checker: do not deref non-pointer types in `fn_signature_using_aliases` (#12340)

vlib/v/ast/types.v

@@ -1150,7 +1150,9 @@ pub fn (t &Table) fn_signature_using_aliases(func &Fn, import_aliases map[string
 		param := func.params[i]
 		mut typ := param.typ
 		if param.is_mut {
-			typ = typ.deref()
+			if param.typ.is_ptr() {
+				typ = typ.deref()
+			}
 			sb.write_string('mut ')
 		}
 		if !opts.type_only {

vlib/v/checker/tests/interface_sameness_check_for_mutable_methods.out

@@ -0,0 +1,8 @@
+vlib/v/checker/tests/interface_sameness_check_for_mutable_methods.vv:15:6: error: `St` incorrectly implements method `method` of interface `Interface`: expected return type `u32`
+   13 |
+   14 | fn bar() {
+   15 |     foo(St{})
+      |         ~~~~
+   16 | }
+Details: main.Interface has `fn method(mut x main.Interface) u32`
+         main.St has `fn method(st main.St) int`

vlib/v/checker/tests/interface_sameness_check_for_mutable_methods.vv

@@ -0,0 +1,16 @@
+interface Interface {
+mut:
+	method() u32
+}
+
+struct St {}
+
+fn (st St) method() int {
+	panic('')
+}
+
+fn foo(_ Interface) {}
+
+fn bar() {
+	foo(St{})
+}