diff --git a/runtime/doc/vim9.txt b/runtime/doc/vim9.txt
index 7c4a64f..f9fd136 100644
--- a/runtime/doc/vim9.txt
+++ b/runtime/doc/vim9.txt
@@ -426,11 +426,14 @@
 	2 && 0   == 0
 	[] && 2  == []
 
-When using `..` for string concatenation the arguments are always converted to
-string. >
+When using `..` for string concatenation arguments of simple types are always
+converted to string. >
 	'hello ' .. 123  == 'hello 123'
 	'hello ' .. v:true  == 'hello true'
 
+Simple types are string, float, special and bool.  For other types |string()|
+can be used.
+
 In Vim9 script one can use "true" for v:true and "false" for v:false.
 
 
@@ -805,6 +808,9 @@
 	   ...
 <   This goes in .../import/someother.vim.
 
+When compiling a `:def` function and a function in an autoload script is
+encountered, the script is not loaded until the `:def` function is called.
+
 
 Import in legacy Vim script ~
 
diff --git a/src/eval.c b/src/eval.c
index 99554e7..07b30c7 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -2712,7 +2712,7 @@
 	    return FAIL;
 	}
 	*arg = skipwhite_and_linebreak(*arg + oplen, evalarg);
-	if (eval6(arg, &var2, evalarg, op == '.') == FAIL)
+	if (eval6(arg, &var2, evalarg, !in_vim9script() && op == '.') == FAIL)
 	{
 	    clear_tv(rettv);
 	    return FAIL;
@@ -2727,8 +2727,22 @@
 	    {
 		char_u	buf1[NUMBUFLEN], buf2[NUMBUFLEN];
 		char_u	*s1 = tv_get_string_buf(rettv, buf1);
-		char_u	*s2 = tv_get_string_buf_chk(&var2, buf2);
+		char_u	*s2 = NULL;
 
+		if (in_vim9script() && (var2.v_type == VAR_VOID
+			|| var2.v_type == VAR_CHANNEL
+			|| var2.v_type == VAR_JOB))
+		    emsg(_(e_inval_string));
+#ifdef FEAT_FLOAT
+		else if (var2.v_type == VAR_FLOAT)
+		{
+		    vim_snprintf((char *)buf2, NUMBUFLEN, "%g",
+							    var2.vval.v_float);
+		    s2 = buf2;
+		}
+#endif
+		else
+		    s2 = tv_get_string_buf_chk(&var2, buf2);
 		if (s2 == NULL)		// type error ?
 		{
 		    clear_tv(rettv);
diff --git a/src/evalfunc.c b/src/evalfunc.c
index bbc3404..d95a1db 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -1046,7 +1046,7 @@
     {"test_settime",	1, 1, FEARG_1,	  ret_void,	f_test_settime},
     {"test_srand_seed",	0, 1, FEARG_1,	  ret_void,	f_test_srand_seed},
     {"test_unknown",	0, 0, 0,	  ret_any,	f_test_unknown},
-    {"test_void",	0, 0, 0,	  ret_any,	f_test_void},
+    {"test_void",	0, 0, 0,	  ret_void,	f_test_void},
     {"timer_info",	0, 1, FEARG_1,	  ret_list_dict_any, TIMER_FUNC(f_timer_info)},
     {"timer_pause",	2, 2, FEARG_1,	  ret_void,	TIMER_FUNC(f_timer_pause)},
     {"timer_start",	2, 3, FEARG_1,	  ret_number,	TIMER_FUNC(f_timer_start)},
diff --git a/src/proto/vim9execute.pro b/src/proto/vim9execute.pro
index eac8979..755b7dd 100644
--- a/src/proto/vim9execute.pro
+++ b/src/proto/vim9execute.pro
@@ -1,4 +1,5 @@
 /* vim9execute.c */
+void to_string_error(vartype_T vartype);
 int call_def_function(ufunc_T *ufunc, int argc_arg, typval_T *argv, partial_T *partial, typval_T *rettv);
 void ex_disassemble(exarg_T *eap);
 int tv2bool(typval_T *tv);
diff --git a/src/testdir/test_vim9_disassemble.vim b/src/testdir/test_vim9_disassemble.vim
index 3c7724d..fe8afad 100644
--- a/src/testdir/test_vim9_disassemble.vim
+++ b/src/testdir/test_vim9_disassemble.vim
@@ -708,7 +708,7 @@
         'return "X" .. a .. "X"\_s*' ..
         '\d PUSHS "X"\_s*' ..
         '\d LOAD arg\[-1\]\_s*' ..
-        '\d 2STRING stack\[-1\]\_s*' ..
+        '\d 2STRING_ANY stack\[-1\]\_s*' ..
         '\d CONCAT\_s*' ..
         '\d PUSHS "X"\_s*' ..
         '\d CONCAT\_s*' ..
@@ -964,7 +964,7 @@
         'let res = g:aa .. "bb".*' ..
         '\d LOADG g:aa.*' ..
         '\d PUSHS "bb".*' ..
-        '\d 2STRING stack\[-2].*' ..
+        '\d 2STRING_ANY stack\[-2].*' ..
         '\d CONCAT.*' ..
         '\d STORE $.*',
         instr)
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index 795ea75..81d3cb9 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -921,6 +921,14 @@
   assert_equal('123 hello', 123 .. ' hello')
   assert_equal('123456', 123 .. 456)
 
+  assert_equal('av:true', 'a' .. true)
+  assert_equal('av:false', 'a' .. false)
+  assert_equal('av:null', 'a' .. v:null)
+  assert_equal('av:none', 'a' .. v:none)
+  if has('float')
+    assert_equal('a0.123', 'a' .. 0.123)
+  endif
+
   assert_equal([1, 2, 3, 4], [1, 2] + [3, 4])
   assert_equal(0z11223344, 0z1122 + 0z3344)
   assert_equal(0z112201ab, 0z1122
@@ -1013,6 +1021,56 @@
       echo 'a'.. 'b'
   END
   CheckScriptFailure(lines, 'E1004:')
+
+  # check valid string concatenation
+  lines =<< trim END
+      vim9script
+      assert_equal('one123', 'one' .. 123)
+      assert_equal('onev:true', 'one' .. true)
+      assert_equal('onev:null', 'one' .. v:null)
+      assert_equal('onev:none', 'one' .. v:none)
+      if has('float')
+        assert_equal('a0.123', 'a' .. 0.123)
+      endif
+  END
+  CheckScriptSuccess(lines)
+
+  # check invalid string concatenation
+  lines =<< trim END
+      vim9script
+      echo 'a' .. [1]
+  END
+  CheckScriptFailure(lines, 'E730:')
+  lines =<< trim END
+      vim9script
+      echo 'a' .. #{a: 1}
+  END
+  CheckScriptFailure(lines, 'E731:')
+  lines =<< trim END
+      vim9script
+      echo 'a' .. test_void()
+  END
+  CheckScriptFailure(lines, 'E908:')
+  lines =<< trim END
+      vim9script
+      echo 'a' .. 0z33
+  END
+  CheckScriptFailure(lines, 'E976:')
+  lines =<< trim END
+      vim9script
+      echo 'a' .. function('len')
+  END
+  CheckScriptFailure(lines, 'E729:')
+  lines =<< trim END
+      vim9script
+      echo 'a' .. test_null_job()
+  END
+  CheckScriptFailure(lines, 'E908:')
+  lines =<< trim END
+      vim9script
+      echo 'a' .. test_null_channel()
+  END
+  CheckScriptFailure(lines, 'E908:')
 enddef
 
 def Test_expr5_float()
@@ -1063,6 +1121,15 @@
   call CheckDefFailure(["let x = [3] + 0z1122"], 'E1051')
   call CheckDefFailure(["let x = 'asdf' + 0z1122"], 'E1051')
   call CheckDefFailure(["let x = 6 + xxx"], 'E1001')
+
+  call CheckDefFailure(["let x = 'a' .. [1]"], 'E1105')
+  call CheckDefFailure(["let x = 'a' .. #{a: 1}"], 'E1105')
+  call CheckDefFailure(["let x = 'a' .. test_void()"], 'E1105')
+  call CheckDefFailure(["let x = 'a' .. 0z32"], 'E1105')
+  call CheckDefFailure(["let x = 'a' .. function('len')"], 'E1105')
+  call CheckDefFailure(["let x = 'a' .. function('len', ['a'])"], 'E1105')
+  call CheckDefFailure(["let x = 'a' .. test_null_job()"], 'E1105')
+  call CheckDefFailure(["let x = 'a' .. test_null_channel()"], 'E1105')
 endfunc
 
 " test multiply, divide, modulo
diff --git a/src/version.c b/src/version.c
index 1100818..f764e98 100644
--- a/src/version.c
+++ b/src/version.c
@@ -755,6 +755,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1435,
+/**/
     1434,
 /**/
     1433,
diff --git a/src/vim9.h b/src/vim9.h
index 77f3427..8789414 100644
--- a/src/vim9.h
+++ b/src/vim9.h
@@ -93,7 +93,7 @@
     ISN_CATCH,	    // drop v:exception
     ISN_ENDTRY,	    // take entry off from ec_trystack
 
-    // moreexpression operations
+    // more expression operations
     ISN_ADDLIST,
     ISN_ADDBLOB,
 
@@ -124,6 +124,7 @@
     ISN_STRINGMEMBER, // dict.member using isn_arg.string
     ISN_2BOOL,	    // convert value to bool, invert if isn_arg.number != 0
     ISN_2STRING,    // convert value to string at isn_arg.number on stack
+    ISN_2STRING_ANY, // like ISN_2STRING but check type
     ISN_NEGATENR,   // apply "-" to number
 
     ISN_CHECKNR,    // check value can be used as a number
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 889a5d5..99f8e22 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -374,19 +374,49 @@
 
 /*
  * If type at "offset" isn't already VAR_STRING then generate ISN_2STRING.
+ * But only for simple types.
  */
     static int
 may_generate_2STRING(int offset, cctx_T *cctx)
 {
     isn_T	*isn;
+    isntype_T	isntype = ISN_2STRING;
     garray_T	*stack = &cctx->ctx_type_stack;
     type_T	**type = ((type_T **)stack->ga_data) + stack->ga_len + offset;
 
-    if ((*type)->tt_type == VAR_STRING)
-	return OK;
-    *type = &t_string;
+    switch ((*type)->tt_type)
+    {
+	// nothing to be done
+	case VAR_STRING: return OK;
 
-    if ((isn = generate_instr(cctx, ISN_2STRING)) == NULL)
+	// conversion possible
+	case VAR_SPECIAL:
+	case VAR_BOOL:
+	case VAR_NUMBER:
+	case VAR_FLOAT:
+			 break;
+
+	// conversion possible (with runtime check)
+	case VAR_ANY:
+	case VAR_UNKNOWN:
+			 isntype = ISN_2STRING_ANY;
+			 break;
+
+	// conversion not possible
+	case VAR_VOID:
+	case VAR_BLOB:
+	case VAR_FUNC:
+	case VAR_PARTIAL:
+	case VAR_LIST:
+	case VAR_DICT:
+	case VAR_JOB:
+	case VAR_CHANNEL:
+			 to_string_error((*type)->tt_type);
+			 return FAIL;
+    }
+
+    *type = &t_string;
+    if ((isn = generate_instr(cctx, isntype)) == NULL)
 	return FAIL;
     isn->isn_arg.number = offset;
 
@@ -7005,6 +7035,7 @@
 
 	case ISN_2BOOL:
 	case ISN_2STRING:
+	case ISN_2STRING_ANY:
 	case ISN_ADDBLOB:
 	case ISN_ADDLIST:
 	case ISN_BCALL:
diff --git a/src/vim9execute.c b/src/vim9execute.c
index 00adff3..8575e58 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -72,6 +72,12 @@
 // Get pointer to item relative to the bottom of the stack, -1 is the last one.
 #define STACK_TV_BOT(idx) (((typval_T *)ectx->ec_stack.ga_data) + ectx->ec_stack.ga_len + idx)
 
+    void
+to_string_error(vartype_T vartype)
+{
+    semsg(_("E1105: Cannot convert %s to string"), vartype_name(vartype));
+}
+
 /*
  * Return the number of arguments, including optional arguments and any vararg.
  */
@@ -2476,12 +2482,26 @@
 		break;
 
 	    case ISN_2STRING:
+	    case ISN_2STRING_ANY:
 		{
 		    char_u *str;
 
 		    tv = STACK_TV_BOT(iptr->isn_arg.number);
 		    if (tv->v_type != VAR_STRING)
 		    {
+			if (iptr->isn_type == ISN_2STRING_ANY)
+			{
+			    switch (tv->v_type)
+			    {
+				case VAR_SPECIAL:
+				case VAR_BOOL:
+				case VAR_NUMBER:
+				case VAR_FLOAT:
+				case VAR_BLOB:	break;
+				default:	to_string_error(tv->v_type);
+						goto on_error;
+			    }
+			}
 			str = typval_tostring(tv);
 			clear_tv(tv);
 			tv->v_type = VAR_STRING;
@@ -3127,6 +3147,9 @@
 	    case ISN_2STRING: smsg("%4d 2STRING stack[%lld]", current,
 					 (long long)(iptr->isn_arg.number));
 			      break;
+	    case ISN_2STRING_ANY: smsg("%4d 2STRING_ANY stack[%lld]", current,
+					 (long long)(iptr->isn_arg.number));
+			      break;
 
 	    case ISN_SHUFFLE: smsg("%4d SHUFFLE %d up %d", current,
 					 iptr->isn_arg.shuffle.shfl_item,