Always use "new" string->float conversions

This ensures that 1_2_3 type numbers are handled correctly
whether or not the argument type is known.

Fixes https://github.com/cython/cython/issues/3958 (and thus the
currently broken builtin_float test)

Cython/Compiler/Optimize.py

@@ -2195,6 +2195,7 @@ class OptimizeBuiltinCalls(Visitor.NodeRefCleanupMixin,
                         args=[func_arg],
                         py_name='float',
                         is_temp=node.is_temp,
+                        utility_code = UtilityCode.load_cached("pynumber_float", "TypeConversion.c"),
                         result_is_used=node.result_is_used,
                     ).coerce_to(node.type, self.current_env())
         return node

Cython/Utility/Optimize.c

@@ -713,7 +713,6 @@ static double __Pyx_SlowPyString_AsDouble(PyObject *obj) {
 
 static const char* __Pyx__PyBytes_AsDouble_Copy(const char* start, char* buffer, Py_ssize_t length) {
     Py_ssize_t i;
-    char *digit = buffer;
     for (i=0; i < length; i++) {
         if (start[i] != '_') *buffer++ = start[i];
     }

Cython/Utility/TypeConversion.c

@@ -126,7 +126,8 @@ static CYTHON_INLINE Py_hash_t __Pyx_PyIndex_AsHash_t(PyObject*);
 #else
 #define __Pyx_PyNumber_Int(x) (PyInt_CheckExact(x) ? __Pyx_NewRef(x) : PyNumber_Int(x))
 #endif
-#define __Pyx_PyNumber_Float(x) (PyFloat_CheckExact(x) ? __Pyx_NewRef(x) : PyNumber_Float(x))
+// __Pyx_PyNumber_Float is now in it's own section since it has dependencies (needed to make
+// string conversion work the same in all circumstances)
 
 #if PY_MAJOR_VERSION < 3 && __PYX_DEFAULT_STRING_ENCODING_IS_ASCII
 static int __Pyx_sys_getdefaultencoding_not_ascii;
@@ -463,6 +464,27 @@ static CYTHON_INLINE PyObject * __Pyx_PyInt_FromSize_t(size_t ival) {
     return PyInt_FromSize_t(ival);
 }
 
+/////////////// pynumber_float.proto ///////////////
+
+static CYTHON_INLINE PyObject* __Pyx_PyNumber_Float(PyObject* obj); /* proto */
+
+/////////////// pynumber_float ///////////////
+//@requires: Optimize.c::pybytes_as_double
+//@requires: Optimize.c::pyunicode_as_double
+
+static CYTHON_INLINE PyObject* __Pyx_PyNumber_Float(PyObject* obj) {
+    if (PyFloat_CheckExact(obj)) {
+        return __Pyx_NewRef(obj);
+    } else if (PyUnicode_CheckExact(obj)) {
+        return PyFloat_FromDouble(__Pyx_PyUnicode_AsDouble(obj));
+    } else if (PyBytes_CheckExact(obj)) {
+        return PyFloat_FromDouble(__Pyx_PyBytes_AsDouble(obj));
+    } else if (PyByteArray_CheckExact(obj)) {
+        return PyFloat_FromDouble(__Pyx_PyByteArray_AsDouble(obj));
+    } else {
+        return PyNumber_Float(obj);
+    }
+}
 
 /////////////// GCCDiagnostics.proto ///////////////