From 0eb4b5cc0ec494f4b8de6933d2eddf97b12a77f7 Mon Sep 17 00:00:00 2001
From: John Firebaugh <john.firebaugh@gmail.com>
Date: Mon, 8 Jan 2018 14:46:44 -0800
Subject: [core] Omit inferred type annotations for 'coalesce' arguments

---
 src/mbgl/style/expression/coalesce.cpp | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

(limited to 'src/mbgl/style/expression/coalesce.cpp')

diff --git a/src/mbgl/style/expression/coalesce.cpp b/src/mbgl/style/expression/coalesce.cpp
index bfde3c7581..0373c9626c 100644
--- a/src/mbgl/style/expression/coalesce.cpp
+++ b/src/mbgl/style/expression/coalesce.cpp
@@ -1,4 +1,5 @@
 #include <mbgl/style/expression/coalesce.hpp>
+#include <mbgl/style/expression/check_subtype.hpp>
 
 namespace mbgl {
 namespace style {
@@ -36,14 +37,15 @@ ParseResult Coalesce::parse(const Convertible& value, ParsingContext& ctx) {
     }
  
     optional<type::Type> outputType;
-    if (ctx.getExpected() && *ctx.getExpected() != type::Value) {
-        outputType = ctx.getExpected();
+    optional<type::Type> expectedType = ctx.getExpected();
+    if (expectedType && *expectedType != type::Value) {
+        outputType = expectedType;
     }
 
     Coalesce::Args args;
     args.reserve(length - 1);
     for (std::size_t i = 1; i < length; i++) {
-        auto parsed = ctx.parse(arrayMember(value, i), i, outputType);
+        auto parsed = ctx.parse(arrayMember(value, i), i, outputType, ParsingContext::omitTypeAnnotations);
         if (!parsed) {
             return parsed;
         }
@@ -52,9 +54,19 @@ ParseResult Coalesce::parse(const Convertible& value, ParsingContext& ctx) {
         }
         args.push_back(std::move(*parsed));
     }
- 
     assert(outputType);
-    return ParseResult(std::make_unique<Coalesce>(*outputType, std::move(args)));
+
+    // Above, we parse arguments without inferred type annotation so that
+    // they don't produce a runtime error for `null` input, which would
+    // preempt the desired null-coalescing behavior.
+    // Thus, if any of our arguments would have needed an annotation, we
+    // need to wrap the enclosing coalesce expression with it instead.
+    bool needsAnnotation = expectedType &&
+        std::any_of(args.begin(), args.end(), [&] (const auto& arg) {
+            return type::checkSubtype(*expectedType, arg->getType());
+        });
+
+    return ParseResult(std::make_unique<Coalesce>(needsAnnotation ? type::Value : *outputType, std::move(args)));
 }
 
 } // namespace expression
-- 
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