Fix Union.from_yaml to prioritize String for quoted scalar (#15405)

spec/std/yaml/serialization_spec.cr

@@ -390,6 +390,18 @@ describe "YAML serialization" do
       Bytes.from_yaml("!!binary aGVsbG8=").should eq("hello".to_slice)
     end
 
+    describe "Union.from_yaml" do
+      it "String priorization" do
+        (Int32 | String).from_yaml(%(42)).should eq 42
+        (Int32 | String).from_yaml(%("42")).should eq "42"
+
+        (String | UInt32).from_yaml(%(42)).should eq 42
+        (String | UInt32).from_yaml(%("42")).should eq "42"
+
+        (Int32 | UInt32).from_yaml(%("42")).should eq 42
+      end
+    end
+
     describe "parse exceptions" do
       it "has correct location when raises in Nil#from_yaml" do
         ex = expect_raises(YAML::ParseException) do

src/yaml/enums.cr

@@ -20,6 +20,10 @@ module YAML
     DOUBLE_QUOTED
     LITERAL
     FOLDED
+
+    def quoted?
+      single_quoted? || double_quoted?
+    end
   end
 
   enum SequenceStyle

src/yaml/from_yaml.cr

@@ -298,6 +298,13 @@ def Union.new(ctx : YAML::ParseContext, node : YAML::Nodes::Node)
     # So, we give a chance first to types in the union to be parsed.
     {% string_type = T.find { |type| type == ::String } %}
 
+    {% if string_type %}
+      if node.as?(YAML::Nodes::Scalar).try(&.style.quoted?)
+        # do prefer String if it's a quoted scalar though
+        return String.new(ctx, node)
+      end
+    {% end %}
+
     {% for type in T %}
       {% unless type == string_type %}
         begin