`<cmath>`: add arithmetic overloads for `std::lerp` (#2113)

Co-authored-by: Casey Carter <Casey@Carter.net>
Co-authored-by: Stephan T. Lavavej <stl@nuwen.net>

stl/inc/cmath

@@ -693,7 +693,7 @@ _GENERIC_MATH2(fdim)
 _GENERIC_MATH2(fmax)
 _GENERIC_MATH2(fmin)
 // fma() is hand-crafted
-// lerp() should be exempt, LWG-3223
+// lerp() is hand-crafted
 // The "classification/comparison functions" (fpclassify(), etc.) are exempt, LWG-1327
 // TRANSITION, VSO-945789, Special Math shouldn't be exempt
 
@@ -1308,9 +1308,6 @@ _NODISCARD constexpr _Ty _Common_lerp(const _Ty _ArgA, const _Ty _ArgB, const _T
     }
 }
 
-// As of 2019-06-17 it is unclear whether the "sufficient additional overloads" clause is intended to target lerp;
-// LWG-3223 is pending.
-
 _NODISCARD constexpr inline float lerp(const float _ArgA, const float _ArgB, const float _ArgT) noexcept {
     return _Common_lerp(_ArgA, _ArgB, _ArgT);
 }
@@ -1323,6 +1320,13 @@ _NODISCARD constexpr inline long double lerp(
     const long double _ArgA, const long double _ArgB, const long double _ArgT) noexcept {
     return _Common_lerp(_ArgA, _ArgB, _ArgT);
 }
+
+template <class _Ty1, class _Ty2, class _Ty3,
+    enable_if_t<is_arithmetic_v<_Ty1> && is_arithmetic_v<_Ty2> && is_arithmetic_v<_Ty3>, int> = 0>
+_NODISCARD constexpr auto lerp(const _Ty1 _ArgA, const _Ty2 _ArgB, const _Ty3 _ArgT) noexcept {
+    using _Tgt = conditional_t<_Is_any_of_v<long double, _Ty1, _Ty2, _Ty3>, long double, double>;
+    return _Common_lerp(static_cast<_Tgt>(_ArgA), static_cast<_Tgt>(_ArgB), static_cast<_Tgt>(_ArgT));
+}
 #endif // _HAS_CXX20
 _STD_END
 #endif // _HAS_CXX17

tests/std/tests/P0811R3_midpoint_lerp/test.cpp

@@ -1023,6 +1023,20 @@ bool test_lerp() {
     return true;
 }
 
+constexpr bool test_gh_2112() {
+    // GH-2112 <cmath>: std::lerp is missing Arithmetic overloads
+    assert(lerp(0, 0, 0) == 0.0);
+    assert(lerp(0.0f, 0.0f, 0.0) == 0.0);
+    assert(lerp(0.0L, 0, 0) == 0.0L);
+
+    STATIC_ASSERT(is_same_v<double, decltype(lerp(0, 0, 0))>);
+    STATIC_ASSERT(is_same_v<long double, decltype(lerp(0.0L, 0, 0))>);
+    STATIC_ASSERT(is_same_v<long double, decltype(lerp(0, 0.0L, 0))>);
+    STATIC_ASSERT(is_same_v<long double, decltype(lerp(0, 0, 0.0L))>);
+
+    return true;
+}
+
 int main() {
     test_constants<float>();
     test_constants<double>();
@@ -1093,4 +1107,7 @@ int main() {
     test_lerp<float>();
     test_lerp<double>();
     test_lerp<long double>();
+
+    test_gh_2112();
+    STATIC_ASSERT(test_gh_2112());
 }