it

spec/compiler/semantic/implicit_block_argument_spec.cr

@@ -41,4 +41,32 @@ describe "Semantic: implicit block argument" do
       foo &{&1, &2, &3}
     )) { tuple_of [int32, char, bool] }
   end
+
+  it "errors if 'it' variable outside of block (assign)" do
+    assert_error %(
+      it = 1
+      ),
+      "implcit block argument 'it' can only be used inside a block"
+  end
+
+  it "errors if 'it' argless call outside of block (assign)" do
+    assert_error %(
+      it
+      ),
+      "implcit block argument 'it' can only be used inside a block"
+  end
+
+  it "uses implicit block argument 'it' with call" do
+    assert_type(%(
+      def foo
+        yield 1
+      end
+
+      def bar(x)
+        x
+      end
+
+      foo &bar(it)
+    )) { int32 }
+  end
 end

src/compiler/crystal/semantic/implicit_block_argument.cr

@@ -21,6 +21,15 @@ module Crystal
       end
     end
 
+    def visit(node : Call)
+      if node.args.empty? && !node.named_args && node.name == "it"
+        @has_implicit_block_arguments = true
+        false
+      else
+        true
+      end
+    end
+
     def visit(node : ASTNode)
       !@has_implicit_block_arguments
     end
@@ -42,7 +51,22 @@ module Crystal
       @block = Block.new
     end
 
-    def transform(node : ExpandableNode | Call)
+    def transform(node : Call)
+      if node.is_a?(Call) && node.args.empty? && !node.named_args && node.name == "it"
+        return replace(node, 1)
+      end
+
+      expanded = node.expanded
+      if expanded
+        node.expanded = expanded.transform(self)
+        node
+      else
+        # TODO: don't go inside Call block_arg
+        super
+      end
+    end
+
+    def transform(node : ExpandableNode)
       expanded = node.expanded
       if expanded
         node.expanded = expanded.transform(self)
@@ -59,8 +83,10 @@ module Crystal
     end
 
     def transform(node : ImplicitBlockArgument)
-      number = node.number
+      replace(node, node.number)
+    end
 
+    def replace(node, number)
       args = @block.args ||= [] of Var
 
       (number - args.size).times do |i|

src/compiler/crystal/semantic/main_visitor.cr

@@ -753,6 +753,10 @@ module Crystal
     end
 
     def type_assign(target : Var, value, node, restriction = nil)
+      if target.name == "it"
+        node.raise "implcit block argument 'it' can only be used inside a block"
+      end
+
       value.accept self
 
       var_name = target.name
@@ -1291,6 +1295,10 @@ module Crystal
     end
 
     def visit(node : Call)
+      if node.args.empty? && !node.named_args && node.name == "it"
+        node.raise "implcit block argument 'it' can only be used inside a block"
+      end
+
       prepare_call(node)
 
       if expand_macro(node)