Allow implicit widening between decimal types and keep explicit narrowing

.github/workflows/codecov.yml

@@ -49,6 +49,7 @@ jobs:
           git submodule update --init libs/predef
           git submodule update --init libs/static_assert
           git submodule update --init libs/test
+          git submodule update --init libs/random
           ./bootstrap.sh
           ./b2 headers
       - name: gcc-gcov-native

include/boost/decimal/decimal128.hpp

@@ -337,7 +337,10 @@ BOOST_DECIMAL_EXPORT class decimal128 final
     explicit constexpr operator std::bfloat16_t() const noexcept;
     #endif
 
-    template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal>, bool> = true>
+    template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal> && (detail::impl::decimal_val_v<Decimal> > detail::impl::decimal_val_v<decimal128>), bool> = true>
+    constexpr operator Decimal() const noexcept;
+
+    template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal> && (detail::impl::decimal_val_v<Decimal> <= detail::impl::decimal_val_v<decimal128>), bool> = true>
     explicit constexpr operator Decimal() const noexcept;
 
     // cmath functions that are easier as friends
@@ -1112,7 +1115,13 @@ constexpr decimal128::operator std::bfloat16_t() const noexcept
 }
 #endif
 
-template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal>, bool>>
+template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal> && (detail::impl::decimal_val_v<Decimal> > detail::impl::decimal_val_v<decimal128>), bool>>
+constexpr decimal128::operator Decimal() const noexcept
+{
+    return to_decimal<Decimal>(*this);
+}
+
+template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal> && (detail::impl::decimal_val_v<Decimal> <= detail::impl::decimal_val_v<decimal128>), bool>>
 constexpr decimal128::operator Decimal() const noexcept
 {
     return to_decimal<Decimal>(*this);

include/boost/decimal/decimal32.hpp

@@ -273,8 +273,9 @@ BOOST_DECIMAL_EXPORT class decimal32 final // NOLINT(cppcoreguidelines-special-m
     explicit constexpr operator detail::uint128_t() const noexcept;
     #endif
 
+    // We allow implict promotions to and decimal type with greater or equal precision (e.g. decimal32_fast)
     template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal>, bool> = true>
-    explicit constexpr operator Decimal() const noexcept;
+    constexpr operator Decimal() const noexcept;
 
     // 3.2.5 initialization from coefficient and exponent:
     #ifdef BOOST_DECIMAL_HAS_CONCEPTS

include/boost/decimal/decimal32_fast.hpp

@@ -16,6 +16,7 @@
 #include <boost/decimal/detail/div_impl.hpp>
 #include <boost/decimal/detail/promote_significand.hpp>
 #include <boost/decimal/detail/ryu/ryu_generic_128.hpp>
+#include <boost/decimal/detail/promotion.hpp>
 
 #ifndef BOOST_DECIMAL_BUILD_MODULE
 #include <limits>
@@ -112,7 +113,7 @@ BOOST_DECIMAL_EXPORT class decimal32_fast final
     BOOST_DECIMAL_REQUIRES_RETURN(detail::is_decimal_floating_point_v, DecimalType, std::uint32_t);
 
 public:
-    constexpr decimal32_fast() noexcept {}
+    constexpr decimal32_fast() noexcept = default;
 
     template <typename T1, typename T2, std::enable_if_t<detail::is_integral_v<T1> && detail::is_integral_v<T2>, bool> = true>
     constexpr decimal32_fast(T1 coeff, T2 exp, bool sign = false) noexcept;
@@ -319,7 +320,10 @@ BOOST_DECIMAL_EXPORT class decimal32_fast final
 
 
     // Conversion to other decimal type
-    template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal>, bool> = true>
+    template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal> && (detail::impl::decimal_val_v<Decimal> > detail::impl::decimal_val_v<decimal32_fast>), bool> = true>
+    constexpr operator Decimal() const noexcept;
+
+    template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal> && (detail::impl::decimal_val_v<Decimal> <= detail::impl::decimal_val_v<decimal32_fast>), bool> = true>
     explicit constexpr operator Decimal() const noexcept;
 
     friend constexpr auto direct_init(std::uint_fast32_t significand, std::uint_fast8_t exponent, bool sign) noexcept -> decimal32_fast;
@@ -1341,7 +1345,13 @@ constexpr decimal32_fast::operator std::bfloat16_t() const noexcept
 }
 #endif
 
-template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal>, bool>>
+template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal> && (detail::impl::decimal_val_v<Decimal> > detail::impl::decimal_val_v<decimal32_fast>), bool>>
+constexpr decimal32_fast::operator Decimal() const noexcept
+{
+    return to_decimal<Decimal>(*this);
+}
+
+template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal> && (detail::impl::decimal_val_v<Decimal> <= detail::impl::decimal_val_v<decimal32_fast>), bool>>
 constexpr decimal32_fast::operator Decimal() const noexcept
 {
     return to_decimal<Decimal>(*this);

include/boost/decimal/decimal64.hpp

@@ -294,7 +294,11 @@ BOOST_DECIMAL_EXPORT class decimal64 final
     #endif
 
 
-    template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal>, bool> = true>
+    // Conversion to other decimal type
+    template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal> && (detail::impl::decimal_val_v<Decimal> > detail::impl::decimal_val_v<decimal64>), bool> = true>
+    constexpr operator Decimal() const noexcept;
+
+    template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal> && (detail::impl::decimal_val_v<Decimal> <= detail::impl::decimal_val_v<decimal64>), bool> = true>
     explicit constexpr operator Decimal() const noexcept;
 
     // 3.2.6 Conversion to floating-point type
@@ -951,8 +955,13 @@ constexpr decimal64::operator detail::uint128_t() const noexcept
 
 #endif
 
+template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal> && (detail::impl::decimal_val_v<Decimal> > detail::impl::decimal_val_v<decimal64>), bool>>
+constexpr decimal64::operator Decimal() const noexcept
+{
+    return to_decimal<Decimal>(*this);
+}
 
-template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal>, bool>>
+template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal> && (detail::impl::decimal_val_v<Decimal> <= detail::impl::decimal_val_v<decimal64>), bool>>
 constexpr decimal64::operator Decimal() const noexcept
 {
     return to_decimal<Decimal>(*this);

include/boost/decimal/decimal64_fast.hpp

@@ -17,6 +17,7 @@
 #include <boost/decimal/detail/div_impl.hpp>
 #include <boost/decimal/detail/promote_significand.hpp>
 #include <boost/decimal/detail/ryu/ryu_generic_128.hpp>
+#include <boost/decimal/detail/promotion.hpp>
 
 #ifndef BOOST_DECIMAL_BUILD_MODULE
 
@@ -267,7 +268,11 @@ BOOST_DECIMAL_EXPORT class decimal64_fast final
     explicit constexpr operator std::bfloat16_t() const noexcept;
     #endif
 
-    template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal>, bool> = true>
+    // Conversion to other decimal type
+    template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal> && (detail::impl::decimal_val_v<Decimal> > detail::impl::decimal_val_v<decimal64_fast>), bool> = true>
+    constexpr operator Decimal() const noexcept;
+
+    template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal> && (detail::impl::decimal_val_v<Decimal> <= detail::impl::decimal_val_v<decimal64_fast>), bool> = true>
     explicit constexpr operator Decimal() const noexcept;
 
     // Unary Operators
@@ -930,7 +935,13 @@ constexpr decimal64_fast::operator std::bfloat16_t() const noexcept
 }
 #endif
 
-template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal>, bool>>
+template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal> && (detail::impl::decimal_val_v<Decimal> > detail::impl::decimal_val_v<decimal64_fast>), bool>>
+constexpr decimal64_fast::operator Decimal() const noexcept
+{
+    return to_decimal<Decimal>(*this);
+}
+
+template <BOOST_DECIMAL_DECIMAL_FLOATING_TYPE Decimal, std::enable_if_t<detail::is_decimal_floating_point_v<Decimal> && (detail::impl::decimal_val_v<Decimal> <= detail::impl::decimal_val_v<decimal64_fast>), bool>>
 constexpr decimal64_fast::operator Decimal() const noexcept
 {
     return to_decimal<Decimal>(*this);

test/Jamfile

@@ -122,8 +122,14 @@ run test_git_issue_266.cpp ;
 run test_git_issue_271.cpp ;
 run test_hash.cpp ;
 run test_hermite.cpp ;
+compile-fail test_illegal_decimal32_fast_implicit_conversions.cpp ;
+compile-fail test_illegal_decimal32_implicit_conversions.cpp ;
+compile-fail test_illegal_decimal64_fast_implicit_conversions.cpp ;
+compile-fail test_illegal_decimal64_implicit_conversions.cpp ;
+compile-fail test_illegal_decimal128_implicit_conversions.cpp ;
 run test_implicit_integral_conversion.cpp ;
 run test_laguerre.cpp ;
+run test_legal_implicit_conversions.cpp ;
 run test_legendre.cpp ;
 run test_literals.cpp ;
 run test_limits.cpp ;

test/cover/make_gcov_02_files.gmk

@@ -8,13 +8,19 @@
 
 FILES_PRJ     := $(basename $(wildcard $(PATH_SRC)/*.cpp))
 
-FILES_EXCLUDE := $(PATH_SRC)/concepts_test.cpp               \
-                 $(PATH_SRC)/link_1.cpp                      \
-                 $(PATH_SRC)/link_2.cpp                      \
-                 $(PATH_SRC)/link_3.cpp                      \
-                 $(PATH_SRC)/test_bad_evaluation_method.cpp  \
-                 $(PATH_SRC)/test_explicit_floats.cpp        \
-                 $(PATH_SRC)/test_from_chars.cpp
+FILES_EXCLUDE := $(PATH_SRC)/concepts_test.cpp                                      \
+                 $(PATH_SRC)/link_1.cpp                                             \
+                 $(PATH_SRC)/link_2.cpp                                             \
+                 $(PATH_SRC)/link_3.cpp                                             \
+                 $(PATH_SRC)/test_bad_evaluation_method.cpp                         \
+                 $(PATH_SRC)/test_explicit_floats.cpp                               \
+                 $(PATH_SRC)/test_from_chars.cpp                                    \
+                 $(PATH_SRC)/test_illegal_decimal32_fast_implicit_conversions.cpp   \
+                 $(PATH_SRC)/test_illegal_decimal32_implicit_conversions.cpp        \
+                 $(PATH_SRC)/test_illegal_decimal64_fast_implicit_conversions.cpp   \
+                 $(PATH_SRC)/test_illegal_decimal64_implicit_conversions.cpp        \
+                 $(PATH_SRC)/test_illegal_decimal128_implicit_conversions.cpp       \
+
 
 FILES_EXCLUDE := $(basename $(FILES_EXCLUDE))
 

test/test_illegal_decimal128_implicit_conversions.cpp

@@ -0,0 +1,22 @@
+// Copyright 2025 Matt Borland
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/decimal.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+template <typename From, typename To>
+void test_implicit()
+{
+    const From from_val {2, 1};
+    const To to_val = from_val;
+
+    BOOST_TEST_EQ(from_val, to_val);
+}
+
+int main()
+{
+    test_implicit<boost::decimal::decimal128_fast, boost::decimal::decimal128>();
+
+    return boost::report_errors();
+}

test/test_illegal_decimal32_fast_implicit_conversions.cpp

@@ -0,0 +1,25 @@
+// Copyright 2025 Matt Borland
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/decimal.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+template <typename From, typename To>
+void test_implicit()
+{
+    const From from_val {2, 1};
+    const To to_val = from_val;
+
+    BOOST_TEST_EQ(from_val, to_val);
+}
+
+int main()
+{
+    test_implicit<boost::decimal::decimal64, boost::decimal::decimal32_fast>();
+    test_implicit<boost::decimal::decimal64_fast, boost::decimal::decimal32_fast>();
+    test_implicit<boost::decimal::decimal128, boost::decimal::decimal32_fast>();
+    test_implicit<boost::decimal::decimal128_fast, boost::decimal::decimal32_fast>();
+
+    return boost::report_errors();
+}

test/test_illegal_decimal32_implicit_conversions.cpp

@@ -0,0 +1,26 @@
+// Copyright 2025 Matt Borland
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/decimal.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+template <typename From, typename To>
+void test_implicit()
+{
+    const From from_val {2, 1};
+    const To to_val = from_val;
+
+    BOOST_TEST_EQ(from_val, to_val);
+}
+
+int main()
+{
+    test_implicit<boost::decimal::decimal32_fast, boost::decimal::decimal32>();
+    test_implicit<boost::decimal::decimal64, boost::decimal::decimal32>();
+    test_implicit<boost::decimal::decimal64_fast, boost::decimal::decimal32>();
+    test_implicit<boost::decimal::decimal128, boost::decimal::decimal32>();
+    test_implicit<boost::decimal::decimal128_fast, boost::decimal::decimal32>();
+
+    return boost::report_errors();
+}

test/test_illegal_decimal64_fast_implicit_conversions.cpp

@@ -0,0 +1,23 @@
+// Copyright 2025 Matt Borland
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/decimal.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+template <typename From, typename To>
+void test_implicit()
+{
+    const From from_val {2, 1};
+    const To to_val = from_val;
+
+    BOOST_TEST_EQ(from_val, to_val);
+}
+
+int main()
+{
+    test_implicit<boost::decimal::decimal128, boost::decimal::decimal64_fast>();
+    test_implicit<boost::decimal::decimal128_fast, boost::decimal::decimal64_fast>();
+
+    return boost::report_errors();
+}

test/test_illegal_decimal64_implicit_conversions.cpp

@@ -0,0 +1,24 @@
+// Copyright 2025 Matt Borland
+// Distributed under the Boost Software License, Version 1.0.
+// https://www.boost.org/LICENSE_1_0.txt
+
+#include <boost/decimal.hpp>
+#include <boost/core/lightweight_test.hpp>
+
+template <typename From, typename To>
+void test_implicit()
+{
+    const From from_val {2, 1};
+    const To to_val = from_val;
+
+    BOOST_TEST_EQ(from_val, to_val);
+}
+
+int main()
+{
+    test_implicit<boost::decimal::decimal64_fast, boost::decimal::decimal64>();
+    test_implicit<boost::decimal::decimal128, boost::decimal::decimal64>();
+    test_implicit<boost::decimal::decimal128_fast, boost::decimal::decimal64>();
+
+    return boost::report_errors();
+}