Don't duplicate match checks for inherited trait bodies

Due to how default method bodies are implemented, we only want to
do match checks within the context of the trait, not in the context
of the class that has inherited the method.

Closes #4612

Author: SeanTAllen

.release-notes/4628.md

@@ -0,0 +1,28 @@
+## Don't duplicate match checks for inherited trait bodies
+
+Due to how we were doing match checks, some usages of match in default method bodies of traits and interfaces could end up failing checking once they were "inherited" by the implementing class.
+
+For example, the following failed to compile:
+
+```pony
+primitive Prim
+  fun ignore() => None
+
+trait A
+  fun union(): (Prim | None)
+
+  fun match_it(): Bool =>
+    match union()
+    | let p: Prim =>
+      p.ignore()
+      true
+    | None =>
+      false
+    end
+
+class B is A
+  fun union(): Prim =>
+    Prim
+```
+
+We've updated to only do the match checks checking the trait's default method bodies.

src/libponyc/expr/match.c

@@ -187,48 +187,62 @@ bool expr_match(pass_opt_t* opt, ast_t* ast)
     type = control_type_add_branch(opt, type, cases);
   }
 
-  // analyze exhaustiveness
-  ast_t* exhaustive_at = is_match_exhaustive(opt, expr_type, cases);
-
-  if(exhaustive_at == NULL)
+  // If the method definition containing the match site had its body inherited
+  // from a trait, we don't want to check exhaustiveness here -
+  // it should only be checked in the context of the original trait.
+  bool skip_exhaustive = false;
+  ast_t* body_donor = (ast_t*)ast_data(opt->check.frame->method);
+  if ((body_donor != NULL) && (ast_id(body_donor) == TK_TRAIT)
+    && (ast_id(opt->check.frame->type) != TK_TRAIT))
   {
-    // match might not be exhaustive
-    if ((ast_id(else_clause) == TK_NONE))
-    {
-      // If we have no else clause, and the match is not found to be exhaustive,
-      // we must generate an implicit else clause that returns None as the value.
-      ast_scope(else_clause);
-      ast_setid(else_clause, TK_SEQ);
-
-      BUILD(ref, else_clause,
-        NODE(TK_TYPEREF,
-          NONE
-          ID("None")
-          NONE));
-      ast_add(else_clause, ref);
-
-      if(!expr_typeref(opt, &ref) || !expr_seq(opt, else_clause))
-        return false;
-    }
+    skip_exhaustive = true;
   }
-  else
+
+  if(!skip_exhaustive)
   {
-    // match is exhaustive
-    if(ast_sibling(exhaustive_at) != NULL)
+    // analyze exhaustiveness
+    ast_t* exhaustive_at = is_match_exhaustive(opt, expr_type, cases);
+
+    if(exhaustive_at == NULL)
     {
-      // we have unreachable cases
-      ast_error(opt->check.errors, ast, "match contains unreachable cases");
-      ast_error_continue(opt->check.errors, ast_sibling(exhaustive_at),
-        "first unreachable case expression");
-      return false;
+      // match might not be exhaustive
+      if ((ast_id(else_clause) == TK_NONE))
+      {
+        // If we have no else clause, and the match is not found to be exhaustive,
+        // we must generate an implicit else clause that returns None as the value.
+        ast_scope(else_clause);
+        ast_setid(else_clause, TK_SEQ);
+
+        BUILD(ref, else_clause,
+          NODE(TK_TYPEREF,
+            NONE
+            ID("None")
+            NONE));
+        ast_add(else_clause, ref);
+
+        if(!expr_typeref(opt, &ref) || !expr_seq(opt, else_clause))
+          return false;
+      }
     }
-    else if((ast_id(else_clause) != TK_NONE))
+    else
     {
-      ast_error(opt->check.errors, ast,
-        "match is exhaustive, the else clause is unreachable");
-      ast_error_continue(opt->check.errors, else_clause,
-        "unreachable code");
-      return false;
+      // match is exhaustive
+      if(ast_sibling(exhaustive_at) != NULL)
+      {
+        // we have unreachable cases
+        ast_error(opt->check.errors, ast, "match contains unreachable cases");
+        ast_error_continue(opt->check.errors, ast_sibling(exhaustive_at),
+          "first unreachable case expression");
+        return false;
+      }
+      else if((ast_id(else_clause) != TK_NONE))
+      {
+        ast_error(opt->check.errors, ast,
+          "match is exhaustive, the else clause is unreachable");
+        ast_error_continue(opt->check.errors, else_clause,
+          "unreachable code");
+        return false;
+      }
     }
   }
 
@@ -504,81 +518,96 @@ bool expr_case(pass_opt_t* opt, ast_t* ast)
 
   ast_settype(ast, pattern_type);
 
+   // If the method definition containing the match site had its body inherited
+  // from a trait, we don't want to check it here -
+  // it should only be checked in the context of the original trait.
+  bool skip = false;
+  ast_t* body_donor = (ast_t*)ast_data(opt->check.frame->method);
+  if ((body_donor != NULL) && (ast_id(body_donor) == TK_TRAIT)
+    && (ast_id(opt->check.frame->type) != TK_TRAIT))
+  {
+    skip = true;
+  }
+
   bool ok = true;
-  errorframe_t info = NULL;
 
-  switch(is_matchtype_with_consumed_pattern(match_type, pattern_type, &info, opt))
+  if (!skip)
   {
-    case MATCHTYPE_ACCEPT:
-      break;
+    errorframe_t info = NULL;
 
-    case MATCHTYPE_REJECT:
+    switch(is_matchtype_with_consumed_pattern(match_type, pattern_type, &info, opt))
     {
-      errorframe_t frame = NULL;
-      ast_error_frame(&frame, pattern, "this pattern can never match");
-      ast_error_frame(&frame, match_type, "match type: %s",
-        ast_print_type(match_type));
-      // TODO report unaliased type when body is consume !
-      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;
-    }
+      case MATCHTYPE_ACCEPT:
+        break;
 
-    case MATCHTYPE_DENY_CAP:
-    {
-      errorframe_t frame = NULL;
-      ast_error_frame(&frame, pattern,
-        "this capture violates capabilities, because the match would "
-        "need to differentiate by capability at runtime instead of matching "
-        "on type alone");
-      ast_error_frame(&frame, match_type, "the match type allows for more than "
-        "one possibility with the same type as pattern type, but different "
-        "capabilities. 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);
-      ast_error_frame(&frame, match_expr,
-        "the match expression with the inadequate capability is here");
-      errorframe_report(&frame, opt->check.errors);
-
-      ok = false;
-      break;
-    }
+      case MATCHTYPE_REJECT:
+      {
+        errorframe_t frame = NULL;
+        ast_error_frame(&frame, pattern, "this pattern can never match");
+        ast_error_frame(&frame, match_type, "match type: %s",
+          ast_print_type(match_type));
+        // TODO report unaliased type when body is consume !
+        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;
+      }
 
-    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;
-    }
-  }
+      case MATCHTYPE_DENY_CAP:
+      {
+        errorframe_t frame = NULL;
+        ast_error_frame(&frame, pattern,
+          "this capture violates capabilities, because the match would "
+          "need to differentiate by capability at runtime instead of matching "
+          "on type alone");
+        ast_error_frame(&frame, match_type, "the match type allows for more than "
+          "one possibility with the same type as pattern type, but different "
+          "capabilities. 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);
+        ast_error_frame(&frame, match_expr,
+          "the match expression with the inadequate capability is here");
+        errorframe_report(&frame, opt->check.errors);
+
+        ok = false;
+        break;
+      }
 
-  if(ast_id(guard) != TK_NONE)
-  {
-    ast_t* guard_type = ast_type(guard);
+      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(is_typecheck_error(guard_type))
+    if(ast_id(guard) != TK_NONE)
     {
-      ok = false;
-    } else if(!is_bool(guard_type)) {
-      ast_error(opt->check.errors, guard,
-        "guard must be a boolean expression");
+      ast_t* guard_type = ast_type(guard);
+
+      if(is_typecheck_error(guard_type))
+      {
+        ok = false;
+      } else if(!is_bool(guard_type)) {
+        ast_error(opt->check.errors, guard,
+          "guard must be a boolean expression");
+      }
     }
   }
   return ok;

test/full-program-tests/regression-4612/main.pony

@@ -0,0 +1,26 @@
+"""
+This won't compile if there is a regression for issue #4612.
+"""
+
+actor Main
+  new create(env: Env) =>
+    env.out.print("streets ahead")
+
+primitive Prim
+  fun ignore() => None
+
+trait A
+  fun union(): (Prim | None)
+
+  fun match_it(): Bool =>
+    match union()
+    | let p: Prim =>
+      p.ignore()
+      true
+    | None =>
+      false
+    end
+
+class B is A
+  fun union(): Prim =>
+    Prim