helpful error upon misuse of macro

Author: MilesCranmer

base/runtime_internals.jl

@@ -173,6 +173,8 @@ false
 """
 ispublic(m::Module, s::Symbol) = ccall(:jl_module_public_p, Cint, (Any, Any), m, s) != 0
 
+_function_macro_error() = (@noinline; error("@__FUNCTION__ can only be used within a function"))
+
 """
     @__FUNCTION__ -> Function
 
@@ -217,7 +219,10 @@ julia> factorial(5)
 ```
 """
 macro __FUNCTION__()
-    return esc(:(var"#self#"))
+    quote
+        $(esc(Expr(:isdefined, :var"#self#"))) || $(esc(_function_macro_error))()
+        $(esc(:var"#self#"))
+    end
 end
 
 # TODO: this is vaguely broken because it only works for explicit calls to

test/loading.jl

@@ -141,6 +141,14 @@ end
             c = CallableStruct(5)
             @test_throws UndefVarError c()
         end
+
+        @testset "Error upon misuse" begin
+            @gensym A
+            @test_throws(
+                "@__FUNCTION__ can only be used within a function",
+                @eval(module $A; @__FUNCTION__; end)
+            )
+        end
     end
 end