patch 8.2.1176: Vim9: not enough type checking in Vim9 script

Problem:    Vim9: not enough type checking in Vim9 script.
Solution:   Use same type checking as in a :def function.

src/eval.c

@@ -2460,8 +2460,16 @@ eval4(char_u **arg, typval_T *rettv, evalarg_T *evalarg)
 	}
 	if (evalarg != NULL && (evalarg->eval_flags & EVAL_EVALUATE))
 	{
-	    int ret = typval_compare(rettv, &var2, type, ic);
+	    int ret;
 
+	    if (in_vim9script() && check_compare_types(
+						   type, rettv, &var2) == FAIL)
+	    {
+		ret = FAIL;
+		clear_tv(rettv);
+	    }
+	    else
+		ret = typval_compare(rettv, &var2, type, ic);
 	    clear_tv(&var2);
 	    return ret;
 	}

src/proto/vim9compile.pro

@@ -3,6 +3,7 @@ int check_defined(char_u *p, size_t len, cctx_T *cctx);
 void clear_type_list(garray_T *gap);
 type_T *typval2type(typval_T *tv);
 int check_type(type_T *expected, type_T *actual, int give_msg);
+int check_compare_types(exptype_T type, typval_T *tv1, typval_T *tv2);
 char_u *skip_type(char_u *start);
 type_T *parse_type(char_u **arg, garray_T *type_gap);
 char *vartype_name(vartype_T type);

src/testdir/test_vim9_expr.vim

@@ -557,7 +557,7 @@ def RetVoid()
 enddef
 
 def Test_expr4_vimscript()
-  " only checks line continuation
+  " check line continuation
   let lines =<< trim END
       vim9script
       let var = 0
@@ -599,6 +599,25 @@ def Test_expr4_vimscript()
       assert_equal(1, var)
   END
   CheckScriptSuccess(lines)
+
+  " spot check mismatching types
+  lines =<< trim END
+      vim9script
+      echo '' == 0
+  END
+  CheckScriptFailure(lines, 'E1072:')
+
+  lines =<< trim END
+      vim9script
+      echo v:true > v:false
+  END
+  CheckScriptFailure(lines, 'Cannot compare bool with bool')
+
+  lines =<< trim END
+      vim9script
+      echo 123 is 123
+  END
+  CheckScriptFailure(lines, 'Cannot use "is" with number')
 enddef
 
 func Test_expr4_fails()

src/version.c

@@ -754,6 +754,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1176,
 /**/
     1175,
 /**/

src/vim9compile.c

@@ -820,6 +820,14 @@ get_compare_isn(exptype_T exptype, vartype_T type1, vartype_T type2)
     return isntype;
 }
 
+    int
+check_compare_types(exptype_T type, typval_T *tv1, typval_T *tv2)
+{
+    if (get_compare_isn(type, tv1->v_type, tv2->v_type) == ISN_DROP)
+	return FAIL;
+    return OK;
+}
+
 /*
  * Generate an ISN_COMPARE* instruction with a boolean result.
  */
@@ -4296,7 +4304,7 @@ compile_expr4(char_u **arg, cctx_T *cctx, ppconst_T *ppconst)
 	    // Both sides are a constant, compute the result now.
 	    // First check for a valid combination of types, this is more
 	    // strict than typval_compare().
-	    if (get_compare_isn(type, tv1->v_type, tv2->v_type) == ISN_DROP)
+	    if (check_compare_types(type, tv1, tv2) == FAIL)
 		ret = FAIL;
 	    else
 	    {