checker: disallow $for i in struct.values and $for i in enum.fields

vlib/v/checker/comptime.v

@@ -287,6 +287,10 @@ fn (mut c Checker) comptime_for(mut node ast.ComptimeFor) {
 			}
 			c.comptime_for_field_var = ''
 			c.inside_comptime_for_field = false
+		} else if c.table.generic_type_names(node.typ).len == 0 && sym.kind != .placeholder {
+			c.error('iterating over .fields is supported only for structs and interfaces, and ${sym.name} is neither',
+				node.typ_pos)
+			return
 		}
 	} else if node.kind == .values {
 		if sym.kind == .enum_ {
@@ -301,6 +305,10 @@ fn (mut c Checker) comptime_for(mut node ast.ComptimeFor) {
 				c.comptime_fields_type[node.val_var] = node.typ
 				c.stmts(mut node.stmts)
 			}
+		} else {
+			c.error('iterating over .values is supported only for enums, and ${sym.name} is not an enum',
+				node.typ_pos)
+			return
 		}
 	} else {
 		c.stmts(mut node.stmts)

vlib/v/checker/tests/for_comptime_enum_fields_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/for_comptime_enum_fields_err.vv:6:11: error: iterating over .fields is supported only for structs and interfaces, and Test is neither
+    4 | }
+    5 | 
+    6 | $for i in Test.fields {
+      |           ~~~~
+    7 |     println(i)
+    8 | }

vlib/v/checker/tests/for_comptime_enum_fields_err.vv

@@ -0,0 +1,8 @@
+enum Test {
+	one
+	two
+}
+
+$for i in Test.fields {
+	println(i)
+}

vlib/v/checker/tests/for_comptime_struct_values_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/for_comptime_struct_values_err.vv:5:11: error: iterating over .values is supported only for enums, and Foo is not an enum
+    3 | fn (_ Foo) hello() {}
+    4 | 
+    5 | $for i in Foo.values {
+      |           ~~~
+    6 |     println(i)
+    7 | }

vlib/v/checker/tests/for_comptime_struct_values_err.vv

@@ -0,0 +1,7 @@
+struct Foo{}
+
+fn (_ Foo) hello() {}
+
+$for i in Foo.values {
+	println(i)
+}