patch 9.0.1741: No type checking in interfaces

Problem: No type checking in interfaces
Solution: Implement member type check in vim9 interfaces

Most of the code is a small refactoring to allow the use of a where_T
for signaling the type mismatch, the type checking itself is pretty
simple.

Improve where_T error reports

Let the caller explicitly define the kind of location it's referring to
and free the WT_ARGUMENT enum from its catch-all role.

Implement type checking for interface methods

Follows closely the logic used for type-checking the members.

closes: #12844

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: LemonBoy <thatlemon@gmail.com>
diff --git a/src/vim9execute.c b/src/vim9execute.c
index eb663a1..6c20878 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -5272,7 +5272,7 @@
 		    // Useful when used in unpack assignment.  Reset at
 		    // ISN_DROP.
 		    ectx->ec_where.wt_index = gi->gi_index + 1;
-		    ectx->ec_where.wt_variable = TRUE;
+		    ectx->ec_where.wt_kind = WT_VARIABLE;
 		}
 		break;
 
@@ -5442,20 +5442,20 @@
 	    case ISN_CHECKTYPE:
 		{
 		    checktype_T *ct = &iptr->isn_arg.type;
-		    int		save_wt_variable = ectx->ec_where.wt_variable;
 		    int		r;
+		    where_T	where = WHERE_INIT;
 
 		    tv = STACK_TV_BOT((int)ct->ct_off);
 		    SOURCING_LNUM = iptr->isn_lnum;
-		    if (!ectx->ec_where.wt_variable)
-			ectx->ec_where.wt_index = ct->ct_arg_idx;
-		    ectx->ec_where.wt_variable = ct->ct_is_var;
-		    r = check_typval_type(ct->ct_type, tv, ectx->ec_where);
-		    ectx->ec_where.wt_variable = save_wt_variable;
+		    if (ct->ct_arg_idx > 0)
+		    {
+			where.wt_index = ct->ct_arg_idx;
+			where.wt_kind = ct->ct_is_var ? WT_VARIABLE : WT_ARGUMENT;
+			where.wt_func_name = ectx->ec_where.wt_func_name;
+		    }
+		    r = check_typval_type(ct->ct_type, tv, where);
 		    if (r == FAIL)
 			goto on_error;
-		    if (!ectx->ec_where.wt_variable)
-			ectx->ec_where.wt_index = 0;
 
 		    // number 0 is FALSE, number 1 is TRUE
 		    if (tv->v_type == VAR_NUMBER
@@ -5736,8 +5736,7 @@
 	    case ISN_DROP:
 		--ectx->ec_stack.ga_len;
 		clear_tv(STACK_TV_BOT(0));
-		ectx->ec_where.wt_index = 0;
-		ectx->ec_where.wt_variable = FALSE;
+		ectx->ec_where = (where_T)WHERE_INIT;
 		break;
 	}
 	continue;
@@ -6170,8 +6169,7 @@
     emsg_silent_def = emsg_silent;
     did_emsg_def = 0;
 
-    ectx.ec_where.wt_index = 0;
-    ectx.ec_where.wt_variable = FALSE;
+    ectx.ec_where = (where_T)WHERE_INIT;
 
     /*
      * Execute the instructions until done.