fix(zscript): disallow casting custom objects to unrelated types

A bug in type conversion allowed this code to compile:

```c++
Object obj = new Object();
int number = obj;
int numbers[] = {obj};
```

The above code now produces a compiler error.

Author: connorjclark

src/parser/SemanticAnalyzer.cpp

@@ -1979,13 +1979,41 @@ void SemanticAnalyzer::caseArrayLiteral(ASTArrayLiteral& host, void*)
 				program.getTypeStore().getCanonicalType(
 						DataTypeArray(elementType)));
 	}
-
-	// Otherwise, default to Untyped -Em
 	else
 	{
+		// No explicit type, try to infer one from elements.
+		const DataType* type = nullptr;
+		for (auto&& node : host.elements)
+		{
+			auto node_type = type = node->getReadType(scope, this);
+			if (node_type->isUntyped())
+			{
+				type = nullptr;
+				break;
+			}
+
+			if (!type)
+			{
+				type = node_type;
+				continue;
+			}
+
+			// I think this should accept only exactly matching types. A future
+			// change could modify this to accept the base type of two related types.
+			if (!type->canCastTo(*node_type) || !node_type->canCastTo(*type))
+			{
+				type = nullptr;
+				break;
+			}
+		}
+
+		// Fallback is to create an untyped array.
+		if (!type)
+			type = &DataType::UNTYPED;
+
 		host.setReadType(
 				program.getTypeStore().getCanonicalType(
-						DataTypeArray(DataType::UNTYPED)));
+						DataTypeArray(*type)));
 	}
 
 	// If initialized, check that each element can be cast to type.

src/parser/Types.cpp

@@ -673,7 +673,7 @@ bool DataTypeCustom::canCastTo(DataType const& target) const
 			dynamic_cast<DataTypeArray const*>(&target))
 		return canCastTo(getBaseType(*t));
 
-	if(!isClass())
+	if(!isClass() && !isUsrClass())
 	{
 		if (DataTypeSimple const* t =
 				dynamic_cast<DataTypeSimple const*>(&target))

tests/scripts/errors/errors_3.zs

@@ -0,0 +1,28 @@
+#option NO_ERROR_HALT on
+
+class Object {
+}
+
+global script Global
+{
+    void run()
+    {
+		Object obj = new Object();
+
+		// Invalid.
+        int number = obj;
+		obj = number;
+		int numbers[] = {obj};
+		Object objects[] = numbers;
+		Object objects2[] = new Object(); // TODO: this should be invalid.
+
+		// OK.
+		int numbers2[] = {obj, <untyped>1};
+		Object objects3[] = {obj};
+		auto objects4[] = objects3; // TODO: this should be valid.
+		auto numbers3[] = {1, 2, 3}; // TODO: this should be valid.
+
+		// Invalid.
+		numbers3 = objects4; // TODO: this should be invalid.
+    }
+}

tests/scripts/errors/errors_3_expected.txt

@@ -0,0 +1,13 @@
+Compiling 'errors_3.zs'
+Pass 1: Parsing
+Pass 2: Preprocessing
+Pass 3: Registration
+Pass 4: Analyzing Code
+ZQ_BUFFER Line 13 @ Columns 13-25 - Error T017: Cannot cast from Object to int.
+ZQ_BUFFER Line 14 @ Columns 3-15 - Error T017: Cannot cast from int to Object.
+ZQ_BUFFER Line 15 @ Columns 7-24 - Error T017: Cannot cast from Object[] to int[].
+ZQ_BUFFER Line 16 @ Columns 10-29 - Error T017: Cannot cast from int[] to Object[].
+ZQ_BUFFER Line 22 @ Columns 8-29 - Error T017: Cannot cast from Object[] to auto[].
+ZQ_BUFFER Line 23 @ Columns 8-30 - Error T017: Cannot cast from int[] to auto[].
+Failure!
+Compile finished with exit code '17' (compiled with errors)