checker: add missing check for mismatch anon struct to typed struct (#18250)

vlib/v/checker/assign.v

@@ -698,6 +698,9 @@ fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 				}
 			}
 		}
+		if left_sym.kind == .struct_ && right is ast.StructInit && (right as ast.StructInit).is_anon {
+			c.error('cannot assign anonymous `struct` to a typed `struct`', right.pos())
+		}
 	}
 	// this needs to run after the assign stmt left exprs have been run through checker
 	// so that ident.obj is set

vlib/v/checker/tests/anon_struct_assign_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/anon_struct_assign_err.vv:9:12: error: cannot assign anonymous `struct` to a typed `struct`
+    7 | }
+    8 | println(y)
+    9 | y = struct {
+      |            ^
+   10 |     name: 'Def'
+   11 | }

vlib/v/checker/tests/anon_struct_assign_err.vv

@@ -0,0 +1,12 @@
+struct Abc {
+	name string
+}
+
+mut y := Abc{
+	name: 'Abc'
+}
+println(y)
+y = struct {
+	name: 'Def'
+}
+println(y)