEndingCodePoints = [](const std::string& input) -> std::string {
+    static constexpr auto encodeHostEndingCodePoints = [](const std::string& input) -> std::string {
         if (input.find_first_of(":/?#\\") != input.npos)
             return encodeURIComponent(input);
         return input;