checker: improve errors for interface method compatibility (#8097)

vlib/v/checker/checker.v

@@ -1849,9 +1849,9 @@ fn (mut c Checker) type_implements(typ table.Type, inter_typ table.Type, pos tok
 	styp := c.table.type_to_str(typ)
 	for imethod in inter_sym.methods {
 		if method := typ_sym.find_method(imethod.name) {
-			if !imethod.is_same_method_as(method) {
-				sig := c.table.fn_signature(imethod, skip_receiver: true)
-				c.error('`$styp` incorrectly implements method `$imethod.name` of interface `$inter_sym.name`, expected `$sig`',
+			msg := c.table.is_same_method(imethod, method)
+			if msg.len > 0 {
+				c.error('`$styp` incorrectly implements method `$imethod.name` of interface `$inter_sym.name`: $msg',
 					pos)
 				return false
 			}

vlib/v/checker/tests/unimplemented_interface_b.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/unimplemented_interface_b.vv:13:6: error: `Cat` incorrectly implements method `name` of interface `Animal`, expected `name() string`
+vlib/v/checker/tests/unimplemented_interface_b.vv:13:6: error: `Cat` incorrectly implements method `name` of interface `Animal`: expected return type `string`
    11 | fn main() {
    12 |     c := Cat{}
    13 |     foo(c)

vlib/v/checker/tests/unimplemented_interface_c.out

@@ -1,5 +1,5 @@
-vlib/v/checker/tests/unimplemented_interface_c.vv:12:6: error: `Cat` incorrectly implements method `name` of interface `Animal`, expected `name()`
-   10 |
+vlib/v/checker/tests/unimplemented_interface_c.vv:12:6: error: `Cat` incorrectly implements method `name` of interface `Animal`: expected 1 parameter(s), not 2
+   10 | 
    11 | fn main() {
    12 |     foo(Cat{})
       |         ~~~~~

vlib/v/checker/tests/unimplemented_interface_d.out

@@ -1,5 +1,5 @@
-vlib/v/checker/tests/unimplemented_interface_d.vv:12:6: error: `Cat` incorrectly implements method `speak` of interface `Animal`, expected `speak(s string)`
-   10 |
+vlib/v/checker/tests/unimplemented_interface_d.vv:12:6: error: `Cat` incorrectly implements method `speak` of interface `Animal`: expected 2 parameter(s), not 1
+   10 | 
    11 | fn main() {
    12 |     foo(Cat{})
       |         ~~~~~

vlib/v/checker/tests/unimplemented_interface_e.out

@@ -1,5 +1,5 @@
-vlib/v/checker/tests/unimplemented_interface_e.vv:12:6: error: `Cat` incorrectly implements method `speak` of interface `Animal`, expected `speak(s string)`
-   10 |
+vlib/v/checker/tests/unimplemented_interface_e.vv:12:6: error: `Cat` incorrectly implements method `speak` of interface `Animal`: expected `string`, not `&string` for parameter 1
+   10 | 
    11 | fn main() {
    12 |     foo(Cat{})
       |         ~~~~~

vlib/v/checker/tests/unimplemented_interface_f.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/unimplemented_interface_f.vv:11:13: error: `Cat` incorrectly implements method `speak` of interface `Animal`, expected `speak(s string)`
+vlib/v/checker/tests/unimplemented_interface_f.vv:11:13: error: `Cat` incorrectly implements method `speak` of interface `Animal`: expected 2 parameter(s), not 1
     9 | fn main() {
    10 |     mut animals := []Animal{}
    11 |     animals << Cat{}

vlib/v/table/table.v

@@ -129,19 +129,22 @@ pub fn (t &Table) fn_type_source_signature(f &Fn) string {
 	return sig
 }
 
-pub fn (f &Fn) is_same_method_as(func &Fn) bool {
+pub fn (t &Table) is_same_method(f &Fn, func &Fn) string {
 	if f.return_type != func.return_type {
-		return false
+		s := t.type_to_str(f.return_type)
+		return 'expected return type `$s`'
 	}
 	if f.params.len != func.params.len {
-		return false
+		return 'expected $f.params.len parameter(s), not $func.params.len'
 	}
 	for i in 1 .. f.params.len {
 		if f.params[i].typ != func.params[i].typ {
-			return false
+			exps := t.type_to_str(f.params[i].typ)
+			gots := t.type_to_str(func.params[i].typ)
+			return 'expected `$exps`, not `$gots` for parameter $i'
 		}
 	}
-	return true
+	return ''
 }
 
 pub fn (t &Table) find_fn(name string) ?Fn {