cgen: fix error for dereference mut interface in loop (fix #13913) (#13941)

vlib/v/gen/c/fn.v

@@ -775,7 +775,14 @@ fn (mut g Gen) method_call(node ast.CallExpr) {
 		left_cc_type := g.cc_type(node.left_type, false)
 		left_type_name := util.no_dots(left_cc_type)
 		g.write('${c_name(left_type_name)}_name_table[')
-		g.expr(node.left)
+		if node.left.is_auto_deref_var() && node.left_type.nr_muls() > 1 {
+			g.write('(')
+			g.write('*'.repeat(node.left_type.nr_muls() - 1))
+			g.expr(node.left)
+			g.write(')')
+		} else {
+			g.expr(node.left)
+		}
 		dot := if left_is_shared {
 			'->val.'
 		} else if node.left_type.is_ptr() {
@@ -785,7 +792,14 @@ fn (mut g Gen) method_call(node ast.CallExpr) {
 		}
 		mname := c_name(node.name)
 		g.write('${dot}_typ]._method_${mname}(')
-		g.expr(node.left)
+		if node.left.is_auto_deref_var() && node.left_type.nr_muls() > 1 {
+			g.write('(')
+			g.write('*'.repeat(node.left_type.nr_muls() - 1))
+			g.expr(node.left)
+			g.write(')')
+		} else {
+			g.expr(node.left)
+		}
 		g.write('${dot}_object')
 		if node.args.len > 0 {
 			g.write(', ')

vlib/v/tests/dereference_mut_interface_in_loop_test.v

@@ -4,15 +4,15 @@ import rand
 import rand.wyrand
 import rand.splitmix64
 
-fn main() {
+fn test_deref_mut_interface_in_loop() {
 	mut wyrand_rng := &rand.PRNG(&wyrand.WyRandRNG{})
 	mut splitmix_rng := &rand.PRNG(&splitmix64.SplitMix64RNG{})
 
 	mut generators := [wyrand_rng, splitmix_rng]
 	for mut rng in generators {
 		seed_len := rng.block_size() / 32
-		// NB: `seed_len := (*rng).block_size() / 32` does compile
 		dump(seed_len)
 		println(rng.string(15))
+		assert seed_len == 2
 	}
 }

vlib/v/tests/known_errors/testdata/README.md

@@ -0,0 +1,8 @@
+## Run the tests here with: `v vlib/v/tests/known_errors/known_errors_test.v`
+
+The intended use of this, is for providing samples, that currently do NOT compile,
+but that a future compiler improvement WILL be able to compile, and to track,
+whether they were not fixed incidentally, due to an unrelated change/improvement.
+For example, code that triggers generating invalid C code can go here,
+and later when a bug is fixed, can be moved to a proper _test.v or .vv/.out pair, 
+outside of the vlib/v/tests/known_errors/testdata/ folder.