Forbid impossible pattern matching on generic capabilities (ponylang#…

…2499)

This changes the capability rules for pattern matching from

"some possible instantiation of the operand is a subtype of some
possible instantation of the pattern"

to

"every possible instantiation of the operand is a subtype of every
possible instantiation of the pattern".

In practice this makes the compiler issue an error when a pattern can
only ever match for some reification of the generic capability. Code
that uses this kind of pattern matching should now use `iftype`
instead.

This doesn't fix any safety issues.

src/libponyc/type/cap.c

@@ -362,146 +362,6 @@ bool is_cap_sub_cap_bound(token_id sub, token_id subalias, token_id super,
   return false;
 }
 
-bool is_cap_match_cap(token_id operand_cap, token_id operand_eph,
-  token_id pattern_cap, token_id pattern_eph)
-{
-  // Transform the cap based on the aliasing info.
-  cap_aliasing(&operand_cap, &operand_eph);
-  cap_aliasing(&pattern_cap, &pattern_eph);
-
-  if(pattern_eph == TK_EPHEMERAL)
-  {
-    // Operand must be ephemeral.
-    if(operand_eph != TK_EPHEMERAL)
-      return false;
-  }
-
-  if((operand_cap == pattern_cap) || (pattern_cap == TK_TAG))
-    return true;
-
-  // Some instantiaion of the operand refcap must be a subtype of some
-  // instantiation of the pattern refcap.
-  switch(operand_cap)
-  {
-    case TK_ISO:
-    case TK_CAP_SEND:
-    case TK_CAP_ANY:
-      switch(pattern_cap)
-      {
-        case TK_ISO:
-        case TK_TRN:
-        case TK_REF:
-        case TK_VAL:
-        case TK_BOX:
-        case TK_CAP_READ:
-        case TK_CAP_SHARE:
-        case TK_CAP_SEND:
-        case TK_CAP_ALIAS:
-        case TK_CAP_ANY:
-          return true;
-
-        default: {}
-      }
-      break;
-
-    case TK_TRN:
-      switch(pattern_cap)
-      {
-        case TK_REF:
-        case TK_VAL:
-        case TK_BOX:
-        case TK_CAP_READ:
-        case TK_CAP_SHARE:
-        case TK_CAP_SEND:
-        case TK_CAP_ALIAS:
-        case TK_CAP_ANY:
-          return true;
-
-        default: {}
-      }
-      break;
-
-    case TK_REF:
-      switch(pattern_cap)
-      {
-        case TK_BOX:
-        case TK_CAP_READ:
-        case TK_CAP_SHARE:
-        case TK_CAP_SEND:
-        case TK_CAP_ALIAS:
-        case TK_CAP_ANY:
-          return true;
-
-        default: {}
-      }
-      break;
-
-    case TK_VAL:
-    case TK_CAP_SHARE:
-      switch(pattern_cap)
-      {
-        case TK_BOX:
-        case TK_CAP_READ:
-        case TK_CAP_SHARE:
-        case TK_CAP_SEND:
-        case TK_CAP_ALIAS:
-        case TK_CAP_ANY:
-          return true;
-
-        default: {}
-      }
-      break;
-
-    case TK_BOX:
-      switch(pattern_cap)
-      {
-        case TK_CAP_READ:
-        case TK_CAP_SHARE:
-        case TK_CAP_SEND:
-        case TK_CAP_ALIAS:
-        case TK_CAP_ANY:
-          return true;
-
-        default: {}
-      }
-      break;
-
-    case TK_TAG:
-      switch(pattern_cap)
-      {
-        case TK_CAP_SHARE:
-        case TK_CAP_SEND:
-        case TK_CAP_ALIAS:
-        case TK_CAP_ANY:
-          return true;
-
-        default: {}
-      }
-      break;
-
-    case TK_CAP_READ:
-    case TK_CAP_ALIAS:
-      switch(pattern_cap)
-      {
-        case TK_REF:
-        case TK_VAL:
-        case TK_BOX:
-        case TK_CAP_SHARE:
-        case TK_CAP_SEND:
-        case TK_CAP_ALIAS:
-        case TK_CAP_ANY:
-          return true;
-
-        default: {}
-      }
-      break;
-
-    default: {}
-  }
-
-  return false;
-}
-
 bool is_cap_compat_cap(token_id left_cap, token_id left_eph,
   token_id right_cap, token_id right_eph)
 {

src/libponyc/type/cap.h

@@ -29,13 +29,6 @@ bool is_cap_sub_cap_constraint(token_id sub, token_id subalias, token_id super,
 bool is_cap_sub_cap_bound(token_id sub, token_id subalias, token_id super,
   token_id supalias);
 
-/**
- * Some possible instantiation of the operand is a subtype of some possible
- * instantiation of the pattern.
- */
-bool is_cap_match_cap(token_id operand_cap, token_id operand_eph,
-  token_id pattern_cap, token_id pattern_eph);
-
 /**
  * Every possible instantiation of the left side is locally compatible with
  * every possible instantiation of the right side. This relationship is

src/libponyc/type/matchtype.c

@@ -277,7 +277,7 @@ static matchtype_t is_nominal_match_entity(ast_t* operand, ast_t* pattern,
 
   // If the operand does provide the pattern, but the operand refcap can't
   // match the pattern refcap, deny the match.
-  if(!is_cap_match_cap(ast_id(o_cap), ast_id(o_eph), tcap, teph))
+  if(!is_cap_sub_cap(ast_id(o_cap), ast_id(o_eph), tcap, teph))
     return MATCHTYPE_DENY;
 
   // Otherwise, accept the match.
@@ -320,7 +320,7 @@ static matchtype_t is_entity_match_trait(ast_t* operand, ast_t* pattern,
 
   // If the operand does provide the pattern, but the operand refcap can't
   // match the pattern refcap, deny the match.
-  if(!is_cap_match_cap(ast_id(o_cap), ast_id(o_eph), tcap, teph))
+  if(!is_cap_sub_cap(ast_id(o_cap), ast_id(o_eph), tcap, teph))
     return MATCHTYPE_DENY;
 
   // Otherwise, accept the match.
@@ -335,7 +335,7 @@ static matchtype_t is_trait_match_trait(ast_t* operand, ast_t* pattern,
   AST_GET_CHILDREN(pattern, p_pkg, p_id, p_typeargs, p_cap, p_eph);
 
   // If the operand refcap can't match the pattern refcap, deny the match.
-  if(!is_cap_match_cap(ast_id(o_cap), ast_id(o_eph),
+  if(!is_cap_sub_cap(ast_id(o_cap), ast_id(o_eph),
     ast_id(p_cap), ast_id(p_eph)))
     return MATCHTYPE_DENY;