add builtin function name to error

Author: IanButterworth

src/method.c

@@ -1299,7 +1299,7 @@ JL_DLLEXPORT jl_method_t* jl_method_def(jl_svec_t *argdata,
     }
     ft = jl_rewrap_unionall(ft, argtype);
     if (!external_mt && !jl_has_empty_intersection(ft, (jl_value_t*)jl_builtin_type)) // disallow adding methods to Any, Function, Builtin, and subtypes, or Unions of those
-        jl_error("cannot add methods to a builtin function");
+        jl_errorf("cannot add methods to a builtin function `%s`", jl_symbol_name(name));
 
     m = jl_new_method_uninit(module);
     m->external_mt = (jl_value_t*)external_mt;

test/core.jl

@@ -2661,13 +2661,16 @@ struct D14919 <: Function; end
 @test B14919()() == "It's a brand new world"
 @test C14919()() == D14919()() == "Boo."
 
-let ex = ErrorException("cannot add methods to a builtin function")
+let ex_t = ErrorException, ex_r = r"cannot add methods to a builtin function"
     for f in (:(Core.Any), :(Core.Function), :(Core.Builtin), :(Base.Callable), :(Union{Nothing,F} where F), :(typeof(Core.getfield)), :(Core.IntrinsicFunction))
-        @test_throws ex @eval (::$f)() = 1
+        @test_throws ex_t @eval (::$f)() = 1
+        @test_throws ex_r @eval (::$f)() = 1
     end
-    @test_throws ex @eval (::Union{Nothing,F})() where {F<:Function} = 1
+    @test_throws ex_t @eval (::Union{Nothing,F})() where {F<:Function} = 1
+    @test_throws ex_r @eval (::Union{Nothing,F})() where {F<:Function} = 1
     for f in (:(Core.getfield),)
-        @test_throws ex @eval $f() = 1
+        @test_throws ex_t @eval $f() = 1
+        @test_throws ex_r @eval $f() = 1
     end
 end