[core] Handle NaN input to interpolate and step

src/mbgl/style/expression/step.cpp

@@ -2,6 +2,7 @@
 #include <mbgl/style/expression/get_covering_stops.hpp>
 #include <mbgl/util/string.hpp>
 
+#include <cmath>
 
 namespace mbgl {
 namespace style {
@@ -9,8 +10,14 @@ namespace expression {
 
 EvaluationResult Step::evaluate(const EvaluationContext& params) const {
     const EvaluationResult evaluatedInput = input->evaluate(params);
-    if (!evaluatedInput) { return evaluatedInput.error(); }
+    if (!evaluatedInput) {
+        return evaluatedInput.error();
+    }
+
     float x = *fromExpressionValue<float>(*evaluatedInput);
+    if (std::isnan(x)) {
+        return EvaluationError { "Input is not a number." };
+    }
 
     if (stops.empty()) {
         return EvaluationError { "No stops in step curve." };

src/mbgl/style/expression/value.cpp

@@ -108,13 +108,9 @@ Value ValueConverter<float>::toExpressionValue(const float value) {
 }
 
 optional<float> ValueConverter<float>::fromExpressionValue(const Value& value) {
-    if (value.template is<double>()) {
-        double v = value.template get<double>();
-        if (v <= std::numeric_limits<float>::max()) {
-            return static_cast<float>(v);
-        }
-    }
-    return optional<float>();
+    return value.template is<double>()
+        ? static_cast<float>(value.template get<double>())
+        : optional<float>();
 }