cgen: fix fn (d Struct) a[T]() T { return d } when T is a sumtype (#25644)

Author: spytheman

vlib/v/gen/c/struct.v

@@ -41,9 +41,13 @@ fn (mut g Gen) struct_init(node ast.StructInit) {
 	mut sym := g.table.final_sym(unwrapped_typ)
 	if sym.kind == .sum_type {
 		if node.typ.has_flag(.generic) && unwrapped_typ.is_ptr() {
-			g.write('&(')
-			g.write(g.type_default_sumtype(unwrapped_typ.set_nr_muls(0), sym))
-			g.write(')')
+			// handle promotions to a sumtype for generic functions like this one: `fn (d Struct) a[T]() T { return d }`
+			// the value should be on the heap, since it is not known where it will be used:
+			sumtype_type := unwrapped_typ.set_nr_muls(0)
+			sumtype_name := g.styp(sumtype_type)
+			g.write('HEAP(${sumtype_name}, (')
+			g.write(g.type_default_sumtype(sumtype_type, sym))
+			g.write('))')
 		} else {
 			g.write(g.type_default_sumtype(unwrapped_typ, sym))
 		}

vlib/v/tests/sumtypes/sumtype_value_pointer_test.v

@@ -0,0 +1,17 @@
+struct Struct {}
+
+type SumType = Struct | int
+
+fn (d Struct) a[T]() T {
+	return T{}
+}
+
+fn test_main() {
+	s := Struct{}
+	x := s.a[&SumType]()
+	assert x is Struct
+	println('first assert passed')
+	println(x)
+	assert x is Struct
+	println('second assert passed')
+}