don't implicitly convert doubles to ints

docs/changelog.rst

@@ -5,6 +5,8 @@ Changelog
 
 1.8 (Not yet released)
 ----------------------
+* Prevent implicit conversion of floating point values to integral types in
+  function arguments
 * Transparent conversion of sparse and dense Eigen data types
 * Fixed incorrect default return value policy for functions returning a shared
   pointer

example/issues.cpp

@@ -104,4 +104,8 @@ void init_issues(py::module &m) {
 
     // (no id): should not be able to pass 'None' to a reference argument
     m2.def("print_element", [](ElementA &el) { std::cout << el.value() << std::endl; });
+
+    // (no id): don't cast doubles to ints
+    m2.def("expect_float", [](float f) { return f; });
+    m2.def("expect_int", [](int i) { return i; });
 }

example/issues.py

@@ -8,6 +8,7 @@ from example.issues import DispatchIssue, dispatch_issue_go
 from example.issues import Placeholder, return_vec_of_reference_wrapper
 from example.issues import iterator_passthrough
 from example.issues import ElementList, ElementA, print_element
+from example.issues import expect_float, expect_int
 import gc
 
 print_cchar("const char *")
@@ -47,3 +48,10 @@ try:
     print_element(None)
 except Exception as e:
     print("Failed as expected: " + str(e))
+
+try:
+    print(expect_int(5.2))
+except Exception as e:
+    print("Failed as expected: " + str(e))
+
+print(expect_float(12))

example/issues.ref

@@ -8,3 +8,7 @@ Yay..
 Failed as expected: Incompatible function arguments. The following argument types are supported:
     1. (example.issues.ElementA) -> NoneType
 
+Failed as expected: Incompatible function arguments. The following argument types are supported:
+    1. (int) -> int
+
+12.0

include/pybind11/cast.h

@@ -326,11 +326,15 @@ public:
         } if (std::is_floating_point<T>::value) {
             py_value = (py_type) PyFloat_AsDouble(src.ptr());
         } else if (sizeof(T) <= sizeof(long)) {
+            if (PyFloat_Check(src.ptr()))
+                return false;
             if (std::is_signed<T>::value)
                 py_value = (py_type) PyLong_AsLong(src.ptr());
             else
                 py_value = (py_type) PyLong_AsUnsignedLong(src.ptr());
         } else {
+            if (PyFloat_Check(src.ptr()))
+                return false;
             if (std::is_signed<T>::value)
                 py_value = (py_type) PYBIND11_LONG_AS_LONGLONG(src.ptr());
             else