Adding Crystal samples to address https://github.com/github/github/issues/24802

samples/Crystal/declare_var_spec.cr

@@ -0,0 +1,79 @@
+#!/usr/bin/env bin/crystal --run
+require "../../spec_helper"
+
+describe "Type inference: declare var" do
+  it "types declare var" do
+    assert_type("a :: Int32") { int32 }
+  end
+
+  it "types declare var and reads it" do
+    assert_type("a :: Int32; a") { int32 }
+  end
+
+  it "types declare var and changes its type" do
+    assert_type("a :: Int32; while 1 == 2; a = 'a'; end; a") { union_of(int32, char) }
+  end
+
+  it "declares instance var which appears in initialize" do
+    result = assert_type("
+      class Foo
+        @x :: Int32
+      end
+
+      Foo.new") { types["Foo"] }
+
+    mod = result.program
+
+    foo = mod.types["Foo"] as NonGenericClassType
+    foo.instance_vars["@x"].type.should eq(mod.int32)
+  end
+
+  it "declares instance var of generic class" do
+    result = assert_type("
+      class Foo(T)
+        @x :: T
+      end
+
+      Foo(Int32).new") do
+        foo = types["Foo"] as GenericClassType
+        foo_i32 = foo.instantiate([int32] of Type | ASTNode)
+        foo_i32.lookup_instance_var("@x").type.should eq(int32)
+        foo_i32
+    end
+  end
+
+  it "declares instance var of generic class after reopen" do
+    result = assert_type("
+      class Foo(T)
+      end
+
+      f = Foo(Int32).new
+
+      class Foo(T)
+        @x :: T
+      end
+
+      f") do
+        foo = types["Foo"] as GenericClassType
+        foo_i32 = foo.instantiate([int32] of Type | ASTNode)
+        foo_i32.lookup_instance_var("@x").type.should eq(int32)
+        foo_i32
+    end
+  end
+
+  it "declares an instance variable in initialize" do
+    assert_type("
+      class Foo
+        def initialize
+          @x :: Int32
+        end
+
+        def x
+          @x
+        end
+      end
+
+      Foo.new.x
+      ") { int32 }
+  end
+end