Math: assert valid binomialCoefficient() input instead of returning 0.

src/Magnum/Math/Functions.cpp

@@ -45,7 +45,8 @@ UnsignedInt log2(UnsignedInt number) {
 }
 
 UnsignedLong binomialCoefficient(const UnsignedInt n, UnsignedInt k) {
-    if(k > n) return 0;
+    CORRADE_ASSERT(n >= k,
+        "Math::binomialCoefficient(): k can't be greater than n in (" << Corrade::Utility::Debug::nospace << n << "choose" << k << Corrade::Utility::Debug::nospace << ")", {});
 
     /* k and n - k gives the same value, optimize the calculation to do fewer
        steps */

src/Magnum/Math/Test/FunctionsTest.cpp

@@ -55,6 +55,7 @@ struct FunctionsTest: Corrade::TestSuite::Tester {
     void fmod();
 
     void binomialCoefficient();
+    void binomialCoefficientInvalidInput();
     void binomialCoefficientOverflow();
 
     void sqrt();
@@ -120,6 +121,7 @@ FunctionsTest::FunctionsTest() {
               &FunctionsTest::fmod,
 
               &FunctionsTest::binomialCoefficient,
+              &FunctionsTest::binomialCoefficientInvalidInput,
               &FunctionsTest::binomialCoefficientOverflow,
 
               &FunctionsTest::sqrt,
@@ -310,6 +312,17 @@ void FunctionsTest::binomialCoefficient() {
     CORRADE_COMPARE(Math::binomialCoefficient(62, 31), 465428353255261088ull);
 }
 
+void FunctionsTest::binomialCoefficientInvalidInput() {
+    #ifdef CORRADE_NO_ASSERT
+    CORRADE_SKIP("CORRADE_NO_ASSERT defined, can't test assertions");
+    #endif
+
+    std::ostringstream out;
+    Error redirectError{&out};
+    Math::binomialCoefficient(15, 16);
+    CORRADE_COMPARE(out.str(), "Math::binomialCoefficient(): k can't be greater than n in (15 choose 16)\n");
+}
+
 void FunctionsTest::binomialCoefficientOverflow() {
     #ifdef CORRADE_NO_ASSERT
     CORRADE_SKIP("CORRADE_NO_ASSERT defined, can't test assertions");