checker, cgen: fix printing in 'for v in a' (#14146)

vlib/v/checker/for.v

@@ -95,6 +95,9 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 				node.scope.update_var_type(node.key_var, key_type)
 			}
 			mut value_type := c.table.value_type(typ)
+			if sym.kind == .string {
+				value_type = ast.byte_type
+			}
 			if value_type == ast.void_type || typ.has_flag(.optional) {
 				if typ != ast.void_type {
 					c.error('for in: cannot index `${c.table.type_to_str(typ)}`', node.cond.pos())

vlib/v/gen/c/for.v

@@ -299,14 +299,15 @@ fn (mut g Gen) for_in_stmt(node ast.ForInStmt) {
 		} else {
 			node.cond
 		}
+		field_accessor := if node.cond_type.is_ptr() { '->' } else { '.' }
 		i := if node.key_var in ['', '_'] { g.new_tmp_var() } else { node.key_var }
 		g.write('for (int $i = 0; $i < ')
 		g.expr(cond)
-		g.writeln('.len; ++$i) {')
+		g.writeln('${field_accessor}len; ++$i) {')
 		if node.val_var != '_' {
-			g.write('\tbyte ${c_name(node.val_var)} = ')
+			g.write('\tu8 ${c_name(node.val_var)} = ')
 			g.expr(cond)
-			g.writeln('.str[$i];')
+			g.writeln('${field_accessor}str[$i];')
 		}
 	} else if node.kind == .struct_ {
 		cond_type_sym := g.table.sym(node.cond_type)

vlib/v/tests/inout/printing_for_v_in_a.out

@@ -0,0 +1,11 @@
+97
+98
+99
+abc
+23
+77
+abc
+> k: abc | v: xyz
+> k: def | v: jkl
+abc
+abc

vlib/v/tests/inout/printing_for_v_in_a.vv

@@ -0,0 +1,30 @@
+interface Any {}
+
+fn abc(a Any) {
+	if a is string {
+		for x in a {
+			println(x)
+		}
+	}
+	if a is []u8 {
+		for x in a {
+			println(x)
+		}
+	}
+	if a is map[string]string {
+		for k, v in a {
+			println('> k: $k | v: $v')
+		}
+	}
+	println(@FN)
+}
+
+fn main() {
+	abc('abc')
+	abc([u8(23), 77])
+	abc({
+		'abc': 'xyz'
+		'def': 'jkl'
+	})
+	abc(123)
+}