From 0916a1ad080868a47a68eca044be3af17a1ef87f Mon Sep 17 00:00:00 2001
From: Anand Thakker <github@anandthakker.net>
Date: Thu, 5 Apr 2018 21:08:31 -0400
Subject: Fuller fix

---
 .../conversion/data_driven_property_value.hpp      | 16 ++-------
 include/mbgl/style/expression/let.hpp              |  3 ++
 src/mbgl/style/expression/parsing_context.cpp      | 41 ++++++++++++++--------
 3 files changed, 33 insertions(+), 27 deletions(-)

diff --git a/include/mbgl/style/conversion/data_driven_property_value.hpp b/include/mbgl/style/conversion/data_driven_property_value.hpp
index 466d4b6402..b4297816df 100644
--- a/include/mbgl/style/conversion/data_driven_property_value.hpp
+++ b/include/mbgl/style/conversion/data_driven_property_value.hpp
@@ -44,19 +44,9 @@ struct Converter<DataDrivenPropertyValue<T>> {
             } else if (!featureConstant && !zoomConstant) {
                 return DataDrivenPropertyValue<T>(CompositeFunction<T>(std::move(*expression)));
             } else {
-                // If an expression is neither zoom- nor feature-dependent, it
-                // should have been reduced to a Literal when it was parsed.
-                optional<T> constant;
-                if (auto literal = dynamic_cast<Literal*>(expression->get())) {
-                    // cool, it's pre-folded to a literal
-                    constant = fromExpressionValue<T>(literal->getValue());
-                } else {
-                    // we didn't manage to fold to a literal during parsing, so evaluate it now
-                    EvaluationContext params(nullptr);
-                    EvaluationResult evaluated((*expression)->evaluate(params));
-                    assert(evaluated);
-                    constant = fromExpressionValue<T>(*evaluated);
-                }
+                auto literal = dynamic_cast<Literal*>(expression->get());
+                assert(literal);
+                optional<T> constant = fromExpressionValue<T>(literal->getValue());
                 if (!constant) {
                     return {};
                 }
diff --git a/include/mbgl/style/expression/let.hpp b/include/mbgl/style/expression/let.hpp
index 75d2adda62..d0210d8bba 100644
--- a/include/mbgl/style/expression/let.hpp
+++ b/include/mbgl/style/expression/let.hpp
@@ -70,6 +70,9 @@ public:
 
     mbgl::Value serialize() const override;
     std::string getOperator() const override { return "var"; }
+    
+    const std::shared_ptr<Expression>& getBoundExpression() const { return value; }
+    
 private:
     std::string name;
     std::shared_ptr<Expression> value;
diff --git a/src/mbgl/style/expression/parsing_context.cpp b/src/mbgl/style/expression/parsing_context.cpp
index 364c3f740a..713c0b5d27 100644
--- a/src/mbgl/style/expression/parsing_context.cpp
+++ b/src/mbgl/style/expression/parsing_context.cpp
@@ -32,23 +32,36 @@ namespace style {
 namespace expression {
 
 bool isConstant(const Expression& expression) {
-    if (dynamic_cast<const Var*>(&expression)) {
-        return false;
+    if (auto varExpression = dynamic_cast<const Var*>(&expression)) {
+        return isConstant(*varExpression->getBoundExpression());
     }
-    
+
     if (auto compound = dynamic_cast<const CompoundExpressionBase*>(&expression)) {
         if (compound->getName() == "error") {
             return false;
         }
     }
+
+    bool isTypeAnnotation = dynamic_cast<const Coercion*>(&expression) ||
+        dynamic_cast<const Assertion*>(&expression) ||
+        dynamic_cast<const ArrayAssertion*>(&expression);
     
-    bool literalArgs = true;
+    bool childrenConstant = true;
     expression.eachChild([&](const Expression& child) {
-        if (!dynamic_cast<const Literal*>(&child)) {
-            literalArgs = false;
+        // We can _almost_ assume that if `expressions` children are constant,
+        // they would already have been evaluated to Literal values when they
+        // were parsed.  Type annotations are the exception, because they might
+        // have been inferred and added after a child was parsed.
+
+        // So we recurse into isConstant() for the children of type annotations,
+        // but otherwise simply check whether they are Literals.
+        if (isTypeAnnotation) {
+            childrenConstant = childrenConstant && isConstant(child);
+        } else {
+            childrenConstant = childrenConstant && dynamic_cast<const Literal*>(&child);
         }
     });
-    if (!literalArgs) {
+    if (!childrenConstant) {
         return false;
     }
     
@@ -141,13 +154,13 @@ ParseResult ParsingContext::parse(const Convertible& value, TypeAnnotationOption
         return parsed;
     }
 
-    if (expected) {
-        auto array = [&](std::unique_ptr<Expression> expression) {
-            std::vector<std::unique_ptr<Expression>> args;
-            args.push_back(std::move(expression));
-            return args;
-        };
+    auto array = [&](std::unique_ptr<Expression> expression) {
+        std::vector<std::unique_ptr<Expression>> args;
+        args.push_back(std::move(expression));
+        return args;
+    };
 
+    if (expected) {
         const type::Type actual = (*parsed)->getType();
         if ((*expected == type::String || *expected == type::Number || *expected == type::Boolean || *expected == type::Object) && actual == type::Value) {
             if (typeAnnotationOption == includeTypeAnnotations) {
@@ -169,7 +182,7 @@ ParseResult ParsingContext::parse(const Convertible& value, TypeAnnotationOption
         }
     }
 
-    // If an expression's arguments are all literals, we can evaluate
+    // If an expression's arguments are all constant, we can evaluate
     // it immediately and replace it with a literal value in the
     // parsed result.
     if (!dynamic_cast<Literal *>(parsed->get()) && isConstant(**parsed)) {
-- 
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