diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5039626e2..5f23b7581 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -63,8 +63,8 @@ if (BOLT_ENABLE_TESTS)
   add_subdirectory(deps/googletest EXCLUDE_FROM_ALL)
   add_executable(
     alltests
-    src/TestText.cc
-    src/TestChecker.cc
+    test/TestText.cc
+    test/TestChecker.cc
   )
   target_link_libraries(
     alltests
diff --git a/include/bolt/DiagnosticEngine.hpp b/include/bolt/DiagnosticEngine.hpp
index 7276f91a2..272a0b35e 100644
--- a/include/bolt/DiagnosticEngine.hpp
+++ b/include/bolt/DiagnosticEngine.hpp
@@ -54,6 +54,10 @@ namespace bolt {
       Diagnostics.clear();
     }
 
+    std::size_t countDiagnostics() const noexcept {
+      return Diagnostics.size();
+    }
+
     ~DiagnosticStore();
 
   };
diff --git a/include/bolt/Diagnostics.hpp b/include/bolt/Diagnostics.hpp
index 888913636..e89b3b596 100644
--- a/include/bolt/Diagnostics.hpp
+++ b/include/bolt/Diagnostics.hpp
@@ -99,6 +99,14 @@ namespace bolt {
     inline UnificationErrorDiagnostic(Type* OrigLeft, Type* OrigRight, TypePath LeftPath, TypePath RightPath, Node* Source):
       Diagnostic(DiagnosticKind::UnificationError), OrigLeft(OrigLeft), OrigRight(OrigRight), LeftPath(LeftPath), RightPath(RightPath), Source(Source) {}
 
+    inline Type* getLeft() const {
+      return OrigLeft->resolve(LeftPath);
+    }
+
+    inline Type* getRight() const {
+      return OrigRight->resolve(RightPath);
+    }
+
     inline Node* getNode() const override {
       return Source;
     }
diff --git a/src/Diagnostics.cc b/src/Diagnostics.cc
index 6740f1f6e..5937e7a33 100644
--- a/src/Diagnostics.cc
+++ b/src/Diagnostics.cc
@@ -288,9 +288,9 @@ namespace bolt {
   }
 
   DiagnosticStore::~DiagnosticStore() {
-    for (auto D: Diagnostics) {
-      delete D;
-    }
+    // for (auto D: Diagnostics) {
+    //   delete D;
+    // }
   }
 
   ConsoleDiagnostics::ConsoleDiagnostics(std::ostream& Out):
diff --git a/src/TestChecker.cc b/src/TestChecker.cc
deleted file mode 100644
index 663630477..000000000
--- a/src/TestChecker.cc
+++ /dev/null
@@ -1,33 +0,0 @@
-
-#include "gtest/gtest.h"
-
-#include "bolt/CST.hpp"
-#include "bolt/Diagnostics.hpp"
-#include "bolt/Scanner.hpp"
-#include "bolt/Parser.hpp"
-#include "bolt/Checker.hpp"
-
-using namespace bolt;
-
-auto checkExpression(std::string Input) {
-  ConsoleDiagnostics DS;
-  TextFile T { "#<anonymous>", Input };
-  VectorStream<std::string, Char> Chars { Input, EOF };
-  Scanner S(T, Chars);
-  Punctuator PT(S);
-  Parser P(T, PT);
-  LanguageConfig Config;
-  auto SF = P.parseSourceFile();
-  Checker C(Config, DS);
-  C.check(SF);
-  return std::make_tuple(
-    static_cast<ExpressionStatement*>(SF->Elements[0])->Expression,
-    C
-  );
-}
-
-TEST(CheckerTest, InfersIntFromIntegerLiteral) {
-  auto [Expression, Checker] = checkExpression("1");
-  ASSERT_EQ(Checker.getType(Expression), Checker.getIntType());
-}
-
diff --git a/src/main.cc b/src/main.cc
index e1fdb7433..b63f2a5ed 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -103,7 +103,7 @@ int main(int Argc, const char* Argv[]) {
   std::sort(DS.Diagnostics.begin(), DS.Diagnostics.end(), lessThan);
 
   for (auto D: DS.Diagnostics) {
-    DE.printDiagnostic(*D);
+    DE.addDiagnostic(D);
   }
 
   if (DE.hasError()) {
diff --git a/test/TestChecker.cc b/test/TestChecker.cc
new file mode 100644
index 000000000..86393f11d
--- /dev/null
+++ b/test/TestChecker.cc
@@ -0,0 +1,53 @@
+
+#include "gtest/gtest.h"
+
+#include "bolt/CST.hpp"
+#include "bolt/Diagnostics.hpp"
+#include "bolt/DiagnosticEngine.hpp"
+#include "bolt/Scanner.hpp"
+#include "bolt/Parser.hpp"
+#include "bolt/Checker.hpp"
+
+using namespace bolt;
+
+auto checkSourceFile(std::string Input) {
+  DiagnosticStore DS;
+  TextFile T { "#<anonymous>", Input };
+  VectorStream<std::string, Char> Chars { Input, EOF };
+  Scanner S(T, Chars);
+  Punctuator PT(S);
+  Parser P(T, PT, DS);
+  LanguageConfig Config;
+  auto SF = P.parseSourceFile();
+  SF->setParents();
+  Checker C(Config, DS);
+  C.check(SF);
+  return std::make_tuple(SF, C, DS);
+}
+
+auto checkExpression(std::string Input) {
+  auto [SF, C, DS] = checkSourceFile(Input);
+  return std::make_tuple(
+    static_cast<ExpressionStatement*>(SF->Elements[0])->Expression,
+    C,
+    DS
+  );
+}
+
+TEST(CheckerTest, InfersIntFromIntegerLiteral) {
+  auto [Expr, Checker, DS] = checkExpression("1");
+  ASSERT_EQ(DS.countDiagnostics(), 0);
+  ASSERT_EQ(Checker.getType(Expr), Checker.getIntType());
+}
+
+TEST(CheckerTest, TestIllegalTypingVariable) {
+  auto [SF, C, DS] = checkSourceFile("let a: Int = \"foo\"");
+  ASSERT_EQ(DS.countDiagnostics(), 1);
+  auto D1 = DS.Diagnostics[0];
+  ASSERT_EQ(D1->getKind(), DiagnosticKind::UnificationError);
+  auto Diag = static_cast<UnificationErrorDiagnostic*>(D1);
+  // TODO these types have to be sorted first
+  ASSERT_EQ(Diag->getLeft(), C.getIntType());
+  ASSERT_EQ(Diag->getRight(), C.getStringType());
+}
+
diff --git a/src/TestText.cc b/test/TestText.cc
similarity index 81%
rename from src/TestText.cc
rename to test/TestText.cc
index 7cf5f2465..b7256a5c0 100644
--- a/src/TestText.cc
+++ b/test/TestText.cc
@@ -1,7 +1,7 @@
 
 #include "gtest/gtest.h"
 
-#include "bolt/Text.hpp"
+#include "bolt/CST.hpp"
 
 using namespace bolt;
 
@@ -23,11 +23,11 @@ TEST(TextFileTest, ReportsCorrectLine) {
 
 TEST(TextFileTest, ReportsCorrectStartOffset) {
   TextFile T1 { "foo.txt", "bar\nbaz\nbax\n" };
-  ASSERT_EQ(T1.getStartOffset(1), 0);
-  ASSERT_EQ(T1.getStartOffset(2), 4);
-  ASSERT_EQ(T1.getStartOffset(3), 8);
-  ASSERT_EQ(T1.getStartOffset(4), 12);
-  ASSERT_EQ(T1.getStartOffset(5), 12);
+  ASSERT_EQ(T1.getStartOffsetOfLine(1), 0);
+  ASSERT_EQ(T1.getStartOffsetOfLine(2), 4);
+  ASSERT_EQ(T1.getStartOffsetOfLine(3), 8);
+  ASSERT_EQ(T1.getStartOffsetOfLine(4), 12);
+  ASSERT_EQ(T1.getStartOffsetOfLine(5), 12);
 }
 
 TEST(TextFileTest, ReportsCorrectColumn) {