Improve error message for match on structs

.release-notes/3746.md

@@ -0,0 +1,55 @@
+## Improve error messages when matching on struct types
+
+A struct type doesn't have a type descriptor, which means that they cannot be used in match or "as" statements. Before this change, the compiler would incorrectly warn that matching against a struct wasn't possible due to a violation of capabilities, which was confusing. With this release, the compiler will now show a more helpful error message, explicitly mentioning that struct types can't be used in union types.
+
+As an example, the following piece of Pony code:
+
+```pony
+struct Rect
+
+actor Main
+  new create(env: Env) =>
+    let a: (Rect | None) = None
+    match a
+    | let a': Rect => None
+    | None => None
+    end
+```
+
+would fail to compile on ponyc 0.40.0 with the following error message:
+
+```
+Error:
+main.pony:7:7: this capture violates capabilities, because the match would need to differentiate by capability at runtime instead of matching on type alone
+    | let a': Rect => None
+      ^
+    Info:
+    main.pony:5:18: the match type allows for more than one possibility with the same type as pattern type, but different capabilities. match type: (Rect ref | None val)
+        let a: (Rect | None) = None
+                     ^
+    main.pony:7:7: pattern type: Rect ref
+        | let a': Rect => None
+          ^
+    main.pony:7:15: matching (Rect ref | None val) with Rect ref could violate capabilities
+        | let a': Rect => None
+                  ^
+```
+
+Starting with this release, the error message is:
+
+```
+Error:
+main.pony:7:7: this capture cannot match, since the type Rect ref is a struct and lacks a type descriptor
+    | let a': Rect => None
+      ^
+    Info:
+    main.pony:5:18: a struct cannot be part of a union type. match type: (Rect ref | None val)
+        let a: (Rect | None) = None
+                     ^
+    main.pony:7:7: pattern type: Rect ref
+        | let a': Rect => None
+          ^
+    main.pony:7:15: matching (Rect ref | None val) with Rect ref is not possible, since a struct lacks a type descriptor
+        | let a': Rect => None
+                  ^
+```

src/libponyc/expr/match.c

@@ -528,7 +528,7 @@ bool expr_case(pass_opt_t* opt, ast_t* ast)
       break;
     }
 
-    case MATCHTYPE_DENY:
+    case MATCHTYPE_DENY_CAP:
     {
       errorframe_t frame = NULL;
       ast_error_frame(&frame, pattern,
@@ -547,6 +547,24 @@ bool expr_case(pass_opt_t* opt, ast_t* ast)
       ok = false;
       break;
     }
+
+    case MATCHTYPE_DENY_NODESC:
+    {
+      errorframe_t frame = NULL;
+      ast_error_frame(&frame, pattern,
+        "this capture cannot match, since the type %s "
+        "is a struct and lacks a type descriptor",
+        ast_print_type(pattern_type));
+      ast_error_frame(&frame, match_type,
+        "a struct cannot be part of a union type. match type: %s",
+        ast_print_type(match_type));
+      ast_error_frame(&frame, pattern, "pattern type: %s",
+        ast_print_type(pattern_type));
+      errorframe_append(&frame, &info);
+      errorframe_report(&frame, opt->check.errors);
+      ok = false;
+      break;
+    }
   }
 
   if(ast_id(guard) != TK_NONE)

src/libponyc/expr/operator.c

@@ -500,7 +500,8 @@ static bool add_as_type(pass_opt_t* opt, ast_t* ast, ast_t* expr,
 
       ast_t* expr_type = ast_type(expr);
       errorframe_t info = NULL;
-      if(is_matchtype(expr_type, type, &info, opt) == MATCHTYPE_DENY)
+      matchtype_t is_match = is_matchtype(expr_type, type, &info, opt);
+      if(is_match == MATCHTYPE_DENY_CAP)
       {
         errorframe_t frame = NULL;
         ast_error_frame(&frame, ast,
@@ -512,6 +513,21 @@ static bool add_as_type(pass_opt_t* opt, ast_t* ast, ast_t* expr,
         errorframe_append(&frame, &info);
         errorframe_report(&frame, opt->check.errors);
 
+        return false;
+      } else if(is_match == MATCHTYPE_DENY_NODESC){
+        errorframe_t frame = NULL;
+        ast_error_frame(&frame, ast,
+          "matching variable of type %s with %s is not possible, "
+          "since a struct lacks a type descriptor",
+          ast_print_type(expr_type), ast_print_type(type));
+        ast_error_frame(&frame, type,
+          "match type: %s", ast_print_type(type));
+        ast_error_frame(&frame, expr,
+          "a struct cannot be part of a union type. "
+          "pattern type: %s", ast_print_type(expr_type));
+        errorframe_append(&frame, &info);
+        errorframe_report(&frame, opt->check.errors);
+
         return false;
       }