cgen: fix missing aggregate rec_type (#17335)

vlib/v/gen/c/fn.v

@@ -952,6 +952,19 @@ fn (mut g Gen) method_call(node ast.CallExpr) {
 			else {}
 		}
 	}
+
+	if node.from_embed_types.len == 0 && node.left is ast.Ident {
+		if node.left.obj is ast.Var {
+			if node.left.obj.smartcasts.len > 0 {
+				unwrapped_rec_type = g.unwrap_generic(node.left.obj.smartcasts.last())
+				cast_sym := g.table.sym(unwrapped_rec_type)
+				if cast_sym.info is ast.Aggregate {
+					unwrapped_rec_type = cast_sym.info.types[g.aggregate_type_idx]
+				}
+			}
+		}
+	}
+
 	if g.inside_comptime_for_field {
 		mut node_ := unsafe { node }
 		comptime_args = g.change_comptime_args(mut node_)

vlib/v/tests/method_call_on_aggregate_test.v

@@ -0,0 +1,27 @@
+type SumType = AA | BB
+
+struct AA {}
+
+struct BB {}
+
+fn (a &AA) foo() int {
+	println('AA.foo()')
+	return 200
+}
+
+fn (b &BB) foo() int {
+	println('BB.foo()')
+	return 100
+}
+
+fn test_main() {
+	x := SumType(BB{})
+	match x {
+		AA, BB {
+			ret := x.foo()
+			assert ret == 100
+			return
+		}
+	}
+	assert false
+}