interfaces: error on implemention of own interface method & on duplicate normal methods

vlib/v/checker/checker.v

@@ -5841,9 +5841,17 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 			c.error('unknown type `$sym.name`', node.receiver_pos)
 			return
 		}
-		// if sym.has_method(node.name) {
-		// c.warn('duplicate method `$node.name`', node.pos)
-		// }
+		// make sure interface does not implement its own interface methods
+		if sym.kind == .interface_ && sym.has_method(node.name) {
+			if sym.info is table.Interface {
+				info := sym.info as table.Interface
+				// if the method is in info.methods then it is an interface method
+				if info.has_method(node.name) {
+					c.error('interface `$sym.name` cannot implement its own interface method `$node.name`',
+						node.pos)
+				}
+			}
+		}
 		// needed for proper error reporting during vweb route checking
 		sym.methods[node.method_idx].source_fn = voidptr(node)
 	}

vlib/v/checker/tests/interface_implementing_own_interface_method.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/interface_implementing_own_interface_method.vv:5:1: error: interface `Animal` cannot implement its own interface method `speak`
+    3 | }
+    4 | 
+    5 | fn (a Animal) speak() {
+      | ~~~~~~~~~~~~~~~~~~~~~
+    6 |     println('speaking')
+    7 | }

vlib/v/checker/tests/interface_implementing_own_interface_method.vv

@@ -0,0 +1,7 @@
+interface Animal {
+    speak()
+}
+
+fn (a Animal) speak() {
+    println('speaking')
+}

vlib/v/parser/fn.v

@@ -221,11 +221,20 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 			}
 		}
 		type_sym := p.table.get_type_symbol(rec.typ)
-		// interfaces are handled in the checker, methods can not be defined on them this way
-		if is_method && (type_sym.has_method(name) && type_sym.kind != .interface_) {
-			p.error_with_pos('duplicate method `$name`', pos)
-			return ast.FnDecl{
-				scope: 0
+		if is_method {
+			mut is_duplicate := type_sym.has_method(name)
+			// make sure this is a normal method and not an interface method
+			if type_sym.kind == .interface_ && is_duplicate {
+				if type_sym.info is table.Interface {
+					// if the method is in info then its an interface method
+					is_duplicate = !type_sym.info.has_method(name)
+				}
+			}
+			if is_duplicate {
+				p.error_with_pos('duplicate method `$name`', pos)
+				return ast.FnDecl{
+					scope: 0
+				}
 			}
 		}
 		// cannot redefine buildin function
@@ -276,7 +285,6 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 			})
 		}
 	}
-	mut end_pos := p.prev_tok.position()
 	// Return type
 	mut return_type := table.void_type
 	// don't confuse token on the next line: fn decl, [attribute]
@@ -287,6 +295,7 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 	}
 	mut type_sym_method_idx := 0
 	no_body := p.tok.kind != .lcbr
+	end_pos := p.prev_tok.position()
 	// Register
 	if is_method {
 		mut type_sym := p.table.get_type_symbol(rec.typ)
@@ -348,7 +357,6 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 			language: language
 		})
 	}
-	end_pos = p.prev_tok.position()
 	// Body
 	p.cur_fn_name = name
 	mut stmts := []ast.Stmt{}

vlib/v/parser/tests/interface_duplicate_interface_method.out

@@ -0,0 +1,6 @@
+vlib/v/parser/tests/interface_duplicate_interface_method.vv:4:2: error: duplicate method `fun`
+    2 | interface Abc {
+    3 |     fun()
+    4 |     fun()
+      |     ~~~
+    5 | }

vlib/v/parser/tests/interface_duplicate_interface_method.vv

@@ -0,0 +1,5 @@
+// duplicate interface methods in decleration
+interface Abc {
+	fun()
+	fun()
+}

vlib/v/parser/tests/interface_duplicate_method.out

@@ -1,6 +1,5 @@
-vlib/v/parser/tests/interface_duplicate_method.vv:3:2: error: duplicate method `fun`
-    1 | interface Abc {
-    2 |     fun()
-    3 |     fun()
-      |     ~~~
-    4 | }
+vlib/v/parser/tests/interface_duplicate_method.vv:5:12: error: duplicate method `foo`
+    3 | // duplicate normal method definitions on interface
+    4 | fn (a Abc) foo() {}
+    5 | fn (a Abc) foo() {}
+      |            ~~~

vlib/v/parser/tests/interface_duplicate_method.vv

@@ -1,4 +1,5 @@
-interface Abc {
-	fun()
-	fun()
-}
+interface Abc {}
+
+// duplicate normal method definitions on interface
+fn (a Abc) foo() {}
+fn (a Abc) foo() {}

vlib/v/table/types.v

@@ -993,6 +993,22 @@ pub fn (i &Interface) find_field(name string) ?Field {
 	return none
 }
 
+pub fn (i &Interface) find_method(name string) ?Fn {
+	for method in i.methods {
+		if method.name == name {
+			return method
+		}
+	}
+	return none
+}
+
+pub fn (i &Interface) has_method(name string) bool {
+	if _ := i.find_method(name) {
+		return true
+	}
+	return false
+}
+
 pub fn (s Struct) find_field(name string) ?Field {
 	for field in s.fields {
 		if field.name == name {