refine returned match errors

compiler/types/types.odin

@@ -552,6 +552,73 @@ can_sum_widen :: proc(from, to: Type, store: ^Store) -> bool {
 	return true
 }
 
+// Reports whether every selected variant from `from` is represented identically in
+// `to`. The checker uses this only after control flow has proven that the source value
+// is one of `selected`; it is not a general implicit narrowing rule.
+selected_sum_fits :: proc(from, to: Type, selected: []u32, store: ^Store) -> bool {
+	if len(selected) == 0 {
+		return false
+	}
+	from_variants: [dynamic]Sum_Variant
+	from_variants.allocator = store.allocator
+	defer delete(from_variants)
+	to_variants: [dynamic]Sum_Variant
+	to_variants.allocator = store.allocator
+	defer delete(to_variants)
+	if !append_sum_variants(store, from, &from_variants) ||
+	   !append_sum_variants(store, to, &to_variants) {
+		return false
+	}
+	for name in selected {
+		source_variant: Sum_Variant
+		source_found := false
+		for variant in from_variants {
+			if variant.name == name {
+				source_variant = variant
+				source_found = true
+				break
+			}
+		}
+		if !source_found {
+			return false
+		}
+		matched := false
+		for variant in to_variants {
+			if variant.id == source_variant.id && variant.payload == source_variant.payload {
+				matched = true
+				break
+			}
+		}
+		if !matched {
+			return false
+		}
+	}
+	return true
+}
+
+// Sum_Project is emitted only with a checker proof. This weaker structural predicate is
+// retained by the backend as a defensive check that the two sums share a valid variant.
+can_sum_project :: proc(from, to: Type, store: ^Store) -> bool {
+	from_variants: [dynamic]Sum_Variant
+	from_variants.allocator = store.allocator
+	defer delete(from_variants)
+	to_variants: [dynamic]Sum_Variant
+	to_variants.allocator = store.allocator
+	defer delete(to_variants)
+	if !append_sum_variants(store, from, &from_variants) ||
+	   !append_sum_variants(store, to, &to_variants) {
+		return false
+	}
+	for source_variant in from_variants {
+		for target_variant in to_variants {
+			if target_variant.id == source_variant.id && target_variant.payload == source_variant.payload {
+				return true
+			}
+		}
+	}
+	return false
+}
+
 define_struct :: proc(store: ^Store, id: Type, fields: []Field, c_layout, opaque: bool) -> bool {
 	return define_record(store, id, fields, c_layout, opaque)
 }