cgen: fix sumtype as cast with calling twice (#17196)

cmd/tools/vtest-self.v

@@ -230,6 +230,7 @@ const (
 		'do_not_remove',
 		'vlib/v/tests/const_fixed_array_containing_references_to_itself_test.v', // error C2099: initializer is not a constant
 		'vlib/v/tests/const_and_global_with_same_name_test.v', // error C2099: initializer is not a constant
+		'vlib/v/tests/sumtype_as_cast_test.v', // error: cannot support compound statement expression ({expr; expr; expr;})
 	]
 	skip_on_windows               = [
 		'do_not_remove',

vlib/v/gen/c/cgen.v

@@ -6088,23 +6088,43 @@ fn (mut g Gen) as_cast(node ast.AsCast) {
 	mut expr_type_sym := g.table.sym(g.unwrap_generic(node.expr_type))
 	if mut expr_type_sym.info is ast.SumType {
 		dot := if node.expr_type.is_ptr() { '->' } else { '.' }
-		if sym.info is ast.FnType {
+		if node.expr is ast.CallExpr && !g.is_cc_msvc {
-			g.write('/* as */ (${styp})__as_cast(')
+			tmp_var := g.new_tmp_var()
+			expr_styp := g.typ(node.expr_type)
+			g.write('({ ${expr_styp} ${tmp_var} = ')
+			g.expr(node.expr)
+			g.write('; ')
+			if sym.info is ast.FnType {
+				g.write('/* as */ (${styp})__as_cast(')
+			} else {
+				g.write('/* as */ *(${styp}*)__as_cast(')
+			}
+			g.write(tmp_var)
+			g.write(dot)
+			g.write('_${sym.cname},')
+			g.write(tmp_var)
+			g.write(dot)
+			sidx := g.type_sidx(unwrapped_node_typ)
+			g.write('_typ, ${sidx}); }) /*expected idx: ${sidx}, name: ${sym.name} */ ')
 		} else {
-			g.write('/* as */ *(${styp}*)__as_cast(')
+			if sym.info is ast.FnType {
+				g.write('/* as */ (${styp})__as_cast(')
+			} else {
+				g.write('/* as */ *(${styp}*)__as_cast(')
+			}
+			g.write('(')
+			g.expr(node.expr)
+			g.write(')')
+			g.write(dot)
+			g.write('_${sym.cname},')
+			g.write('(')
+			g.expr(node.expr)
+			g.write(')')
+			g.write(dot)
+			// g.write('typ, /*expected:*/$node.typ)')
+			sidx := g.type_sidx(unwrapped_node_typ)
+			g.write('_typ, ${sidx}) /*expected idx: ${sidx}, name: ${sym.name} */ ')
 		}
-		g.write('(')
-		g.expr(node.expr)
-		g.write(')')
-		g.write(dot)
-		g.write('_${sym.cname},')
-		g.write('(')
-		g.expr(node.expr)
-		g.write(')')
-		g.write(dot)
-		// g.write('typ, /*expected:*/$node.typ)')
-		sidx := g.type_sidx(unwrapped_node_typ)
-		g.write('_typ, ${sidx}) /*expected idx: ${sidx}, name: ${sym.name} */ ')
 
 		// fill as cast name table
 		for variant in expr_type_sym.info.variants {

vlib/v/tests/sumtype_as_cast_test.v

@@ -0,0 +1,18 @@
+type Sum = int | string
+
+struct Count {
+mut:
+	count int
+}
+
+fn (mut c Count) ret_sum() Sum {
+	c.count++
+	return c.count
+}
+
+fn test_sumtype_as_cast() {
+	mut cnt := Count{22}
+	_ := cnt.ret_sum() as int
+	println(cnt)
+	assert cnt.count == 23
+}