checker: prevent invalid SelectorExprs

CHANGELOG.md

@@ -21,6 +21,7 @@ _unreleased_
 ### Error and Type Checks
 - If conditions must be of type bool
 - Cannot assign from a void function call
+- Prevent selecting fields of unsupported types (e.g. enums)
 - Prevent some cases of identifier redefinition
   - Any identifier by a function name
   - Function argument by a variable

lib/bait/checker/expr.bt

@@ -347,7 +347,8 @@ fun (mut c Checker) selector_expr(mut node ast.SelectorExpr) ast.Type {
 		return ast.VOID_TYPE
 	}
 
-	return ast.ANY_TYPE
+	c.error('cannot select from ${c.table.type_name(node.expr_type)}', node.pos)
+	return ast.PLACEHOLDER_TYPE
 }
 
 fun (c Checker) string_literal(node ast.StringLiteral) ast.Type {

lib/bait/preference/preference.bt

@@ -164,7 +164,7 @@ fun backend_from_string(s string) Backend {
 		'c' { .c }
 		else {
 			panic('Invalid backend: ${s}')
-			.js
+			Backend.js // TODO remove once @noreturn works properly
 		}
 	}
 }

tests/out/error/selector/cannot_select.in.bt

@@ -0,0 +1,7 @@
+enum Color {
+	red
+	green
+}
+
+c := Color.red
+_ := c.my_field

tests/out/error/selector/cannot_select.out

@@ -0,0 +1 @@
+tests/out/error/selector/cannot_select.in.bt:7:7 error: cannot select from Color