diff --git a/src/build-scripts/ci-startup.bash b/src/build-scripts/ci-startup.bash
index 9693d0244a..2522c28bc6 100755
--- a/src/build-scripts/ci-startup.bash
+++ b/src/build-scripts/ci-startup.bash
@@ -37,7 +37,7 @@ export OpenImageIO_CI=true
 export USE_NINJA=${USE_NINJA:=1}
 export CMAKE_GENERATOR=${CMAKE_GENERATOR:=Ninja}
 export CMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE:=Release}
-export CMAKE_CXX_STANDARD=${CMAKE_CXX_STANDARD:=11}
+export CMAKE_CXX_STANDARD=${CMAKE_CXX_STANDARD:=14}
 
 export LOCAL_DEPS_DIR=${LOCAL_DEPS_DIR:=$HOME/ext}
 export PATH=${LOCAL_DEPS_DIR}/dist/bin:$PATH
diff --git a/src/cmake/compiler.cmake b/src/cmake/compiler.cmake
index 83d6788731..e8f3061678 100644
--- a/src/cmake/compiler.cmake
+++ b/src/cmake/compiler.cmake
@@ -35,7 +35,7 @@ set (CMAKE_CXX_STANDARD_REQUIRED ON)
 set (CMAKE_CXX_EXTENSIONS OFF)
 message (STATUS "Building with C++${CMAKE_CXX_STANDARD}, downstream minimum C++${DOWNSTREAM_CXX_STANDARD}")
 if (CMAKE_CXX_STANDARD VERSION_LESS CMAKE_CXX_MINIMUM)
-    message (ERROR "C++${CMAKE_CXX_STANDARD} is not supported, minimum is C++${CMAKE_CXX_MINIMUM}")
+    message (FATAL_ERROR "C++${CMAKE_CXX_STANDARD} is not supported, minimum is C++${CMAKE_CXX_MINIMUM}")
 endif ()
 
 ###########################################################################
diff --git a/src/include/OpenImageIO/fmath.h b/src/include/OpenImageIO/fmath.h
index 35b2ce53d3..eb276480b9 100644
--- a/src/include/OpenImageIO/fmath.h
+++ b/src/include/OpenImageIO/fmath.h
@@ -145,7 +145,7 @@ using std::isnan;
 /// Quick test for whether an integer is a power of 2.
 ///
 template<typename T>
-inline OIIO_HOSTDEVICE OIIO_CONSTEXPR14 bool
+inline OIIO_HOSTDEVICE constexpr bool
 ispow2(T x) noexcept
 {
     // Numerous references for this bit trick are on the web.  The
@@ -158,7 +158,7 @@ ispow2(T x) noexcept
 
 /// Round up to next higher power of 2 (return x if it's already a power
 /// of 2).
-inline OIIO_HOSTDEVICE OIIO_CONSTEXPR14 int
+inline OIIO_HOSTDEVICE constexpr int
 ceil2(int x) noexcept
 {
     // Here's a version with no loops.
@@ -181,7 +181,7 @@ ceil2(int x) noexcept
 
 /// Round down to next lower power of 2 (return x if it's already a power
 /// of 2).
-inline OIIO_HOSTDEVICE OIIO_CONSTEXPR14 int
+inline OIIO_HOSTDEVICE constexpr int
 floor2(int x) noexcept
 {
     // Make all bits past the first 1 also be 1, i.e. 0001xxxx -> 00011111
diff --git a/src/include/OpenImageIO/hash.h b/src/include/OpenImageIO/hash.h
index 7cab6e0a08..7f4e4be400 100644
--- a/src/include/OpenImageIO/hash.h
+++ b/src/include/OpenImageIO/hash.h
@@ -320,7 +320,7 @@ OIIO_HOSTDEVICE inline constexpr uint128_t Uint128(uint64_t lo, uint64_t hi) {
     return lo + (((uint128_t)hi) << 64);
 }
 
-OIIO_HOSTDEVICE inline OIIO_CONSTEXPR14 void
+OIIO_HOSTDEVICE inline constexpr void
 CopyUint128(uint128_t &dst, const uint128_t src) {
     dst = src;
 }
@@ -336,11 +336,11 @@ OIIO_HOSTDEVICE inline constexpr uint64_t Uint128High64(const uint128_t x) {
     return x.second;
 }
 
-OIIO_HOSTDEVICE inline OIIO_CONSTEXPR14 uint128_t Uint128(uint64_t lo, uint64_t hi) {
+OIIO_HOSTDEVICE inline constexpr uint128_t Uint128(uint64_t lo, uint64_t hi) {
     return uint128_t(lo, hi);
 }
 
-OIIO_HOSTDEVICE inline OIIO_CONSTEXPR14 void
+OIIO_HOSTDEVICE inline constexpr void
 CopyUint128(uint128_t &dst, const uint128_t src) {
     dst.first  = src.first;
     dst.second = src.second;
@@ -402,7 +402,7 @@ uint128_t OIIO_API Hash128WithSeed(const char* s, size_t len, uint128_t seed);
 // This is intended to be a reasonably good hash function.
 // May change from time to time, may differ on different platforms, may differ
 // depending on NDEBUG.
-OIIO_HOSTDEVICE inline OIIO_CONSTEXPR14 uint64_t Hash128to64(uint128_t x) {
+OIIO_HOSTDEVICE inline constexpr uint64_t Hash128to64(uint128_t x) {
   // Murmur-inspired hashing.
   const uint64_t kMul = 0x9ddfea08eb382d69ULL;
   uint64_t a = (Uint128Low64(x) ^ Uint128High64(x)) * kMul;
@@ -426,7 +426,7 @@ uint128_t OIIO_API Fingerprint128(const char* s, size_t len);
 
 // This is intended to be a good fingerprinting primitive.
 // See below for more overloads.
-OIIO_HOSTDEVICE inline OIIO_CONSTEXPR14 uint64_t Fingerprint(uint128_t x) {
+OIIO_HOSTDEVICE inline constexpr uint64_t Fingerprint(uint128_t x) {
   // Murmur-inspired hashing.
   const uint64_t kMul = 0x9ddfea08eb382d69ULL;
   uint64_t a = (Uint128Low64(x) ^ Uint128High64(x)) * kMul;
@@ -440,7 +440,7 @@ OIIO_HOSTDEVICE inline OIIO_CONSTEXPR14 uint64_t Fingerprint(uint128_t x) {
 }
 
 // This is intended to be a good fingerprinting primitive.
-OIIO_HOSTDEVICE inline OIIO_CONSTEXPR14 uint64_t Fingerprint(uint64_t x) {
+OIIO_HOSTDEVICE inline constexpr uint64_t Fingerprint(uint64_t x) {
   // Murmur-inspired hashing.
   const uint64_t kMul = 0x9ddfea08eb382d69ULL;
   uint64_t b = x * kMul;
diff --git a/src/include/OpenImageIO/platform.h b/src/include/OpenImageIO/platform.h
index 9ef60c8a8f..99523f8bf9 100644
--- a/src/include/OpenImageIO/platform.h
+++ b/src/include/OpenImageIO/platform.h
@@ -658,20 +658,14 @@ inline void aligned_delete(T* t) {
 }
 
 
-
-#if OIIO_CPLUSPLUS_VERSION >= 14
-    using std::enable_if_t;    // Use C++14 std::enable_if_t
-#else
-    // Define enable_if_t for C++11
-    template <bool B, class T = void>
-    using enable_if_t = typename std::enable_if<B, T>::type;
-#endif
+// DEPRECATED(2.6)
+using std::enable_if_t;
 
 // An enable_if helper to be used in template parameters which results in
 // much shorter symbols: https://godbolt.org/z/sWw4vP
 // Borrowed from fmtlib.
 #ifndef OIIO_ENABLE_IF
-#   define OIIO_ENABLE_IF(...) OIIO::enable_if_t<(__VA_ARGS__), int> = 0
+#   define OIIO_ENABLE_IF(...) std::enable_if_t<(__VA_ARGS__), int> = 0
 #endif
 
 OIIO_NAMESPACE_END
diff --git a/src/include/OpenImageIO/span.h b/src/include/OpenImageIO/span.h
index d9d9725c3f..6c4efa1f94 100644
--- a/src/include/OpenImageIO/span.h
+++ b/src/include/OpenImageIO/span.h
@@ -233,7 +233,7 @@ using cspan = span<const T, Extent>;
 
 /// Compare all elements of two spans for equality
 template <class T, oiio_span_size_type X, class U, oiio_span_size_type Y>
-OIIO_CONSTEXPR14 bool operator== (span<T,X> l, span<U,Y> r) {
+constexpr bool operator== (span<T,X> l, span<U,Y> r) {
 #if OIIO_CPLUSPLUS_VERSION >= 20
     return std::equal (l.begin(), l.end(), r.begin(), r.end());
 #else
@@ -247,7 +247,7 @@ OIIO_CONSTEXPR14 bool operator== (span<T,X> l, span<U,Y> r) {
 
 /// Compare all elements of two spans for inequality
 template <class T, oiio_span_size_type X, class U, oiio_span_size_type Y>
-OIIO_CONSTEXPR14 bool operator!= (span<T,X> l, span<U,Y> r) {
+constexpr bool operator!= (span<T,X> l, span<U,Y> r) {
     return !(l == r);
 }
 
@@ -299,7 +299,7 @@ class span_strided {
 
     /// Construct from std::vector<T>.
     template<class Allocator>
-    OIIO_CONSTEXPR14 span_strided (std::vector<T, Allocator> &v)
+    constexpr span_strided (std::vector<T, Allocator> &v)
         : span_strided(v.data(), v.size(), 1) {}
 
     /// Construct from const std::vector<T>. This turns const std::vector<T>
@@ -359,7 +359,7 @@ using cspan_strided = span_strided<const T, Extent>;
 
 /// Compare all elements of two spans for equality
 template <class T, oiio_span_size_type X, class U, oiio_span_size_type Y>
-OIIO_CONSTEXPR14 bool operator== (span_strided<T,X> l, span_strided<U,Y> r) {
+constexpr bool operator== (span_strided<T,X> l, span_strided<U,Y> r) {
     auto lsize = l.size();
     if (lsize != r.size())
         return false;
@@ -371,7 +371,7 @@ OIIO_CONSTEXPR14 bool operator== (span_strided<T,X> l, span_strided<U,Y> r) {
 
 /// Compare all elements of two spans for inequality
 template <class T, oiio_span_size_type X, class U, oiio_span_size_type Y>
-OIIO_CONSTEXPR14 bool operator!= (span_strided<T,X> l, span_strided<U,Y> r) {
+constexpr bool operator!= (span_strided<T,X> l, span_strided<U,Y> r) {
     return !(l == r);
 }
 
diff --git a/src/include/OpenImageIO/string_view.h b/src/include/OpenImageIO/string_view.h
index eb5f928016..7329a1ac41 100644
--- a/src/include/OpenImageIO/string_view.h
+++ b/src/include/OpenImageIO/string_view.h
@@ -160,7 +160,7 @@ class basic_string_view {
     const CharT* c_str() const;
 
     // Assignment
-    OIIO_CONSTEXPR14 basic_string_view& operator=(const basic_string_view& copy) noexcept = default;
+    constexpr basic_string_view& operator=(const basic_string_view& copy) noexcept = default;
 
     /// Convert a string_view to a `std::string`.
     operator std::basic_string<CharT, Traits>() const {
@@ -223,22 +223,22 @@ class basic_string_view {
     constexpr const_pointer data() const noexcept { return m_chars; }
 
     // modifiers
-    OIIO_CONSTEXPR14 void clear() noexcept { init(nullptr, 0); }
-    OIIO_CONSTEXPR14 void remove_prefix(size_type n) noexcept
+    constexpr void clear() noexcept { init(nullptr, 0); }
+    constexpr void remove_prefix(size_type n) noexcept
     {
         if (n > m_len)
             n = m_len;
         m_chars += n;
         m_len -= n;
     }
-    OIIO_CONSTEXPR14 void remove_suffix(size_type n) noexcept
+    constexpr void remove_suffix(size_type n) noexcept
     {
         if (n > m_len)
             n = m_len;
         m_len -= n;
     }
 
-    OIIO_CONSTEXPR14 basic_string_view substr(size_type pos, size_type n = npos) const noexcept
+    constexpr basic_string_view substr(size_type pos, size_type n = npos) const noexcept
     {
         if (pos >= size())
             return basic_string_view();  // start past end -> return empty
@@ -409,7 +409,7 @@ class basic_string_view {
     const CharT* m_chars = nullptr;
     size_t m_len = 0;
 
-    OIIO_CONSTEXPR14 void init(const CharT* chars, size_t len) noexcept
+    constexpr void init(const CharT* chars, size_t len) noexcept
     {
         m_chars = chars;
         m_len   = len;
diff --git a/src/include/OpenImageIO/typedesc.h b/src/include/OpenImageIO/typedesc.h
index ab720bfddc..bcdc7ed212 100644
--- a/src/include/OpenImageIO/typedesc.h
+++ b/src/include/OpenImageIO/typedesc.h
@@ -177,7 +177,7 @@ struct OIIO_UTIL_API TypeDesc {
 
     /// Return the number of elements: 1 if not an array, or the array
     /// length. Invalid to call this for arrays of undetermined size.
-    OIIO_HOSTDEVICE OIIO_CONSTEXPR14 size_t numelements () const noexcept {
+    OIIO_HOSTDEVICE constexpr size_t numelements () const noexcept {
         OIIO_DASSERT_MSG (arraylen >= 0, "Called numelements() on TypeDesc "
                           "of array with unspecified length (%d)", arraylen);
         return (arraylen >= 1 ? arraylen : 1);
@@ -186,7 +186,7 @@ struct OIIO_UTIL_API TypeDesc {
     /// Return the number of basetype values: the aggregate count multiplied
     /// by the array length (or 1 if not an array). Invalid to call this
     /// for arrays of undetermined size.
-    OIIO_HOSTDEVICE OIIO_CONSTEXPR14 size_t basevalues () const noexcept {
+    OIIO_HOSTDEVICE constexpr size_t basevalues () const noexcept {
         return numelements() * aggregate;
     }
 
@@ -219,7 +219,7 @@ struct OIIO_UTIL_API TypeDesc {
 
     /// Return the type of one element, i.e., strip out the array-ness.
     ///
-    OIIO_HOSTDEVICE OIIO_CONSTEXPR14 TypeDesc elementtype () const noexcept {
+    OIIO_HOSTDEVICE constexpr TypeDesc elementtype () const noexcept {
         TypeDesc t (*this);  t.arraylen = 0;  return t;
     }