checker: verify interface is not used as receiver

vlib/v/ast/ast.v

@@ -206,6 +206,7 @@ pub:
 	is_variadic   bool
 	is_anon       bool
 	receiver      Field
+	receiver_pos  token.Position
 	is_method     bool
 	rec_mut       bool // is receiver mutable
 	is_c          bool

vlib/v/checker/checker.v

@@ -1385,12 +1385,15 @@ fn (mut c Checker) stmt(node ast.Stmt) {
 			c.check_expr_opt_call(it.expr, etype, false)
 		}
 		ast.FnDecl {
-			// if it.is_method {
+			if it.is_method {
-			// sym := c.table.get_type_symbol(it.receiver.typ)
+				sym := c.table.get_type_symbol(it.receiver.typ)
+				if sym.kind == .interface_ {
+					c.error('interaces cannot be used as method receiver', it.receiver_pos)
+				}
 				// if sym.has_method(it.name) {
 				// c.warn('duplicate method `$it.name`', it.pos)
 				// }
-			// }
+			}
 			if !it.is_c {
 				// Make sure all types are valid
 				for arg in it.args {
@@ -1748,7 +1751,8 @@ pub fn (mut c Checker) ident(ident mut ast.Ident) table.Type {
 					if sym.info is table.FnType {
 						// anon/local fn assigned to new variable uses this
 						info := sym.info as table.FnType
-						fn_type := table.new_type(c.table.find_or_register_fn_type(info.func, true, true))
+						fn_type := table.new_type(c.table.find_or_register_fn_type(info.func,
+							true, true))
 						ident.kind = .function
 						ident.info = ast.IdentFn{
 							typ: fn_type

vlib/v/checker/tests/no_interface_receiver.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/no_interface_receiver.v:5:5: error: interaces cannot be used as method receiver
+    3 | }
+    4 |
+    5 | fn (a Animal) str() {}
+      |     ~~~~~~~~
+

vlib/v/checker/tests/no_interface_receiver.vv

@@ -0,0 +1,5 @@
+interface Animal {
+	speak()
+}
+
+fn (a Animal) str() {}

vlib/v/parser/fn.v

@@ -109,6 +109,7 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 	// Receiver?
 	mut rec_name := ''
 	mut is_method := false
+	mut receiver_pos := token.Position{}
 	mut rec_type := table.void_type
 	mut rec_mut := false
 	mut args := []table.Arg{}
@@ -119,11 +120,13 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 		if rec_mut {
 			p.next() // `mut`
 		}
+		rec_start_pos := p.tok.position()
 		rec_name = p.check_name()
 		if !rec_mut {
 			rec_mut = p.tok.kind == .key_mut
 		}
 		is_amp := p.tok.kind == .amp
+		receiver_pos = rec_start_pos.extend(p.tok.position())
 		// if rec_mut {
 		// p.check(.key_mut)
 		// }
@@ -256,6 +259,7 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 			name: rec_name
 			typ: rec_type
 		}
+		receiver_pos: receiver_pos
 		is_method: is_method
 		rec_mut: rec_mut
 		is_c: is_c