checker: disallow blank ident in const and global decl

vlib/v/checker/checker.v

@@ -1776,6 +1776,13 @@ fn (mut c Checker) const_decl(mut node ast.ConstDecl) {
 			}
 			c.error('duplicate of a module name `${field.name}`', name_pos)
 		}
+		if const_name == '_' {
+			name_pos := token.Pos{
+				...field.pos
+				len: util.no_cur_mod(field.name, c.mod).len
+			}
+			c.error('cannot use `_` as a const name', name_pos)
+		}
 		c.const_names << field.name
 	}
 	for i, mut field in node.fields {
@@ -2317,7 +2324,7 @@ fn (mut c Checker) global_decl(mut node ast.GlobalDecl) {
 		if sym.kind == .placeholder {
 			c.error('unknown type `${sym.name}`', field.typ_pos)
 		}
-		if field.has_expr {
+		if field.has_expr && field.name != '_' {
 			if field.expr is ast.AnonFn && field.name == 'main' {
 				c.error('the `main` function is the program entry point, cannot redefine it',
 					field.pos)
@@ -2337,6 +2344,9 @@ fn (mut c Checker) global_decl(mut node ast.GlobalDecl) {
 				}
 				v.typ = ast.mktyp(field.typ)
 			}
+			if field.name == '_' {
+				c.error('cannot use `_` as a global name', field.pos)
+			}
 		}
 		c.global_names << field.name
 	}

vlib/v/checker/tests/const_blank_ident_as_name_err.out

@@ -0,0 +1,3 @@
+vlib/v/checker/tests/const_blank_ident_as_name_err.vv:1:7: error: cannot use `_` as a const name
+    1 | const _ = 23
+      |       ^

vlib/v/checker/tests/const_blank_ident_as_name_err.vv

@@ -0,0 +1 @@
+const _ = 23

vlib/v/checker/tests/globals/global_blank_ident_as_name_err.out

@@ -0,0 +1,18 @@
+vlib/v/checker/tests/globals/global_blank_ident_as_name_err.vv:2:5: error: cannot use `_` as a global name
+    1 | __global (
+    2 |     _ = 23
+      |     ^
+    3 |     _ int
+    4 | )
+vlib/v/checker/tests/globals/global_blank_ident_as_name_err.vv:3:5: error: duplicate global `_`
+    1 | __global (
+    2 |     _ = 23
+    3 |     _ int
+      |     ^
+    4 | )
+vlib/v/checker/tests/globals/global_blank_ident_as_name_err.vv:3:5: error: cannot use `_` as a global name
+    1 | __global (
+    2 |     _ = 23
+    3 |     _ int
+      |     ^
+    4 | )

vlib/v/checker/tests/globals/global_blank_ident_as_name_err.vv

@@ -0,0 +1,4 @@
+__global (
+    _ = 23
+    _ int
+)