diff --git a/src/evalfunc.c b/src/evalfunc.c
index d8c621d..39b94de 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -228,7 +228,9 @@
 arg_float_or_nr(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
-		  || type->tt_type == VAR_FLOAT || type->tt_type == VAR_NUMBER)
+	    || type->tt_type == VAR_UNKNOWN
+	    || type->tt_type == VAR_FLOAT
+	    || type->tt_type == VAR_NUMBER)
 	return OK;
     arg_type_mismatch(&t_number, type, context->arg_idx + 1);
     return FAIL;
@@ -313,7 +315,9 @@
 arg_list_or_blob(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
-		     || type->tt_type == VAR_LIST || type->tt_type == VAR_BLOB)
+	    || type->tt_type == VAR_UNKNOWN
+	    || type->tt_type == VAR_LIST
+	    || type->tt_type == VAR_BLOB)
 	return OK;
     arg_type_mismatch(&t_list_any, type, context->arg_idx + 1);
     return FAIL;
@@ -326,7 +330,9 @@
 arg_string_or_nr(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
-	    || type->tt_type == VAR_STRING || type->tt_type == VAR_NUMBER)
+	    || type->tt_type == VAR_UNKNOWN
+	    || type->tt_type == VAR_STRING
+	    || type->tt_type == VAR_NUMBER)
 	return OK;
     arg_type_mismatch(&t_string, type, context->arg_idx + 1);
     return FAIL;
@@ -339,7 +345,9 @@
 arg_buffer(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
-	    || type->tt_type == VAR_STRING || type->tt_type == VAR_NUMBER)
+	    || type->tt_type == VAR_UNKNOWN
+	    || type->tt_type == VAR_STRING
+	    || type->tt_type == VAR_NUMBER)
 	return OK;
     arg_type_mismatch(&t_string, type, context->arg_idx + 1);
     return FAIL;
@@ -352,6 +360,7 @@
 arg_buffer_or_dict_any(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
+	    || type->tt_type == VAR_UNKNOWN
 	    || type->tt_type == VAR_STRING
 	    || type->tt_type == VAR_NUMBER
 	    || type->tt_type == VAR_DICT)
@@ -367,7 +376,9 @@
 arg_lnum(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
-	    || type->tt_type == VAR_STRING || type->tt_type == VAR_NUMBER)
+	    || type->tt_type == VAR_UNKNOWN
+	    || type->tt_type == VAR_STRING
+	    || type->tt_type == VAR_NUMBER)
 	return OK;
     arg_type_mismatch(&t_string, type, context->arg_idx + 1);
     return FAIL;
@@ -379,7 +390,9 @@
     static int
 arg_string_or_list_string(type_T *type, argcontext_T *context)
 {
-    if (type->tt_type == VAR_ANY || type->tt_type == VAR_STRING)
+    if (type->tt_type == VAR_ANY
+	    || type->tt_type == VAR_UNKNOWN
+	    || type->tt_type == VAR_STRING)
 	return OK;
     if (type->tt_type != VAR_LIST)
     {
@@ -401,7 +414,9 @@
 arg_string_or_list_any(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
-	    || type->tt_type == VAR_STRING || type->tt_type == VAR_LIST)
+	    || type->tt_type == VAR_UNKNOWN
+	    || type->tt_type == VAR_STRING
+	    || type->tt_type == VAR_LIST)
 	return OK;
     arg_type_mismatch(&t_string, type, context->arg_idx + 1);
     return FAIL;
@@ -414,7 +429,9 @@
 arg_string_or_blob(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
-	    || type->tt_type == VAR_STRING || type->tt_type == VAR_BLOB)
+	    || type->tt_type == VAR_UNKNOWN
+	    || type->tt_type == VAR_STRING
+	    || type->tt_type == VAR_BLOB)
 	return OK;
     arg_type_mismatch(&t_string, type, context->arg_idx + 1);
     return FAIL;
@@ -427,7 +444,9 @@
 arg_list_or_dict(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
-		     || type->tt_type == VAR_LIST || type->tt_type == VAR_DICT)
+	    || type->tt_type == VAR_UNKNOWN
+	    || type->tt_type == VAR_LIST
+	    || type->tt_type == VAR_DICT)
 	return OK;
     arg_type_mismatch(&t_list_any, type, context->arg_idx + 1);
     return FAIL;
@@ -440,9 +459,10 @@
 arg_list_or_dict_or_blob(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
-		     || type->tt_type == VAR_LIST
-		     || type->tt_type == VAR_DICT
-		     || type->tt_type == VAR_BLOB)
+	    || type->tt_type == VAR_UNKNOWN
+	    || type->tt_type == VAR_LIST
+	    || type->tt_type == VAR_DICT
+	    || type->tt_type == VAR_BLOB)
 	return OK;
     arg_type_mismatch(&t_list_any, type, context->arg_idx + 1);
     return FAIL;
@@ -455,10 +475,11 @@
 arg_list_or_dict_or_blob_or_string(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
-		     || type->tt_type == VAR_LIST
-		     || type->tt_type == VAR_DICT
-		     || type->tt_type == VAR_BLOB
-		     || type->tt_type == VAR_STRING)
+	    || type->tt_type == VAR_UNKNOWN
+	    || type->tt_type == VAR_LIST
+	    || type->tt_type == VAR_DICT
+	    || type->tt_type == VAR_BLOB
+	    || type->tt_type == VAR_STRING)
 	return OK;
     arg_type_mismatch(&t_list_any, type, context->arg_idx + 1);
     return FAIL;
@@ -471,9 +492,10 @@
 arg_string_list_or_blob(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
-		     || type->tt_type == VAR_LIST
-		     || type->tt_type == VAR_BLOB
-		     || type->tt_type == VAR_STRING)
+	    || type->tt_type == VAR_UNKNOWN
+	    || type->tt_type == VAR_LIST
+	    || type->tt_type == VAR_BLOB
+	    || type->tt_type == VAR_STRING)
 	return OK;
     arg_type_mismatch(&t_list_any, type, context->arg_idx + 1);
     return FAIL;
@@ -495,6 +517,7 @@
 arg_chan_or_job(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
+	    || type->tt_type == VAR_UNKNOWN
 	    || type->tt_type == VAR_CHANNEL
 	    || type->tt_type == VAR_JOB)
 	return OK;
@@ -557,9 +580,10 @@
 arg_str_or_nr_or_list(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
-		|| type->tt_type == VAR_STRING
-		|| type->tt_type == VAR_NUMBER
-		|| type->tt_type == VAR_LIST)
+	    || type->tt_type == VAR_UNKNOWN
+	    || type->tt_type == VAR_STRING
+	    || type->tt_type == VAR_NUMBER
+	    || type->tt_type == VAR_LIST)
 	return OK;
     arg_type_mismatch(&t_string, type, context->arg_idx + 1);
     return FAIL;
@@ -572,8 +596,9 @@
 arg_dict_any_or_string(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
-		|| type->tt_type == VAR_DICT
-		|| type->tt_type == VAR_STRING)
+	    || type->tt_type == VAR_UNKNOWN
+	    || type->tt_type == VAR_DICT
+	    || type->tt_type == VAR_STRING)
 	return OK;
     arg_type_mismatch(&t_string, type, context->arg_idx + 1);
     return FAIL;
@@ -603,6 +628,7 @@
 arg_get1(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
+	    || type->tt_type == VAR_UNKNOWN
 	    || type->tt_type == VAR_BLOB
 	    || type->tt_type == VAR_LIST
 	    || type->tt_type == VAR_DICT
@@ -622,6 +648,7 @@
 arg_len1(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
+	    || type->tt_type == VAR_UNKNOWN
 	    || type->tt_type == VAR_STRING
 	    || type->tt_type == VAR_NUMBER
 	    || type->tt_type == VAR_BLOB
@@ -657,6 +684,7 @@
 arg_repeat1(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
+	    || type->tt_type == VAR_UNKNOWN
 	    || type->tt_type == VAR_STRING
 	    || type->tt_type == VAR_NUMBER
 	    || type->tt_type == VAR_LIST)
@@ -674,6 +702,7 @@
 arg_slice1(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
+	    || type->tt_type == VAR_UNKNOWN
 	    || type->tt_type == VAR_LIST
 	    || type->tt_type == VAR_BLOB
 	    || type->tt_type == VAR_STRING)
@@ -691,6 +720,7 @@
 arg_count1(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
+	    || type->tt_type == VAR_UNKNOWN
 	    || type->tt_type == VAR_STRING
 	    || type->tt_type == VAR_LIST
 	    || type->tt_type == VAR_DICT)
@@ -708,6 +738,7 @@
 arg_cursor1(type_T *type, argcontext_T *context)
 {
     if (type->tt_type == VAR_ANY
+	    || type->tt_type == VAR_UNKNOWN
 	    || type->tt_type == VAR_NUMBER
 	    || type->tt_type == VAR_STRING
 	    || type->tt_type == VAR_LIST)
diff --git a/src/globals.h b/src/globals.h
index c69306a..45561de 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -404,9 +404,16 @@
 
 
 // Commonly used types.
+// "unknown" is used for when the type is really unknown, e.g. global
+// variables.  Also for when a function may or may not return something.
 EXTERN type_T t_unknown INIT6(VAR_UNKNOWN, 0, 0, TTFLAG_STATIC, NULL, NULL);
+
+// "any" is used for when the type is mixed.  Excludes "void".
 EXTERN type_T t_any INIT6(VAR_ANY, 0, 0, TTFLAG_STATIC, NULL, NULL);
+
+// "void" is used for a function not returning anything.
 EXTERN type_T t_void INIT6(VAR_VOID, 0, 0, TTFLAG_STATIC, NULL, NULL);
+
 EXTERN type_T t_bool INIT6(VAR_BOOL, 0, 0, TTFLAG_STATIC, NULL, NULL);
 EXTERN type_T t_special INIT6(VAR_SPECIAL, 0, 0, TTFLAG_STATIC, NULL, NULL);
 EXTERN type_T t_number INIT6(VAR_NUMBER, 0, 0, TTFLAG_STATIC, NULL, NULL);
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index d17d010..81fa7e4 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -547,6 +547,39 @@
       defcompile
   END
   CheckScriptFailure(lines, 'E1001: Variable not found: b')
+
+  # using script variable requires matching type or type cast
+  lines =<< trim END
+      vim9script
+      var a: any
+      def Func(arg: string = a)
+        echo arg
+      enddef
+      defcompile
+  END
+  CheckScriptFailure(lines, 'E1013: Argument 1: type mismatch, expected string but got any')
+
+  lines =<< trim END
+      vim9script
+      var a: any
+      def Func(arg: string = <string>a)
+        echo arg
+      enddef
+      a = 'works'
+      Func()
+  END
+  CheckScriptSuccess(lines)
+
+  # using global variable does not require type cast
+  lines =<< trim END
+      vim9script
+      def Func(arg: string = g:str)
+        echo arg
+      enddef
+      g:str = 'works'
+      Func()
+  END
+  CheckScriptSuccess(lines)
 enddef
 
 def FuncWithComment(  # comment
diff --git a/src/version.c b/src/version.c
index 90b35d3..2c46608 100644
--- a/src/version.c
+++ b/src/version.c
@@ -750,6 +750,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3866,
+/**/
     3865,
 /**/
     3864,
diff --git a/src/vim9cmds.c b/src/vim9cmds.c
index 6d8935a..5c7a938 100644
--- a/src/vim9cmds.c
+++ b/src/vim9cmds.c
@@ -843,8 +843,11 @@
 	// If we know the type of "var" and it is a not a supported type we can
 	// give an error now.
 	vartype = ((type_T **)stack->ga_data)[stack->ga_len - 1];
-	if (vartype->tt_type != VAR_LIST && vartype->tt_type != VAR_STRING
-		&& vartype->tt_type != VAR_BLOB && vartype->tt_type != VAR_ANY)
+	if (vartype->tt_type != VAR_LIST
+		&& vartype->tt_type != VAR_STRING
+		&& vartype->tt_type != VAR_BLOB
+		&& vartype->tt_type != VAR_ANY
+		&& vartype->tt_type != VAR_UNKNOWN)
 	{
 	    semsg(_(e_for_loop_on_str_not_supported),
 					       vartype_name(vartype->tt_type));
diff --git a/src/vim9expr.c b/src/vim9expr.c
index 66d0fc4..6fe8e2a 100644
--- a/src/vim9expr.c
+++ b/src/vim9expr.c
@@ -90,7 +90,7 @@
     vartype = (*typep)->tt_type;
     idxtype = ((type_T **)stack->ga_data)[stack->ga_len - 1];
     // If the index is a string, the variable must be a Dict.
-    if (*typep == &t_any && idxtype == &t_string)
+    if ((*typep == &t_any || *typep == &t_unknown) && idxtype == &t_string)
 	vartype = VAR_DICT;
     if (vartype == VAR_STRING || vartype == VAR_LIST || vartype == VAR_BLOB)
     {
@@ -156,7 +156,7 @@
 		return FAIL;
 	}
     }
-    else if (vartype == VAR_LIST || *typep == &t_any)
+    else if (vartype == VAR_LIST || *typep == &t_any || *typep == &t_unknown)
     {
 	if (is_slice)
 	{
@@ -415,7 +415,7 @@
 		// Global, Buffer-local, Window-local and Tabpage-local
 		// variables can be defined later, thus we don't check if it
 		// exists, give an error at runtime.
-		res = generate_LOAD(cctx, isn_type, 0, name, &t_any);
+		res = generate_LOAD(cctx, isn_type, 0, name, &t_unknown);
 	    }
 	}
     }
@@ -1428,7 +1428,10 @@
     if (type == &t_bool)
 	return OK;
 
-    if (type == &t_any || type == &t_number || type == &t_number_bool)
+    if (type == &t_any
+	    || type == &t_unknown
+	    || type == &t_number
+	    || type == &t_number_bool)
 	// Number 0 and 1 are OK to use as a bool.  "any" could also be a bool.
 	// This requires a runtime type check.
 	return generate_COND2BOOL(cctx);
@@ -2155,9 +2158,10 @@
 
 	generate_ppconst(cctx, ppconst);
 	actual = ((type_T **)stack->ga_data)[stack->ga_len - 1];
-	if (check_type(want_type, actual, FALSE, where) == FAIL)
+	if (check_type_maybe(want_type, actual, FALSE, where) != OK)
 	{
-	    if (need_type(actual, want_type, -1, 0, cctx, FALSE, FALSE) == FAIL)
+	    if (need_type(actual, want_type, -1, 0, cctx, FALSE, FALSE)
+								       == FAIL)
 		return FAIL;
 	}
     }
diff --git a/src/vim9instr.c b/src/vim9instr.c
index 48c90dc..06d6b90 100644
--- a/src/vim9instr.c
+++ b/src/vim9instr.c
@@ -168,9 +168,10 @@
     static int
 check_number_or_float(vartype_T type1, vartype_T type2, char_u *op)
 {
-    if (!((type1 == VAR_NUMBER || type1 == VAR_FLOAT || type1 == VAR_ANY)
+    if (!((type1 == VAR_NUMBER || type1 == VAR_FLOAT
+				   || type1 == VAR_ANY || type1 == VAR_UNKNOWN)
 	    && (type2 == VAR_NUMBER || type2 == VAR_FLOAT
-							 || type2 == VAR_ANY)))
+				 || type2 == VAR_ANY || type2 == VAR_UNKNOWN)))
     {
 	if (*op == '+')
 	    emsg(_(e_wrong_argument_type_for_plus));
@@ -204,7 +205,9 @@
 
     if (vartype != VAR_LIST && vartype != VAR_BLOB
 	    && type1->tt_type != VAR_ANY
+	    && type1->tt_type != VAR_UNKNOWN
 	    && type2->tt_type != VAR_ANY
+	    && type2->tt_type != VAR_UNKNOWN
 	    && check_number_or_float(
 			type1->tt_type, type2->tt_type, (char_u *)"+") == FAIL)
 	return FAIL;
@@ -293,8 +296,10 @@
 		  break;
 
 	case '%': if ((type1->tt_type != VAR_ANY
+			      && type1->tt_type != VAR_UNKNOWN
 					       && type1->tt_type != VAR_NUMBER)
 			  || (type2->tt_type != VAR_ANY
+			      && type2->tt_type != VAR_UNKNOWN
 					      && type2->tt_type != VAR_NUMBER))
 		  {
 		      emsg(_(e_percent_requires_number_arguments));
@@ -1528,7 +1533,7 @@
 
     RETURN_OK_IF_SKIP(cctx);
 
-    if (type->tt_type == VAR_ANY)
+    if (type->tt_type == VAR_ANY || type->tt_type == VAR_UNKNOWN)
 	ret_type = &t_any;
     else if (type->tt_type == VAR_FUNC || type->tt_type == VAR_PARTIAL)
     {
@@ -1620,7 +1625,7 @@
 
     // check for dict type
     type = ((type_T **)stack->ga_data)[stack->ga_len - 1];
-    if (type->tt_type != VAR_DICT && type != &t_any)
+    if (type->tt_type != VAR_DICT && type != &t_any && type != &t_unknown)
     {
 	char *tofree;