Fixes for overlay tables (JuliaLang#41174)

* Add support for at-overlay with parametric function definitions.

* Fix error message with at-MethodTable.

* Fix function signature to avoid compiler warning.

Author: maleadt

base/experimental.jl

@@ -268,15 +268,21 @@ method tables (e.g., using [`Core.Compiler.OverlayMethodTable`](@ref)).
 """
 macro overlay(mt, def)
     def = macroexpand(__module__, def) # to expand @inline, @generated, etc
-    if !isexpr(def, [:function, :(=)]) || !isexpr(def.args[1], :call)
+    if !isexpr(def, [:function, :(=)])
+        error("@overlay requires a function Expr")
+    end
+    if isexpr(def.args[1], :call)
+        def.args[1].args[1] = Expr(:overlay, mt, def.args[1].args[1])
+    elseif isexpr(def.args[1], :where)
+        def.args[1].args[1].args[1] = Expr(:overlay, mt, def.args[1].args[1].args[1])
+    else
         error("@overlay requires a function Expr")
     end
-    def.args[1].args[1] = Expr(:overlay, mt, def.args[1].args[1])
     esc(def)
 end
 
 let new_mt(name::Symbol, mod::Module) = begin
-        ccall(:jl_check_top_level_effect, Cvoid, (Any, Cstring), mod, name)
+        ccall(:jl_check_top_level_effect, Cvoid, (Any, Cstring), mod, "@MethodTable")
         ccall(:jl_new_method_table, Any, (Any, Any), name, mod)
     end
     @eval macro MethodTable(name::Symbol)

src/toplevel.c

@@ -922,7 +922,7 @@ static void jl_check_open_for(jl_module_t *m, const char* funcname)
     }
 }
 
-JL_DLLEXPORT void jl_check_top_level_effect(jl_value_t *m, char *fname)
+JL_DLLEXPORT void jl_check_top_level_effect(jl_module_t *m, char *fname)
 {
     if (jl_current_task->ptls->in_pure_callback)
         jl_errorf("%s cannot be used in a generated function", fname);

test/compiler/contextual.jl

@@ -153,6 +153,9 @@ end
 # short function def
 @overlay mt cos(x::Float64) = 2
 
+# parametric function def
+@overlay mt tan(x::T) where {T} = 3
+
 end
 
 methods = Base._methods_by_ftype(Tuple{typeof(sin), Float64}, nothing, 1, typemax(UInt))