checker: check fn returning fn type mismatch (fix #15988) (#15997)

vlib/v/checker/check_types.v

@@ -384,8 +384,10 @@ pub fn (mut c Checker) check_matching_function_symbols(got_type_sym &ast.TypeSym
 	}
 	for i, got_arg in got_fn.params {
 		exp_arg := exp_fn.params[i]
-		exp_arg_is_ptr := exp_arg.typ.is_ptr() || exp_arg.typ.is_pointer()
-		got_arg_is_ptr := got_arg.typ.is_ptr() || got_arg.typ.is_pointer()
+		exp_arg_typ := c.unwrap_generic(exp_arg.typ)
+		got_arg_typ := c.unwrap_generic(got_arg.typ)
+		exp_arg_is_ptr := exp_arg_typ.is_ptr() || exp_arg_typ.is_pointer()
+		got_arg_is_ptr := got_arg_typ.is_ptr() || got_arg_typ.is_pointer()
 		if exp_arg_is_ptr != got_arg_is_ptr {
 			exp_arg_pointedness := if exp_arg_is_ptr { 'a pointer' } else { 'NOT a pointer' }
 			got_arg_pointedness := if got_arg_is_ptr { 'a pointer' } else { 'NOT a pointer' }
@@ -397,9 +399,11 @@ pub fn (mut c Checker) check_matching_function_symbols(got_type_sym &ast.TypeSym
 			}
 			return false
 		} else if exp_arg_is_ptr && got_arg_is_ptr {
-			continue
+			if exp_arg_typ.is_pointer() || got_arg_typ.is_pointer() {
+				continue
+			}
 		}
-		if got_arg.typ != exp_arg.typ {
+		if c.table.unaliased_type(got_arg_typ).idx() != c.table.unaliased_type(exp_arg_typ).idx() {
 			return false
 		}
 	}

vlib/v/checker/return.v

@@ -134,12 +134,12 @@ pub fn (mut c Checker) return_stmt(mut node ast.Return) {
 					}
 					continue
 				}
-				if got_typ_sym.kind == .function && exp_typ_sym.kind == .function {
-					if (got_typ_sym.info as ast.FnType).is_anon {
-						continue
-					}
+				got_typ_name := if got_typ_sym.kind == .function {
+					'${c.table.type_to_str(got_typ)}'
+				} else {
+					got_typ_sym.name
 				}
-				c.error('cannot use `$got_typ_sym.name` as type `${c.table.type_to_str(exp_type)}` in return argument',
+				c.error('cannot use `$got_typ_name` as type `${c.table.type_to_str(exp_type)}` in return argument',
 					pos)
 			}
 		}

vlib/v/checker/tests/fn_return_fn_type_mismatch_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/fn_return_fn_type_mismatch_err.vv:22:9: error: cannot use `fn (&WrongType, i64, u64) int` as type `fn (&ExpectedType, i64, u64) int` in return argument
+   20 | pub fn bad_signature() Fn {
+   21 |     // create closure that has nonmatching signature due to first arg. Compiler should reject it
+   22 |     return fn (n &WrongType, a i64, b u64) int {
+      |            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+   23 |         println('bad signature param0=$n')
+   24 |         return 0

vlib/v/checker/tests/fn_return_fn_type_mismatch_err.vv

@@ -0,0 +1,39 @@
+pub struct ExpectedType {
+	a u32
+	b u32
+}
+
+struct WrongType {
+	c string
+	d int
+}
+
+pub type Fn = fn (&ExpectedType, i64, u64) int
+
+pub fn good_signature() Fn {
+	return fn (n &ExpectedType, a i64, b u64) int {
+		println('good signature param0=$n')
+		return 0
+	}
+}
+
+pub fn bad_signature() Fn {
+	// create closure that has nonmatching signature due to first arg. Compiler should reject it
+	return fn (n &WrongType, a i64, b u64) int {
+		println('bad signature param0=$n')
+		return 0
+	}
+}
+
+fn main() {
+	arg := &ExpectedType{
+		a: 3
+		b: 7
+	}
+
+	mut cb := good_signature()
+	cb(arg, 1, 1)
+
+	cb = bad_signature()
+	cb(arg, 1, 1)
+}