Don't use {{yield}} outside a macro

spec/compiler/macro/macro_expander_spec.cr

@@ -152,4 +152,8 @@ describe "MacroExpander" do
   it "does not expand when macro expression is {% ... %}" do
     assert_macro "", %({% 1 %}), [] of ASTNode, ""
   end
+
+  it "can't use `yield` outside a macro" do
+    assert_error %({{yield}}), "can't use `{{yield}}` outside a macro"
+  end
 end

src/compiler/crystal/macros/interpreter.cr

@@ -3,7 +3,7 @@ module Crystal
     getter last : ASTNode
     property free_vars : Hash(String, TypeVar)?
 
-    def self.new(program, scope : Type, path_lookup : Type, a_macro : Macro, call, a_def : Def? = nil)
+    def self.new(program, scope : Type, path_lookup : Type, a_macro : Macro, call, a_def : Def? = nil, in_macro = false)
       vars = {} of String => ASTNode
       splat_index = a_macro.splat_index
       double_splat = a_macro.double_splat
@@ -68,14 +68,15 @@ module Crystal
         vars[macro_block_arg.name] = call_block || Nop.new
       end
 
-      new(program, scope, path_lookup, a_macro.location, vars, call.block, a_def)
+      new(program, scope, path_lookup, a_macro.location, vars, call.block, a_def, in_macro)
     end
 
     record MacroVarKey, name : String, exps : Array(ASTNode)?
 
     def initialize(@program : Program,
                    @scope : Type, @path_lookup : Type, @location : Location?,
-                   @vars = {} of String => ASTNode, @block : Block? = nil, @def : Def? = nil)
+                   @vars = {} of String => ASTNode, @block : Block? = nil, @def : Def? = nil,
+                   @in_macro = false)
       @str = IO::Memory.new(512) # Can't be String::Builder because of `{{debug()}}
       @last = Nop.new
     end
@@ -362,6 +363,10 @@ module Crystal
     end
 
     def visit(node : Yield)
+      unless @in_macro
+        node.raise "can't use `{{yield}}` outside a macro"
+      end
+
       if block = @block
         if node.exps.empty?
           @last = block.body.clone

src/compiler/crystal/macros/macros.cr

@@ -37,13 +37,13 @@ class Crystal::Program
   end
 
   def expand_macro(a_macro : Macro, call : Call, scope : Type, path_lookup : Type? = nil, a_def : Def? = nil)
-    interpreter = MacroInterpreter.new self, scope, path_lookup || scope, a_macro, call, a_def
+    interpreter = MacroInterpreter.new self, scope, path_lookup || scope, a_macro, call, a_def, in_macro: true
     a_macro.body.accept interpreter
     interpreter.to_s
   end
 
   def expand_macro(node : ASTNode, scope : Type, path_lookup : Type? = nil, free_vars = nil, a_def : Def? = nil)
-    interpreter = MacroInterpreter.new self, scope, path_lookup || scope, node.location, def: a_def
+    interpreter = MacroInterpreter.new self, scope, path_lookup || scope, node.location, def: a_def, in_macro: false
     interpreter.free_vars = free_vars
     node.accept interpreter
     interpreter.to_s