Add partially evaluated methods ===(x), !==(x), and isa(T)

[tkf]

This PR adds ===(x), !==(x), and isa(T) that creates a Fix2 like isequal etc. For ===(x) and isa(T), it looks like I needed to add tfuncs.

fix #36163
fix #32018

[tkf]

This is a bit of a long shot for working around analyzegc https://build.julialang.org/#/builders/65/builds/1574/steps/4/logs/stdio

But I wonder if there is a reason why this is not used? Maybe I'm doing something stupid here?

If this is not the right way to do it, maybe I need to do something like this?

diff --git a/src/builtins.c b/src/builtins.c
index 440b69629e..c2576d553d 100644
--- a/src/builtins.c
+++ b/src/builtins.c
@@ -369,6 +369,7 @@ JL_CALLABLE(jl_f_is)
         jl_function_t *fix2 = jl_get_function(jl_base_module, "Fix2");
         if (fix2 == NULL)
             jl_undefined_var_error(jl_symbol("Fix2"));
+        JL_GC_PROMISE_ROOTED(jl_builtin_is);
         return jl_call2(fix2, jl_builtin_is, args[0]);
     }
     if (args[0] == args[1])
@@ -440,6 +441,7 @@ JL_CALLABLE(jl_f_isa)
         jl_function_t *fix2 = jl_get_function(jl_base_module, "Fix2");
         if (fix2 == NULL)
             jl_undefined_var_error(jl_symbol("Fix2"));
+        JL_GC_PROMISE_ROOTED(jl_builtin_isa);
         return jl_call2(fix2, jl_builtin_isa, args[0]);
     }
     JL_TYPECHK(isa, type, args[1]);

[ericphanson]

Should be y -> y === x

[JeffBezanson]

This is tricky. I suspect the only good way to do this is to make === generic (also isa), since otherwise it will need special inlining and/or codegen support. What I would try is

• Rename the current builtin === to something else
• Move Fix2 to Core (boot.jl) and define the two methods of === there
• Add a check to prevent adding more methods to ===
• Inference can still handle Core.:(===) specially, since we know what it does
• We might need to still make === a Builtin by special arrangement?
• Check the impact on the compiler. Might need to e.g. avoid adding backedges to === (that would be a natural part of a general "sealed functions" feature).

[vtjnash]

Add a check to prevent adding more methods to ===

julia> setfield!(typeof(+).name.mt, fieldcount(Core.MethodTable), 0x01)
# this "seals" a method, and can trick it into thinking that `+` is now a subtype of `Core.Builtin`

julia> Base.:(+)(a) = 1
ERROR: cannot add methods to a builtin function

[cossio]

Something holding this back?

[tkf]

It's stuck because I haven't had time to work on what Jeff suggested. I'll close it for now so that others can pick it up without thinking about racing with me.