checker: fix unwrap, when generic structs are used as arguments, in u…

…ncalled methods (fix #20132) (#20135)

vlib/v/checker/struct.v

@@ -900,6 +900,23 @@ or use an explicit `unsafe{ a[..] }`, if you do not want a copy of the slice.',
 				generic_names := struct_sym.info.generic_types.map(c.table.sym(it).name)
 				node.typ = c.table.unwrap_generic_type(node.typ, generic_names, concrete_types)
 			}
+		} else if struct_sym.info.generic_types.len > 0
+			&& struct_sym.info.generic_types.len == struct_sym.info.concrete_types.len
+			&& c.table.cur_concrete_types.len == 0 {
+			parent_type := struct_sym.info.parent_type
+			parent_sym := c.table.sym(parent_type)
+			for method in parent_sym.methods {
+				generic_names := struct_sym.info.generic_types.map(c.table.sym(it).name)
+				for i, param in method.params {
+					if i == 0 || !param.typ.has_flag(.generic) {
+						continue
+					}
+					param_sym := c.table.sym(param.typ)
+					if param_sym.kind in [.struct_, .interface_, .sum_type] {
+						c.table.unwrap_generic_type(param.typ, generic_names, struct_sym.info.concrete_types)
+					}
+				}
+			}
 		}
 	}
 	return node.typ

vlib/v/tests/generics_struct_method_params_is_generics_struct_and_not_called_test.v

@@ -0,0 +1,17 @@
+struct Param[T] {
+}
+
+struct Struct[T] {
+}
+
+pub fn (s Struct[T]) method[T](p Param[T]) {
+}
+
+fn test_main() {
+	_ = Struct[int]{}
+	assert true
+	// NOTE:
+	// Do not test call Struct.method() here,
+	// to test unwrap of generic struct parameters when the method is not called.
+	// test results only need to can compile.
+}