Add iftype conditions (RFC 26) (#1855)

These new conditions allow programmers to get information on subtyping
relationships of type parameters at compile time to conditionally
compile code.

Most of the work is done during the scope pass, where the definition
assigned to a type parameter reference differs based on whether the
reference occurs in the then clause of an iftype condition.

This change implements only a part of RFC 26. Method specialisation is
still to be implemented.

pony.g

@@ -90,6 +90,7 @@ binop
 nextterm
   : 'if' ('\\' ID (',' ID)* '\\')? rawseq 'then' rawseq (elseif | ('else' annotatedrawseq))? 'end'
   | 'ifdef' ('\\' ID (',' ID)* '\\')? infix 'then' rawseq (elseifdef | ('else' annotatedrawseq))? 'end'
+  | 'iftype' ('\\' ID (',' ID)* '\\')? type '<:' type 'then' rawseq (elseiftype | ('else' annotatedrawseq))? 'end'
   | 'match' ('\\' ID (',' ID)* '\\')? rawseq caseexpr* ('else' annotatedrawseq)? 'end'
   | 'while' ('\\' ID (',' ID)* '\\')? rawseq 'do' rawseq ('else' annotatedrawseq)? 'end'
   | 'repeat' ('\\' ID (',' ID)* '\\')? rawseq 'until' annotatedrawseq ('else' annotatedrawseq)? 'end'
@@ -105,6 +106,7 @@ nextterm
 term
   : 'if' ('\\' ID (',' ID)* '\\')? rawseq 'then' rawseq (elseif | ('else' annotatedrawseq))? 'end'
   | 'ifdef' ('\\' ID (',' ID)* '\\')? infix 'then' rawseq (elseifdef | ('else' annotatedrawseq))? 'end'
+  | 'iftype' ('\\' ID (',' ID)* '\\')? type '<:' type 'then' rawseq (elseiftype | ('else' annotatedrawseq))? 'end'
   | 'match' ('\\' ID (',' ID)* '\\')? rawseq caseexpr* ('else' annotatedrawseq)? 'end'
   | 'while' ('\\' ID (',' ID)* '\\')? rawseq 'do' rawseq ('else' annotatedrawseq)? 'end'
   | 'repeat' ('\\' ID (',' ID)* '\\')? rawseq 'until' annotatedrawseq ('else' annotatedrawseq)? 'end'
@@ -125,6 +127,10 @@ caseexpr
   : '|' ('\\' ID (',' ID)* '\\')? pattern? ('if' rawseq)? ('=>' rawseq)?
   ;
 
+elseiftype
+  : 'elseiftype' ('\\' ID (',' ID)* '\\')? type '<:' type 'then' rawseq (elseiftype | ('else' annotatedrawseq))?
+  ;
+
 elseifdef
   : 'elseif' ('\\' ID (',' ID)* '\\')? infix 'then' rawseq (elseifdef | ('else' annotatedrawseq))?
   ;

src/libponyc/ast/frame.c

@@ -112,6 +112,9 @@ bool frame_push(typecheck_t* t, ast_t* ast)
     {
       ast_t* parent = ast_parent(ast);
 
+      if(parent == NULL)
+        return pop;
+
       switch(ast_id(parent))
       {
         case TK_TYPEPARAM:
@@ -259,6 +262,20 @@ bool frame_push(typecheck_t* t, ast_t* ast)
           break;
         }
 
+        case TK_IFTYPE:
+        {
+          AST_GET_CHILDREN(parent, l_type, r_type, body);
+
+          pop = push_frame(t);
+          if(r_type == ast)
+          {
+            t->frame->iftype_constraint = ast;
+          } else if(body == ast) {
+            t->frame->iftype_body = ast;
+          }
+          break;
+        }
+
         default: {}
       }
       break;

src/libponyc/ast/frame.h

@@ -32,6 +32,8 @@ typedef struct typecheck_frame_t
   ast_t* recover;
   ast_t* ifdef_cond;
   ast_t* ifdef_clause;
+  ast_t* iftype_constraint;
+  ast_t* iftype_body;
 
   struct typecheck_frame_t* prev;
 } typecheck_frame_t;

src/libponyc/ast/lexer.c

@@ -80,6 +80,8 @@ static const lextoken_t symbols[] =
 
   { ".>", TK_CHAIN },
 
+  { "<:", TK_SUBTYPE },
+
   { "\\", TK_BACKSLASH },
 
   { "{", TK_LBRACE },
@@ -165,9 +167,11 @@ static const lextoken_t keywords[] =
 
   { "if", TK_IF },
   { "ifdef", TK_IFDEF },
+  { "iftype", TK_IFTYPE },
   { "then", TK_THEN },
   { "else", TK_ELSE },
   { "elseif", TK_ELSEIF },
+  { "elseiftype", TK_ELSEIFTYPE },
   { "end", TK_END },
   { "for", TK_FOR },
   { "in", TK_IN },

src/libponyc/ast/parser.c

@@ -678,6 +678,35 @@ DEF(ifdef);
   REORDER(0, 2, 3, 1);
   DONE();
 
+// ELSEIFTYPE [annotations] type <: type THEN seq [elseiftype | (ELSE seq)]
+DEF(elseiftype);
+  AST_NODE(TK_IFTYPE);
+  SCOPE();
+  SKIP(NULL, TK_ELSEIFTYPE);
+  ANNOTATE(annotations);
+  RULE("type", type);
+  SKIP(NULL, TK_SUBTYPE);
+  RULE("type", type);
+  SKIP(NULL, TK_THEN);
+  RULE("then value", seq);
+  OPT RULE("else clause", elseiftype, elseclause);
+  DONE();
+
+// IFTYPE [annotations] type <: type THEN seq [elseiftype | (ELSE seq)] END
+DEF(iftype);
+  PRINT_INLINE();
+  TOKEN(NULL, TK_IFTYPE);
+  ANNOTATE(annotations);
+  SCOPE();
+  RULE("type", type);
+  SKIP(NULL, TK_SUBTYPE);
+  RULE("type", type);
+  SKIP(NULL, TK_THEN);
+  RULE("then value", seq);
+  OPT RULE("else clause", elseiftype, elseclause);
+  TERMINATE("iftype expression", TK_END);
+  DONE();
+
 // PIPE [annotations] [infix] [WHERE rawseq] [ARROW rawseq]
 DEF(caseexpr);
   AST_NODE(TK_CASE);
@@ -876,18 +905,18 @@ DEF(test_ifdef_flag);
   TOKEN(NULL, TK_ID);
   DONE();
 
-// cond | ifdef | match | whileloop | repeat | forloop | with | try |
+// cond | ifdef | iftype | match | whileloop | repeat | forloop | with | try |
 // recover | consume | pattern | const_expr | test_<various>
 DEF(term);
-  RULE("value", cond, ifdef, match, whileloop, repeat, forloop, with,
+  RULE("value", cond, ifdef, iftype, match, whileloop, repeat, forloop, with,
     try_block, recover, consume, pattern, const_expr, test_noseq,
     test_seq_scope, test_try_block, test_ifdef_flag, test_prefix);
   DONE();
 
-// cond | ifdef | match | whileloop | repeat | forloop | with | try |
+// cond | ifdef | iftype | match | whileloop | repeat | forloop | with | try |
 // recover | consume | pattern | const_expr | test_<various>
 DEF(nextterm);
-  RULE("value", cond, ifdef, match, whileloop, repeat, forloop, with,
+  RULE("value", cond, ifdef, iftype, match, whileloop, repeat, forloop, with,
     try_block, recover, consume, nextpattern, const_expr, test_noseq,
     test_seq_scope, test_try_block, test_ifdef_flag, test_prefix);
   DONE();

src/libponyc/ast/token.h

@@ -82,6 +82,7 @@ typedef enum token_id
   TK_ISECTTYPE,
   TK_EPHEMERAL,
   TK_ALIASED,
+  TK_SUBTYPE,
 
   TK_QUESTION,
   TK_UNARY_MINUS,
@@ -143,9 +144,11 @@ typedef enum token_id
 
   TK_IF,
   TK_IFDEF,
+  TK_IFTYPE,
   TK_THEN,
   TK_ELSE,
   TK_ELSEIF,
+  TK_ELSEIFTYPE,
   TK_END,
   TK_WHILE,
   TK_DO,

src/libponyc/codegen/gencontrol.c

@@ -152,6 +152,16 @@ LLVMValueRef gen_if(compile_t* c, ast_t* ast)
   return GEN_NOTNEEDED;
 }
 
+LLVMValueRef gen_iftype(compile_t* c, ast_t* ast)
+{
+  AST_GET_CHILDREN(ast, subtype, supertype, left, right);
+
+  if(is_subtype_constraint(subtype, supertype, NULL, c->opt))
+    return gen_expr(c, left);
+
+  return gen_expr(c, right);
+}
+
 LLVMValueRef gen_while(compile_t* c, ast_t* ast)
 {
   bool needed = is_result_needed(ast);

src/libponyc/codegen/gencontrol.h

@@ -10,6 +10,8 @@ LLVMValueRef gen_seq(compile_t* c, ast_t* ast);
 
 LLVMValueRef gen_if(compile_t* c, ast_t* ast);
 
+LLVMValueRef gen_iftype(compile_t* c, ast_t* ast);
+
 LLVMValueRef gen_while(compile_t* c, ast_t* ast);
 
 LLVMValueRef gen_repeat(compile_t* c, ast_t* ast);

src/libponyc/codegen/genexpr.c

@@ -63,6 +63,10 @@ LLVMValueRef gen_expr(compile_t* c, ast_t* ast)
       ret = gen_if(c, ast);
       break;
 
+    case TK_IFTYPE:
+      ret = gen_iftype(c, ast);
+      break;
+
     case TK_WHILE:
       ret = gen_while(c, ast);
       break;

src/libponyc/expr/control.c

@@ -10,6 +10,7 @@
 #include "../type/assemble.h"
 #include "../type/subtype.h"
 #include "../type/alias.h"
+#include "../type/typeparam.h"
 #include "ponyassert.h"
 
 bool expr_seq(pass_opt_t* opt, ast_t* ast)
@@ -133,6 +134,44 @@ bool expr_if(pass_opt_t* opt, ast_t* ast)
   return true;
 }
 
+bool expr_iftype(pass_opt_t* opt, ast_t* ast)
+{
+  pony_assert(ast_id(ast) == TK_IFTYPE);
+  AST_GET_CHILDREN(ast, sub, super, left, right);
+
+  ast_t* type = NULL;
+
+  if(!ast_checkflag(left, AST_FLAG_JUMPS_AWAY))
+  {
+    if(is_typecheck_error(ast_type(left)))
+      return false;
+
+    type = control_type_add_branch(opt, type, left);
+  }
+
+  if(!ast_checkflag(right, AST_FLAG_JUMPS_AWAY))
+  {
+    if(is_typecheck_error(ast_type(right)))
+      return false;
+
+    type = control_type_add_branch(opt, type, right);
+  }
+
+  if(type == NULL)
+  {
+    if((ast_id(ast_parent(ast)) == TK_SEQ) && ast_sibling(ast) != NULL)
+    {
+      ast_error(opt->check.errors, ast_sibling(ast), "unreachable code");
+      return false;
+    }
+  }
+
+  ast_settype(ast, type);
+  literal_unify_control(ast, opt);
+
+  return true;
+}
+
 bool expr_while(pass_opt_t* opt, ast_t* ast)
 {
   pony_assert(ast_id(ast) == TK_WHILE);

src/libponyc/expr/control.h

@@ -9,6 +9,7 @@ PONY_EXTERN_C_BEGIN
 
 bool expr_seq(pass_opt_t* opt, ast_t* ast);
 bool expr_if(pass_opt_t* opt, ast_t* ast);
+bool expr_iftype(pass_opt_t* opt, ast_t* ast);
 bool expr_while(pass_opt_t* opt, ast_t* ast);
 bool expr_repeat(pass_opt_t* opt, ast_t* ast);
 bool expr_try(pass_opt_t* opt, ast_t* ast);

src/libponyc/expr/reference.c

@@ -13,6 +13,7 @@
 #include "../type/cap.h"
 #include "../type/reify.h"
 #include "../type/lookup.h"
+#include "../type/typeparam.h"
 #include "../ast/astbuild.h"
 #include "ponyassert.h"
 
@@ -127,6 +128,8 @@ bool expr_fieldref(pass_opt_t* opt, ast_t* ast, ast_t* find, token_id tid)
 
   AST_GET_CHILDREN(find, id, f_type, init);
 
+  f_type = typeparam_current(opt, f_type, ast);
+
   // Viewpoint adapted type of the field.
   ast_t* type = viewpoint_type(l_type, f_type);
 
@@ -368,6 +371,8 @@ bool expr_localref(pass_opt_t* opt, ast_t* ast)
   if(is_typecheck_error(type))
     return false;
 
+  type = typeparam_current(opt, type, ast);
+
   if(!sendable(type))
   {
     if(opt->check.frame->recover != NULL)
@@ -425,6 +430,8 @@ bool expr_paramref(pass_opt_t* opt, ast_t* ast)
   if(is_typecheck_error(type))
     return false;
 
+  type = typeparam_current(opt, type, ast);
+
   if(!sendable(type) && (opt->check.frame->recover != NULL))
   {
     ast_t* parent = ast_parent(ast);

src/libponyc/pass/expr.c

@@ -56,6 +56,13 @@ bool is_result_needed(ast_t* ast)
 
       return is_result_needed(parent);
 
+    case TK_IFTYPE:
+      // Sub/supertype not needed, body/else needed only if parent needed.
+      if((ast_child(parent) == ast) || (ast_childidx(parent, 1) == ast))
+        return false;
+
+      return is_result_needed(parent);
+
     case TK_REPEAT:
       // Cond needed, body/else needed only if parent needed.
       if(ast_childidx(parent, 1) == ast)
@@ -128,6 +135,12 @@ bool is_method_result(typecheck_t* t, ast_t* ast)
         return false;
       break;
 
+    case TK_IFTYPE:
+      // The subtype and the supertype are not the result.
+      if((ast_child(parent) == ast) || (ast_childidx(parent, 1) == ast))
+        return false;
+      break;
+
     case TK_REPEAT:
       // The condition is not the result.
       if(ast_childidx(parent, 1) == ast)
@@ -241,6 +254,7 @@ ast_result_t pass_expr(ast_t** astp, pass_opt_t* options)
     case TK_CALL:       r = expr_call(options, astp); break;
     case TK_IFDEF:
     case TK_IF:         r = expr_if(options, ast); break;
+    case TK_IFTYPE:     r = expr_iftype(options, ast); break;
     case TK_WHILE:      r = expr_while(options, ast); break;
     case TK_REPEAT:     r = expr_repeat(options, ast); break;
     case TK_TRY_NO_CHECK:

src/libponyc/pass/flatten.c

@@ -56,6 +56,7 @@ static ast_result_t flatten_isect(pass_opt_t* opt, ast_t* ast)
   AST_EXTRACT_CHILDREN(ast, left, right);
 
   if((opt->check.frame->constraint == NULL) &&
+    (opt->check.frame->iftype_constraint == NULL) &&
     (opt->check.frame->provides == NULL) &&
     !is_compat_type(left, right))
   {

src/libponyc/pass/names.c

@@ -190,13 +190,18 @@ static bool names_typeparam(pass_opt_t* opt, ast_t** astp, ast_t* def)
     }
   }
 
+  const char* name = ast_name(id);
+
   // Change to a typeparamref.
   REPLACE(astp,
     NODE(TK_TYPEPARAMREF,
       TREE(id)
       TREE(cap)
       TREE(ephemeral)));
 
+  if(opt->check.frame->iftype_body != NULL)
+    def = ast_get(ast, name, NULL);
+
   ast_setdata(*astp, def);
   return true;
 }
@@ -210,7 +215,8 @@ static bool names_type(pass_opt_t* opt, ast_t** astp, ast_t* def)
 
   if(tcap == TK_NONE)
   {
-    if(opt->check.frame->constraint != NULL)
+    if((opt->check.frame->constraint != NULL) ||
+      (opt->check.frame->iftype_constraint != NULL))
     {
       // A primitive constraint is a val, otherwise #any.
       if(ast_id(def) == TK_PRIMITIVE)