cgen: fix passing sumtype parameter in sumtype matching results (fix #15078) (#15767)

vlib/v/gen/c/cgen.v

@@ -2306,7 +2306,7 @@ fn (mut g Gen) expr_with_cast(expr ast.Expr, got_type_raw ast.Type, expected_typ
 			scope := g.file.scope.innermost(expr.pos().pos)
 			if expr is ast.Ident {
 				if v := scope.find_var(expr.name) {
-					if v.smartcasts.len > 0 {
+					if v.smartcasts.len > 0 && unwrapped_expected_type == v.orig_type {
 						is_already_sum_type = true
 					}
 				}

vlib/v/gen/c/fn.v

@@ -1594,6 +1594,23 @@ fn (mut g Gen) call_args(node ast.CallExpr) {
 		if is_variadic && i == expected_types.len - 1 {
 			break
 		}
+		if arg.expr is ast.Ident {
+			if arg.expr.obj is ast.Var {
+				if arg.expr.obj.smartcasts.len > 0 {
+					exp_sym := g.table.sym(expected_types[i])
+					orig_sym := g.table.sym(arg.expr.obj.orig_type)
+					if orig_sym.kind != .interface_ && (exp_sym.kind != .sum_type
+						|| (exp_sym.kind == .sum_type
+						&& expected_types[i] != arg.expr.obj.orig_type)) {
+						expected_types[i] = g.unwrap_generic(arg.expr.obj.smartcasts.last())
+						cast_sym := g.table.sym(expected_types[i])
+						if cast_sym.info is ast.Aggregate {
+							expected_types[i] = cast_sym.info.types[g.aggregate_type_idx]
+						}
+					}
+				}
+			}
+		}
 		use_tmp_var_autofree := g.is_autofree && arg.typ == ast.string_type && arg.is_tmp_autofree
 			&& !g.inside_const && !g.is_builtin_mod
 		// g.write('/* af=$arg.is_tmp_autofree */')

vlib/v/tests/passing_sumtype_parameter_in_sumtype_matching_results_test.v

@@ -0,0 +1,25 @@
+struct Foo {}
+
+type Bar = Foo | int
+type Baz = Foo | f32
+
+fn foo(a Bar) int {
+	return 22
+}
+
+fn bar() Baz {
+	return Foo{}
+}
+
+fn test_passing_sumtype_parameter_in_sumtype_matching_results() {
+	a := bar()
+	mut ret := 0
+	match a {
+		Foo {
+			ret = foo(Bar(a))
+		}
+		f32 {}
+	}
+	println(ret)
+	assert ret == 22
+}