cgen: fix multi-return in if/match exprs (#12139)

vlib/v/gen/c/cgen.v

@@ -4879,7 +4879,10 @@ fn (mut g Gen) cast_expr(node ast.CastExpr) {
 }
 
 fn (mut g Gen) concat_expr(node ast.ConcatExpr) {
-	styp := g.typ(node.return_type)
+	mut styp := g.typ(node.return_type)
+	if g.inside_return {
+		styp = g.typ(g.fn_decl.return_type)
+	}
 	sym := g.table.get_type_symbol(node.return_type)
 	is_multi := sym.kind == .multi_return
 	if !is_multi {
@@ -5163,8 +5166,8 @@ fn (mut g Gen) return_stmt(node ast.Return) {
 	}
 	// regular cases
 	if fn_return_is_multi && node.exprs.len > 0 && !g.expr_is_multi_return_call(node.exprs[0]) {
-		if node.exprs.len == 1 && node.exprs[0] is ast.IfExpr {
-			// use a temporary for `return if cond { x,y } else { a,b }`
+		if node.exprs.len == 1 && (node.exprs[0] is ast.IfExpr || node.exprs[0] is ast.MatchExpr) {
+			// use a temporary for `return if cond { x,y } else { a,b }` or `return match expr { abc { x, y } else { z, w } }`
 			g.write('$ret_typ $tmpvar = ')
 			g.expr(node.exprs[0])
 			g.writeln(';')

vlib/v/tests/fn_multiple_returns_test.v

@@ -82,3 +82,29 @@ fn test_multi_values() {
 	assert x == 'abc'
 	assert y == 'def'
 }
+
+fn match_expr(x bool) (int, int) {
+	return match x {
+		true { 1, 1 }
+		else { 0, 0 }
+	}
+}
+
+fn if_expr(x bool) (int, int) {
+	return if x { 3, 3 } else { 2, 2 }
+}
+
+fn test_multi_return_if_match_expr() {
+	a, b := match_expr(true)
+	c, d := match_expr(false)
+	x, y := if_expr(true)
+	z, w := if_expr(false)
+	assert a == 1
+	assert b == 1
+	assert c == 0
+	assert d == 0
+	assert x == 3
+	assert y == 3
+	assert z == 2
+	assert w == 2
+}