Add StackedMethodTable

Author: vchuravy

src/jlgen.jl

@@ -297,6 +297,57 @@ end # !HAS_INTEGRATED_CACHE
 
 Base.Experimental.@MethodTable(GLOBAL_METHOD_TABLE)
 
+# Implements a priority lookup for method tables, where the first match in the stack get's returned.
+# An alternative to this would be to use a "Union" where we would query the parent method table and
+# do a most-specific match.
+struct StackedMethodTable{MTV<:CC.MethodTableView} <: CC.MethodTableView
+    world::UInt
+    mt::Core.MethodTable
+    parent::MTV
+end
+StackedMethodTable(world::UInt, mt::Core.MethodTable) = StackedMethodTable(world, mt, CC.InternalMethodTable(world))
+StackedMethodTable(world::UInt, mt::Core.MethodTable, parent::Core.MethodTable) = StackedMethodTable(world, mt, StackedMethodTable(world, parent))
+
+function findall(@nospecialize(sig::Type), table::StackedMethodTable; limit::Int=-1)
+    result = CC._findall(sig, table.mt, table.world, limit)
+    result === nothing && return nothing # to many matches
+    nr = CC.length(result)
+    if nr ≥ 1 && CC.getindex(result, nr).fully_covers
+        # no need to fall back to the parent method view
+        return CC.MethodMatchResult(result, true)
+    end
+
+    parent_result = CC.findall(sig, table.parent; limit)::Union{Nothing, CC.MethodMatchResult}
+    parent_result === nothing && return nothing #too many matches
+
+    overlayed = parent_result.overlayed | !CC.isempty(result)
+    parent_result = parent_result.matches::CC.MethodLookupResult
+    
+    # merge the parent match results with the internal method table
+    return CC.MethodMatchResult(
+    CC.MethodLookupResult(
+        CC.vcat(result.matches, parent_result.matches),
+        CC.WorldRange(
+            CC.max(result.valid_worlds.min_world, parent_result.valid_worlds.min_world),
+            CC.min(result.valid_worlds.max_world, parent_result.valid_worlds.max_world)),
+        result.ambig | parent_result.ambig),
+    overlayed)
+end
+
+CC.isoverlayed(::StackedMethodTable) = true
+
+function CC.findsup(@nospecialize(sig::Type), table::StackedMethodTable)
+    match, valid_worlds = CC._findsup(sig, table.mt, table.world)
+    match !== nothing && return match, valid_worlds, true
+    # look up in parent
+    parent_match, parent_valid_worlds, overlayed = CC.findsup(sig, table.parent)
+    return (
+        parent_match,
+        CC.WorldRange(
+            max(valid_worlds.min_world, parent_valid_worlds.min_world),
+            min(valid_worlds.max_world, parent_valid_worlds.max_world)),
+        overlayed)
+end
 
 ## interpreter