From 3d01f3ae3279b806739936f0ffd44598bf20245c Mon Sep 17 00:00:00 2001
From: Rupert Nash <r.nash@epcc.ed.ac.uk>
Date: Fri, 17 Sep 2021 11:03:25 +0100
Subject: [PATCH] Backport changes from
 7bea1e2ac36ac54b648ae5c9d381a59bc69db912 to fix #2273 for 2.x

---
 include/internal/catch_approx.h               | 2 +-
 projects/SelfTest/UsageTests/Approx.tests.cpp | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/include/internal/catch_approx.h b/include/internal/catch_approx.h
index 4522e5ad..2d12efe4 100644
--- a/include/internal/catch_approx.h
+++ b/include/internal/catch_approx.h
@@ -33,7 +33,7 @@ namespace Detail {
         Approx operator-() const;
 
         template <typename T, typename = typename std::enable_if<std::is_constructible<double, T>::value>::type>
-        Approx operator()( T const& value ) {
+        Approx operator()( T const& value ) const {
             Approx approx( static_cast<double>(value) );
             approx.m_epsilon = m_epsilon;
             approx.m_margin = m_margin;
diff --git a/projects/SelfTest/UsageTests/Approx.tests.cpp b/projects/SelfTest/UsageTests/Approx.tests.cpp
index 4029223a..6280599e 100644
--- a/projects/SelfTest/UsageTests/Approx.tests.cpp
+++ b/projects/SelfTest/UsageTests/Approx.tests.cpp
@@ -212,4 +212,11 @@ TEST_CASE( "Comparison with explicitly convertible types", "[Approx]" )
 
 }
 
+TEST_CASE("Approx::operator() is const correct", "[Approx][.approvals]") {
+  const Approx ap = Approx(0.0).margin(0.01);
+
+  // As long as this compiles, the test should be considered passing
+  REQUIRE(1.0 == ap(1.0));
+}
+
 }} // namespace ApproxTests