vim-patch:8.2.1794: no falsy Coalescing operator

Problem:    No falsy Coalescing operator.
Solution:   Add the "??" operator.  Fix mistake with function argument count.

92f26c256e

Cherry-pick tv2bool() into eval/typval.c.
Cherry-pick *??* tag from Vim runtime.

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/eval.c

@@ -2336,6 +2336,7 @@ int eval0(char *arg, typval_T *rettv, exarg_T *eap, evalarg_T *const evalarg)
 
 /// Handle top level expression:
 ///      expr2 ? expr1 : expr1
+///      expr2 ?? expr1
 ///
 /// "arg" must point to the first non-white of the expression.
 /// "arg" is advanced to the next non-white after the recognized expression.
@@ -2352,6 +2353,7 @@ int eval1(char **arg, typval_T *rettv, evalarg_T *const evalarg)
 
   char *p = *arg;
   if (*p == '?') {
+    const bool op_falsy = p[1] == '?';
     evalarg_T *evalarg_used = evalarg;
     evalarg_T local_evalarg;
     if (evalarg == NULL) {
@@ -2365,49 +2367,62 @@ int eval1(char **arg, typval_T *rettv, evalarg_T *const evalarg)
     if (evaluate) {
       bool error = false;
 
-      if (tv_get_number_chk(rettv, &error) != 0) {
+      if (op_falsy) {
+        result = tv2bool(rettv);
+      } else if (tv_get_number_chk(rettv, &error) != 0) {
         result = true;
       }
-      tv_clear(rettv);
+      if (error || !op_falsy || !result) {
+        tv_clear(rettv);
+      }
       if (error) {
         return FAIL;
       }
     }
 
     // Get the second variable.  Recursive!
-    *arg = skipwhite(*arg + 1);
-    evalarg_used->eval_flags = result ? orig_flags : orig_flags & ~EVAL_EVALUATE;
-    if (eval1(arg, rettv, evalarg_used) == FAIL) {
-      evalarg_used->eval_flags = orig_flags;
-      return FAIL;
+    if (op_falsy) {
+      (*arg)++;
     }
-
-    // Check for the ":".
-    p = *arg;
-    if (*p != ':') {
-      emsg(_("E109: Missing ':' after '?'"));
-      if (evaluate && result) {
-        tv_clear(rettv);
-      }
-      evalarg_used->eval_flags = orig_flags;
-      return FAIL;
-    }
-
-    // Get the third variable.  Recursive!
     *arg = skipwhite(*arg + 1);
-    evalarg_used->eval_flags = !result ? orig_flags : orig_flags & ~EVAL_EVALUATE;
+    evalarg_used->eval_flags = (op_falsy ? !result : result)
+                                    ? orig_flags : orig_flags & ~EVAL_EVALUATE;
     typval_T var2;
     if (eval1(arg, &var2, evalarg_used) == FAIL) {
-      if (evaluate && result) {
-        tv_clear(rettv);
-      }
       evalarg_used->eval_flags = orig_flags;
       return FAIL;
     }
-    if (evaluate && !result) {
+    if (!op_falsy || !result) {
       *rettv = var2;
     }
 
+    if (!op_falsy) {
+      // Check for the ":".
+      p = *arg;
+      if (*p != ':') {
+        emsg(_("E109: Missing ':' after '?'"));
+        if (evaluate && result) {
+          tv_clear(rettv);
+        }
+        evalarg_used->eval_flags = orig_flags;
+        return FAIL;
+      }
+
+      // Get the third variable.  Recursive!
+      *arg = skipwhite(*arg + 1);
+      evalarg_used->eval_flags = !result ? orig_flags : orig_flags & ~EVAL_EVALUATE;
+      if (eval1(arg, &var2, evalarg_used) == FAIL) {
+        if (evaluate && result) {
+          tv_clear(rettv);
+        }
+        evalarg_used->eval_flags = orig_flags;
+        return FAIL;
+      }
+      if (evaluate && !result) {
+        *rettv = var2;
+      }
+    }
+
     if (evalarg == NULL) {
       clear_evalarg(&local_evalarg, NULL);
     } else {

src/nvim/eval/typval.c

@@ -4201,3 +4201,34 @@ const char *tv_get_string_buf(const typval_T *const tv, char *const buf)
 
   return res != NULL ? res : "";
 }
+
+/// Return true when "tv" is not falsy: non-zero, non-empty string, non-empty
+/// list, etc.  Mostly like what JavaScript does, except that empty list and
+/// empty dictionary are false.
+bool tv2bool(const typval_T *const tv)
+{
+  switch (tv->v_type) {
+  case VAR_NUMBER:
+    return tv->vval.v_number != 0;
+  case VAR_FLOAT:
+    return tv->vval.v_float != 0.0;
+  case VAR_PARTIAL:
+    return tv->vval.v_partial != NULL;
+  case VAR_FUNC:
+  case VAR_STRING:
+    return tv->vval.v_string != NULL && *tv->vval.v_string != NUL;
+  case VAR_LIST:
+    return tv->vval.v_list != NULL && tv->vval.v_list->lv_len > 0;
+  case VAR_DICT:
+    return tv->vval.v_dict != NULL && tv->vval.v_dict->dv_hashtab.ht_used > 0;
+  case VAR_BOOL:
+    return tv->vval.v_bool == kBoolVarTrue;
+  case VAR_SPECIAL:
+    return tv->vval.v_special == kSpecialVarNull;
+  case VAR_BLOB:
+    return tv->vval.v_blob != NULL && tv->vval.v_blob->bv_ga.ga_len > 0;
+  case VAR_UNKNOWN:
+    break;
+  }
+  return false;
+}