diff --git a/src/dict.c b/src/dict.c
index 54d3110..3824f40 100644
--- a/src/dict.c
+++ b/src/dict.c
@@ -791,8 +791,9 @@
  * Return OK or FAIL.  Returns NOTDONE for {expr}.
  */
     int
-eval_dict(char_u **arg, typval_T *rettv, int evaluate, int literal)
+eval_dict(char_u **arg, typval_T *rettv, int flags, int literal)
 {
+    int		evaluate = flags & EVAL_EVALUATE;
     dict_T	*d = NULL;
     typval_T	tvkey;
     typval_T	tv;
@@ -800,6 +801,7 @@
     dictitem_T	*item;
     char_u	*start = skipwhite(*arg + 1);
     char_u	buf[NUMBUFLEN];
+    int		vim9script = current_sctx.sc_version == SCRIPT_VERSION_VIM9;
 
     /*
      * First check if it's not a curly-braces thing: {expr}.
@@ -808,9 +810,9 @@
      * first item.
      * But {} is an empty Dictionary.
      */
-    if (*start != '}')
+    if (!vim9script && *start != '}')
     {
-	if (eval1(&start, &tv, FALSE) == FAIL)	// recursive!
+	if (eval1(&start, &tv, 0) == FAIL)	// recursive!
 	    return FAIL;
 	if (*start == '}')
 	    return NOTDONE;
@@ -830,7 +832,7 @@
     {
 	if ((literal
 		? get_literal_key(arg, &tvkey)
-		: eval1(arg, &tvkey, evaluate)) == FAIL)	// recursive!
+		: eval1(arg, &tvkey, flags)) == FAIL)	// recursive!
 	    goto failret;
 
 	if (**arg != ':')
@@ -852,7 +854,7 @@
 	}
 
 	*arg = skipwhite(*arg + 1);
-	if (eval1(arg, &tv, evaluate) == FAIL)	// recursive!
+	if (eval1(arg, &tv, flags) == FAIL)	// recursive!
 	{
 	    if (evaluate)
 		clear_tv(&tvkey);
diff --git a/src/eval.c b/src/eval.c
index 2100d80..b3fe650 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -48,12 +48,12 @@
 } forinfo_T;
 
 static int tv_op(typval_T *tv1, typval_T *tv2, char_u  *op);
-static int eval2(char_u **arg, typval_T *rettv, int evaluate);
-static int eval3(char_u **arg, typval_T *rettv, int evaluate);
-static int eval4(char_u **arg, typval_T *rettv, int evaluate);
-static int eval5(char_u **arg, typval_T *rettv, int evaluate);
-static int eval6(char_u **arg, typval_T *rettv, int evaluate, int want_string);
-static int eval7(char_u **arg, typval_T *rettv, int evaluate, int want_string);
+static int eval2(char_u **arg, typval_T *rettv, int flags);
+static int eval3(char_u **arg, typval_T *rettv, int flags);
+static int eval4(char_u **arg, typval_T *rettv, int flags);
+static int eval5(char_u **arg, typval_T *rettv, int flags);
+static int eval6(char_u **arg, typval_T *rettv, int flags, int want_string);
+static int eval7(char_u **arg, typval_T *rettv, int flags, int want_string);
 static int eval7_leader(typval_T *rettv, char_u *start_leader, char_u **end_leaderp);
 
 static int free_unref_items(int copyID);
@@ -173,7 +173,7 @@
 
     if (skip)
 	++emsg_skip;
-    if (eval0(arg, &tv, nextcmd, !skip) == FAIL)
+    if (eval0(arg, &tv, nextcmd, skip ? 0 : EVAL_EVALUATE) == FAIL)
 	*error = TRUE;
     else
     {
@@ -201,7 +201,7 @@
     int		did_emsg_before = did_emsg;
     int		called_emsg_before = called_emsg;
 
-    ret = eval1(arg, rettv, evaluate);
+    ret = eval1(arg, rettv, evaluate ? EVAL_EVALUATE : 0);
     if (ret == FAIL)
     {
 	// Report the invalid expression unless the expression evaluation has
@@ -315,7 +315,7 @@
 
     if (skip)
 	++emsg_skip;
-    if (eval0(arg, &tv, nextcmd, !skip) == FAIL || skip)
+    if (eval0(arg, &tv, nextcmd, skip ? 0 : EVAL_EVALUATE) == FAIL || skip)
 	retval = NULL;
     else
     {
@@ -338,7 +338,7 @@
     typval_T	rettv;
 
     *pp = skipwhite(*pp);
-    return eval1(pp, &rettv, FALSE);
+    return eval1(pp, &rettv, 0);
 }
 
 /*
@@ -360,7 +360,7 @@
     char_u	numbuf[NUMBUFLEN];
 #endif
 
-    if (eval0(arg, &tv, nextcmd, TRUE) == FAIL)
+    if (eval0(arg, &tv, nextcmd, EVAL_EVALUATE) == FAIL)
 	retval = NULL;
     else
     {
@@ -430,7 +430,7 @@
 
     ++emsg_off;
 
-    if (eval1(&p, &rettv, TRUE) == FAIL)
+    if (eval1(&p, &rettv, EVAL_EVALUATE) == FAIL)
 	retval = -1;
     else
     {
@@ -453,7 +453,7 @@
     typval_T	*tv;
 
     tv = ALLOC_ONE(typval_T);
-    if (tv != NULL && eval0(arg, tv, nextcmd, TRUE) == FAIL)
+    if (tv != NULL && eval0(arg, tv, nextcmd, EVAL_EVALUATE) == FAIL)
 	VIM_CLEAR(tv);
 
     return tv;
@@ -578,7 +578,7 @@
 	++sandbox;
     ++textwinlock;
     *cp = NUL;
-    if (eval0(arg, &tv, NULL, TRUE) == FAIL)
+    if (eval0(arg, &tv, NULL, EVAL_EVALUATE) == FAIL)
 	retval = 0;
     else
     {
@@ -766,7 +766,7 @@
 	    else
 	    {
 		empty1 = FALSE;
-		if (eval1(&p, &var1, TRUE) == FAIL)	// recursive!
+		if (eval1(&p, &var1, EVAL_EVALUATE) == FAIL)  // recursive!
 		    return NULL;
 		if (tv_get_string_chk(&var1) == NULL)
 		{
@@ -803,7 +803,8 @@
 		else
 		{
 		    lp->ll_empty2 = FALSE;
-		    if (eval1(&p, &var2, TRUE) == FAIL)	// recursive!
+		    // recursive!
+		    if (eval1(&p, &var2, EVAL_EVALUATE) == FAIL)
 		    {
 			clear_tv(&var1);
 			return NULL;
@@ -1433,7 +1434,8 @@
 
     if (skip)
 	++emsg_skip;
-    if (eval0(skipwhite(expr + 2), &tv, nextcmdp, !skip) == OK)
+    if (eval0(skipwhite(expr + 2), &tv, nextcmdp, skip ? 0 : EVAL_EVALUATE)
+									 == OK)
     {
 	*errp = FALSE;
 	if (!skip)
@@ -1694,9 +1696,10 @@
 	char_u	    *name,
 	int	    name_len,
 	typval_T    *rettv,
-	int	    evaluate,
+	int	    flags,
 	typval_T    *basetv)	// "expr" for "expr->name(arg)"
 {
+    int		evaluate = flags & EVAL_EVALUATE;
     char_u	*s = name;
     int		len = name_len;
     partial_T	*partial;
@@ -1712,7 +1715,7 @@
     // Need to make a copy, in case evaluating the arguments makes
     // the name invalid.
     s = vim_strsave(s);
-    if (s == NULL)
+    if (s == NULL || (flags & EVAL_CONSTANT))
 	ret = FAIL;
     else
     {
@@ -1761,6 +1764,7 @@
  * This calls eval1() and handles error message and nextcmd.
  * Put the result in "rettv" when returning OK and "evaluate" is TRUE.
  * Note: "rettv.v_lock" is not set.
+ * "flags" has EVAL_EVALUATE and similar flags.
  * Return OK or FAIL.
  */
     int
@@ -1768,7 +1772,7 @@
     char_u	*arg,
     typval_T	*rettv,
     char_u	**nextcmd,
-    int		evaluate)
+    int		flags)
 {
     int		ret;
     char_u	*p;
@@ -1776,7 +1780,7 @@
     int		called_emsg_before = called_emsg;
 
     p = skipwhite(arg);
-    ret = eval1(&p, rettv, evaluate);
+    ret = eval1(&p, rettv, flags);
     if (ret == FAIL || !ends_excmd2(arg, p))
     {
 	if (ret != FAIL)
@@ -1787,8 +1791,10 @@
 	 * exception, or we already gave a more specific error.
 	 * Also check called_emsg for when using assert_fails().
 	 */
-	if (!aborting() && did_emsg == did_emsg_before
-					  && called_emsg == called_emsg_before)
+	if (!aborting()
+		&& did_emsg == did_emsg_before
+		&& called_emsg == called_emsg_before
+		&& (flags & EVAL_CONSTANT) == 0)
 	    semsg(_(e_invexpr2), arg);
 	ret = FAIL;
     }
@@ -1810,7 +1816,7 @@
  * Return OK or FAIL.
  */
     int
-eval1(char_u **arg, typval_T *rettv, int evaluate)
+eval1(char_u **arg, typval_T *rettv, int flags)
 {
     int		result;
     typval_T	var2;
@@ -1818,13 +1824,15 @@
     /*
      * Get the first variable.
      */
-    if (eval2(arg, rettv, evaluate) == FAIL)
+    if (eval2(arg, rettv, flags) == FAIL)
 	return FAIL;
 
     if ((*arg)[0] == '?')
     {
+	int evaluate = flags & EVAL_EVALUATE;
+
 	result = FALSE;
-	if (evaluate)
+	if (flags & EVAL_EVALUATE)
 	{
 	    int		error = FALSE;
 
@@ -1836,10 +1844,10 @@
 	}
 
 	/*
-	 * Get the second variable.
+	 * Get the second variable.  Recursive!
 	 */
 	*arg = skipwhite(*arg + 1);
-	if (eval1(arg, rettv, evaluate && result) == FAIL) // recursive!
+	if (eval1(arg, rettv, result ? flags : flags & ~EVAL_EVALUATE) == FAIL)
 	    return FAIL;
 
 	/*
@@ -1854,10 +1862,10 @@
 	}
 
 	/*
-	 * Get the third variable.
+	 * Get the third variable.  Recursive!
 	 */
 	*arg = skipwhite(*arg + 1);
-	if (eval1(arg, &var2, evaluate && !result) == FAIL) // recursive!
+	if (eval1(arg, &var2, !result ? flags : flags & ~EVAL_EVALUATE) == FAIL)
 	{
 	    if (evaluate && result)
 		clear_tv(rettv);
@@ -1880,7 +1888,7 @@
  * Return OK or FAIL.
  */
     static int
-eval2(char_u **arg, typval_T *rettv, int evaluate)
+eval2(char_u **arg, typval_T *rettv, int flags)
 {
     typval_T	var2;
     long	result;
@@ -1890,7 +1898,7 @@
     /*
      * Get the first variable.
      */
-    if (eval3(arg, rettv, evaluate) == FAIL)
+    if (eval3(arg, rettv, flags) == FAIL)
 	return FAIL;
 
     /*
@@ -1900,6 +1908,8 @@
     result = FALSE;
     while ((*arg)[0] == '|' && (*arg)[1] == '|')
     {
+	int evaluate = flags & EVAL_EVALUATE;
+
 	if (evaluate && first)
 	{
 	    if (tv_get_number_chk(rettv, &error) != 0)
@@ -1914,7 +1924,8 @@
 	 * Get the second variable.
 	 */
 	*arg = skipwhite(*arg + 2);
-	if (eval3(arg, &var2, evaluate && !result) == FAIL)
+	if (eval3(arg, &var2, !result ? flags : flags & ~EVAL_EVALUATE)
+								       == FAIL)
 	    return FAIL;
 
 	/*
@@ -1948,7 +1959,7 @@
  * Return OK or FAIL.
  */
     static int
-eval3(char_u **arg, typval_T *rettv, int evaluate)
+eval3(char_u **arg, typval_T *rettv, int flags)
 {
     typval_T	var2;
     long	result;
@@ -1958,7 +1969,7 @@
     /*
      * Get the first variable.
      */
-    if (eval4(arg, rettv, evaluate) == FAIL)
+    if (eval4(arg, rettv, flags) == FAIL)
 	return FAIL;
 
     /*
@@ -1968,6 +1979,8 @@
     result = TRUE;
     while ((*arg)[0] == '&' && (*arg)[1] == '&')
     {
+	int evaluate = flags & EVAL_EVALUATE;
+
 	if (evaluate && first)
 	{
 	    if (tv_get_number_chk(rettv, &error) == 0)
@@ -1982,7 +1995,7 @@
 	 * Get the second variable.
 	 */
 	*arg = skipwhite(*arg + 2);
-	if (eval4(arg, &var2, evaluate && result) == FAIL)
+	if (eval4(arg, &var2, result ? flags : flags & ~EVAL_EVALUATE) == FAIL)
 	    return FAIL;
 
 	/*
@@ -2025,7 +2038,7 @@
  * Return OK or FAIL.
  */
     static int
-eval4(char_u **arg, typval_T *rettv, int evaluate)
+eval4(char_u **arg, typval_T *rettv, int flags)
 {
     typval_T	var2;
     char_u	*p;
@@ -2037,7 +2050,7 @@
     /*
      * Get the first variable.
      */
-    if (eval5(arg, rettv, evaluate) == FAIL)
+    if (eval5(arg, rettv, flags) == FAIL)
 	return FAIL;
 
     p = *arg;
@@ -2105,12 +2118,12 @@
 	 * Get the second variable.
 	 */
 	*arg = skipwhite(p + len);
-	if (eval5(arg, &var2, evaluate) == FAIL)
+	if (eval5(arg, &var2, flags) == FAIL)
 	{
 	    clear_tv(rettv);
 	    return FAIL;
 	}
-	if (evaluate)
+	if (flags & EVAL_EVALUATE)
 	{
 	    int ret = typval_compare(rettv, &var2, type, ic);
 
@@ -2172,7 +2185,7 @@
  * Return OK or FAIL.
  */
     static int
-eval5(char_u **arg, typval_T *rettv, int evaluate)
+eval5(char_u **arg, typval_T *rettv, int flags)
 {
     typval_T	var2;
     int		op;
@@ -2188,7 +2201,7 @@
     /*
      * Get the first variable.
      */
-    if (eval6(arg, rettv, evaluate, FALSE) == FAIL)
+    if (eval6(arg, rettv, flags, FALSE) == FAIL)
 	return FAIL;
 
     /*
@@ -2217,7 +2230,7 @@
 	    // we know that the first operand needs to be a string or number
 	    // without evaluating the 2nd operand.  So check before to avoid
 	    // side effects after an error.
-	    if (evaluate && tv_get_string_chk(rettv) == NULL)
+	    if ((flags & EVAL_EVALUATE) && tv_get_string_chk(rettv) == NULL)
 	    {
 		clear_tv(rettv);
 		return FAIL;
@@ -2230,13 +2243,13 @@
 	if (op == '.' && *(*arg + 1) == '.')  // .. string concatenation
 	    ++*arg;
 	*arg = skipwhite(*arg + 1);
-	if (eval6(arg, &var2, evaluate, op == '.') == FAIL)
+	if (eval6(arg, &var2, flags, op == '.') == FAIL)
 	{
 	    clear_tv(rettv);
 	    return FAIL;
 	}
 
-	if (evaluate)
+	if (flags & EVAL_EVALUATE)
 	{
 	    /*
 	     * Compute the result.
@@ -2358,7 +2371,7 @@
 eval6(
     char_u	**arg,
     typval_T	*rettv,
-    int		evaluate,
+    int		flags,
     int		want_string)  // after "." operator
 {
     typval_T	var2;
@@ -2373,7 +2386,7 @@
     /*
      * Get the first variable.
      */
-    if (eval7(arg, rettv, evaluate, want_string) == FAIL)
+    if (eval7(arg, rettv, flags, want_string) == FAIL)
 	return FAIL;
 
     /*
@@ -2385,7 +2398,7 @@
 	if (op != '*' && op != '/' && op != '%')
 	    break;
 
-	if (evaluate)
+	if (flags & EVAL_EVALUATE)
 	{
 #ifdef FEAT_FLOAT
 	    if (rettv->v_type == VAR_FLOAT)
@@ -2408,10 +2421,10 @@
 	 * Get the second variable.
 	 */
 	*arg = skipwhite(*arg + 1);
-	if (eval7(arg, &var2, evaluate, FALSE) == FAIL)
+	if (eval7(arg, &var2, flags, FALSE) == FAIL)
 	    return FAIL;
 
-	if (evaluate)
+	if (flags & EVAL_EVALUATE)
 	{
 #ifdef FEAT_FLOAT
 	    if (var2.v_type == VAR_FLOAT)
@@ -2528,9 +2541,10 @@
 eval7(
     char_u	**arg,
     typval_T	*rettv,
-    int		evaluate,
+    int		flags,
     int		want_string)	// after "." operator
 {
+    int		evaluate = flags & EVAL_EVALUATE;
     int		len;
     char_u	*s;
     char_u	*start_leader, *end_leader;
@@ -2595,7 +2609,7 @@
     /*
      * List: [expr, expr]
      */
-    case '[':	ret = get_list_tv(arg, rettv, evaluate, TRUE);
+    case '[':	ret = get_list_tv(arg, rettv, flags, TRUE);
 		break;
 
     /*
@@ -2604,7 +2618,7 @@
     case '#':	if ((*arg)[1] == '{')
 		{
 		    ++*arg;
-		    ret = eval_dict(arg, rettv, evaluate, TRUE);
+		    ret = eval_dict(arg, rettv, flags, TRUE);
 		}
 		else
 		    ret = NOTDONE;
@@ -2616,7 +2630,7 @@
      */
     case '{':	ret = get_lambda_tv(arg, rettv, evaluate);
 		if (ret == NOTDONE)
-		    ret = eval_dict(arg, rettv, evaluate, FALSE);
+		    ret = eval_dict(arg, rettv, flags, FALSE);
 		break;
 
     /*
@@ -2649,7 +2663,7 @@
      * nested expression: (expression).
      */
     case '(':	*arg = skipwhite(*arg + 1);
-		ret = eval1(arg, rettv, evaluate);	// recursive!
+		ret = eval1(arg, rettv, flags);	// recursive!
 		if (**arg == ')')
 		    ++*arg;
 		else if (ret == OK)
@@ -2680,7 +2694,7 @@
 	else
 	{
 	    if (**arg == '(')		// recursive!
-		ret = eval_func(arg, s, len, rettv, evaluate, NULL);
+		ret = eval_func(arg, s, len, rettv, flags, NULL);
 	    else if (evaluate)
 		ret = get_var_tv(s, len, rettv, NULL, TRUE, FALSE);
 	    else
@@ -2697,7 +2711,7 @@
     // Handle following '[', '(' and '.' for expr[expr], expr.name,
     // expr(expr), expr->name(expr)
     if (ret == OK)
-	ret = handle_subscript(arg, rettv, evaluate, TRUE,
+	ret = handle_subscript(arg, rettv, flags, TRUE,
 						    start_leader, &end_leader);
 
     /*
@@ -2919,7 +2933,8 @@
 	    ret = FAIL;
 	}
 	else
-	    ret = eval_func(arg, name, len, rettv, evaluate, &base);
+	    ret = eval_func(arg, name, len, rettv,
+					  evaluate ? EVAL_EVALUATE : 0, &base);
     }
 
     // Clear the funcref afterwards, so that deleting it while
@@ -2939,9 +2954,10 @@
 eval_index(
     char_u	**arg,
     typval_T	*rettv,
-    int		evaluate,
+    int		flags,
     int		verbose)	// give error messages
 {
+    int		evaluate = flags & EVAL_EVALUATE;
     int		empty1 = FALSE, empty2 = FALSE;
     typval_T	var1, var2;
     long	i;
@@ -3010,7 +3026,7 @@
 	*arg = skipwhite(*arg + 1);
 	if (**arg == ':')
 	    empty1 = TRUE;
-	else if (eval1(arg, &var1, evaluate) == FAIL)	// recursive!
+	else if (eval1(arg, &var1, flags) == FAIL)	// recursive!
 	    return FAIL;
 	else if (evaluate && tv_get_string_chk(&var1) == NULL)
 	{
@@ -3028,7 +3044,7 @@
 	    *arg = skipwhite(*arg + 1);
 	    if (**arg == ']')
 		empty2 = TRUE;
-	    else if (eval1(arg, &var2, evaluate) == FAIL)	// recursive!
+	    else if (eval1(arg, &var2, flags) == FAIL)	// recursive!
 	    {
 		if (!empty1)
 		    clear_tv(&var1);
@@ -5310,11 +5326,12 @@
 handle_subscript(
     char_u	**arg,
     typval_T	*rettv,
-    int		evaluate,	// do more than finding the end
+    int		flags,		// do more than finding the end
     int		verbose,	// give error messages
     char_u	*start_leader,	// start of '!' and '-' prefixes
     char_u	**end_leaderp)  // end of '!' and '-' prefixes
 {
+    int		evaluate = flags & EVAL_EVALUATE;
     int		ret = OK;
     dict_T	*selfdict = NULL;
 
@@ -5374,7 +5391,7 @@
 	    }
 	    else
 		selfdict = NULL;
-	    if (eval_index(arg, rettv, evaluate, verbose) == FAIL)
+	    if (eval_index(arg, rettv, flags, verbose) == FAIL)
 	    {
 		clear_tv(rettv);
 		ret = FAIL;
@@ -6108,7 +6125,7 @@
 	need_clr_eos = needclr;
 
 	p = arg;
-	if (eval1(&arg, &rettv, !eap->skip) == FAIL)
+	if (eval1(&arg, &rettv, eap->skip ? 0 : EVAL_EVALUATE) == FAIL)
 	{
 	    /*
 	     * Report the invalid expression unless the expression evaluation
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 7579208..2979611 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -2132,7 +2132,7 @@
 	s = skipwhite(s);
 
     p = s;
-    if (s == NULL || eval1(&s, rettv, TRUE) == FAIL)
+    if (s == NULL || eval1(&s, rettv, EVAL_EVALUATE) == FAIL)
     {
 	if (p != NULL && !aborting())
 	    semsg(_(e_invexpr2), p);
diff --git a/src/evalvars.c b/src/evalvars.c
index e747230..fd652ae 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -433,7 +433,7 @@
     if (p_verbose == 0)
 	++emsg_off;
 
-    if (eval1(&p, &rettv, TRUE) == OK)
+    if (eval1(&p, &rettv, EVAL_EVALUATE) == OK)
     {
 	if (rettv.v_type != VAR_LIST)
 	    clear_tv(&rettv);
@@ -701,11 +701,14 @@
 }
 
 /*
- * When "redefine" is TRUE the command will be executed again, redefining the
- * variable is OK then.
+ * When "discovery" is TRUE the ":let" or ":const" is encountered during the
+ * discovery phase of vim9script:
+ * - The command will be executed again, redefining the variable is OK then.
+ * - The expresion argument must be a constant.
+ * - If no constant expression a type must be specified.
  */
     void
-ex_let_const(exarg_T *eap, int redefine)
+ex_let_const(exarg_T *eap, int discovery)
 {
     char_u	*arg = eap->arg;
     char_u	*expr = NULL;
@@ -717,13 +720,14 @@
     char_u	*argend;
     int		first = TRUE;
     int		concat;
+    int		has_assign;
     int		flags = eap->cmdidx == CMD_const ? LET_IS_CONST : 0;
 
     // detect Vim9 assignment without ":let" or ":const"
     if (eap->arg == eap->cmd)
 	flags |= LET_NO_COMMAND;
-    if (redefine)
-	flags |= LET_REDEFINE;
+    if (discovery)
+	flags |= LET_DISCOVERY;
 
     argend = skip_var_list(arg, TRUE, &var_count, &semicolon);
     if (argend == NULL)
@@ -734,8 +738,9 @@
     concat = expr[0] == '.'
 	&& ((expr[1] == '=' && current_sctx.sc_version < 2)
 		|| (expr[1] == '.' && expr[2] == '='));
-    if (*expr != '=' && !((vim_strchr((char_u *)"+-*/%", *expr) != NULL
-						 && expr[1] == '=') || concat))
+    has_assign =  *expr == '=' || (vim_strchr((char_u *)"+-*/%", *expr) != NULL
+							    && expr[1] == '=');
+    if (!has_assign && !concat && !discovery)
     {
 	// ":let" without "=": list variables
 	if (*arg == '[')
@@ -779,32 +784,45 @@
     }
     else
     {
-	op[0] = '=';
-	op[1] = NUL;
-	if (*expr != '=')
-	{
-	    if (vim_strchr((char_u *)"+-*/%.", *expr) != NULL)
-	    {
-		op[0] = *expr;   // +=, -=, *=, /=, %= or .=
-		if (expr[0] == '.' && expr[1] == '.') // ..=
-		    ++expr;
-	    }
-	    expr = skipwhite(expr + 2);
-	}
-	else
-	    expr = skipwhite(expr + 1);
+	int eval_flags;
+	int save_called_emsg = called_emsg;
 
-	if (eap->skip)
-	    ++emsg_skip;
-	i = eval0(expr, &rettv, &eap->nextcmd, !eap->skip);
+	rettv.v_type = VAR_UNKNOWN;
+	i = FAIL;
+	if (has_assign || concat)
+	{
+	    op[0] = '=';
+	    op[1] = NUL;
+	    if (*expr != '=')
+	    {
+		if (vim_strchr((char_u *)"+-*/%.", *expr) != NULL)
+		{
+		    op[0] = *expr;   // +=, -=, *=, /=, %= or .=
+		    if (expr[0] == '.' && expr[1] == '.') // ..=
+			++expr;
+		}
+		expr = skipwhite(expr + 2);
+	    }
+	    else
+		expr = skipwhite(expr + 1);
+
+	    if (eap->skip)
+		++emsg_skip;
+	    eval_flags = eap->skip ? 0 : EVAL_EVALUATE;
+	    if (discovery)
+		eval_flags |= EVAL_CONSTANT;
+	    i = eval0(expr, &rettv, &eap->nextcmd, eval_flags);
+	}
 	if (eap->skip)
 	{
 	    if (i != FAIL)
 		clear_tv(&rettv);
 	    --emsg_skip;
 	}
-	else if (i != FAIL)
+	else if (i != FAIL || (discovery && save_called_emsg == called_emsg))
 	{
+	    // In Vim9 script discovery "let v: bool = Func()" fails but is
+	    // still a valid declaration.
 	    (void)ex_let_vars(eap->arg, &rettv, FALSE, semicolon, var_count,
 								 flags, op);
 	    clear_tv(&rettv);
@@ -1112,7 +1130,7 @@
 		{
 		    // handle d.key, l[idx], f(expr)
 		    arg_subsc = arg;
-		    if (handle_subscript(&arg, &tv, TRUE, TRUE,
+		    if (handle_subscript(&arg, &tv, EVAL_EVALUATE, TRUE,
 							  name, &name) == FAIL)
 			error = TRUE;
 		    else
@@ -1353,7 +1371,12 @@
 	lval_T	lv;
 
 	p = get_lval(arg, tv, &lv, FALSE, FALSE, 0, FNE_CHECK_START);
-	if (p != NULL && lv.ll_name != NULL)
+	if ((flags & LET_DISCOVERY) && tv->v_type == VAR_UNKNOWN
+							 && lv.ll_type == NULL)
+	{
+	    semsg(_("E1091: type missing for %s"), arg);
+	}
+	else if (p != NULL && lv.ll_name != NULL)
 	{
 	    if (endchars != NULL && vim_strchr(endchars,
 					   *skipwhite(lv.ll_name_end)) == NULL)
@@ -2981,7 +3004,7 @@
 
     if (flags & LET_IS_CONST)
 	di->di_tv.v_lock |= VAR_LOCKED;
-    if (flags & LET_REDEFINE)
+    if (flags & LET_DISCOVERY)
 	di->di_flags |= DI_FLAGS_RELOAD;
 }
 
@@ -3288,7 +3311,8 @@
 	if (n)
 	{
 	    // handle d.key, l[idx], f(expr)
-	    n = (handle_subscript(&var, &tv, TRUE, FALSE, name, &name) == OK);
+	    n = (handle_subscript(&var, &tv, EVAL_EVALUATE,
+						    FALSE, name, &name) == OK);
 	    if (n)
 		clear_tv(&tv);
 	}
diff --git a/src/ex_eval.c b/src/ex_eval.c
index 4aa2658..f725343 100644
--- a/src/ex_eval.c
+++ b/src/ex_eval.c
@@ -879,7 +879,8 @@
 {
     typval_T	tv;
 
-    if (eval0(eap->arg, &tv, &eap->nextcmd, !eap->skip) == OK)
+    if (eval0(eap->arg, &tv, &eap->nextcmd, eap->skip ? 0 : EVAL_EVALUATE)
+									 == OK)
 	clear_tv(&tv);
 }
 
diff --git a/src/list.c b/src/list.c
index 6242e52..b9e7e34 100644
--- a/src/list.c
+++ b/src/list.c
@@ -1046,8 +1046,9 @@
  * Return OK or FAIL.
  */
     int
-get_list_tv(char_u **arg, typval_T *rettv, int evaluate, int do_error)
+get_list_tv(char_u **arg, typval_T *rettv, int flags, int do_error)
 {
+    int		evaluate = flags & EVAL_EVALUATE;
     list_T	*l = NULL;
     typval_T	tv;
     listitem_T	*item;
@@ -1062,7 +1063,7 @@
     *arg = skipwhite(*arg + 1);
     while (**arg != ']' && **arg != NUL)
     {
-	if (eval1(arg, &tv, evaluate) == FAIL)	// recursive!
+	if (eval1(arg, &tv, flags) == FAIL)	// recursive!
 	    goto failret;
 	if (evaluate)
 	{
diff --git a/src/testdir/test_vim9_disassemble.vim b/src/testdir/test_vim9_disassemble.vim
index 7213ee4..2df28e2 100644
--- a/src/testdir/test_vim9_disassemble.vim
+++ b/src/testdir/test_vim9_disassemble.vim
@@ -1054,7 +1054,7 @@
     def FuncTwo(): string
       return 'two'
     enddef
-    let g:res_FuncOne = execute('disass FuncOne')
+    let g:res_FuncOne: string = execute('disass FuncOne')
   END
   writefile(lines, 'Xdisassemble')
   source Xdisassemble
diff --git a/src/testdir/test_vim9_script.vim b/src/testdir/test_vim9_script.vim
index e85da5a..c528b80 100644
--- a/src/testdir/test_vim9_script.vim
+++ b/src/testdir/test_vim9_script.vim
@@ -494,7 +494,7 @@
   def Concat(arg: string): string
     return name .. arg
   enddef
-  let g:result = Concat('bie')
+  let g:result: string = Concat('bie')
   let g:localname = name
 
   export const CONST = 1234
@@ -1633,7 +1633,7 @@
   CheckScriptFailure([
       'vim9script',
       'let g:var = 123',
-      'unlet g:var# comment',
+      'unlet g:var# comment1',
       ], 'E108:')
 
   CheckScriptFailure([
@@ -1643,7 +1643,7 @@
 
   CheckScriptSuccess([
       'vim9script',
-      'if 1 # comment',
+      'if 1 # comment2',
       '  echo "yes"',
       'elseif 2 #comment',
       '  echo "no"',
@@ -1652,14 +1652,14 @@
 
   CheckScriptFailure([
       'vim9script',
-      'if 1# comment',
+      'if 1# comment3',
       '  echo "yes"',
       'endif',
       ], 'E15:')
 
   CheckScriptFailure([
       'vim9script',
-      'if 0 # comment',
+      'if 0 # comment4',
       '  echo "yes"',
       'elseif 2#comment',
       '  echo "no"',
@@ -1668,23 +1668,18 @@
 
   CheckScriptSuccess([
       'vim9script',
-      'let # comment',
+      'let v = 1 # comment5',
       ])
 
   CheckScriptFailure([
       'vim9script',
-      'let# comment',
-      ], 'E121:')
-
-  CheckScriptSuccess([
-      'vim9script',
-      'let v:version # comment',
-      ])
+      'let v = 1# comment6',
+      ], 'E15:')
 
   CheckScriptFailure([
       'vim9script',
-      'let v:version# comment',
-      ], 'E121:')
+      'let v:version',
+      ], 'E1091:')
 
   CheckScriptSuccess([
       'vim9script',
@@ -1722,6 +1717,41 @@
   delete('Xfinished')
 enddef
 
+def Test_let_func_call()
+  let lines =<< trim END
+    vim9script
+    func GetValue()
+      if exists('g:count')
+        let g:count += 1
+      else
+        let g:count = 1
+      endif
+      return 'this'
+    endfunc
+    let val: string = GetValue() 
+  END
+  writefile(lines, 'Xfinished')
+  source Xfinished
+  assert_equal(1, g:count)
+
+  unlet g:count
+  delete('Xfinished')
+enddef
+
+def Test_let_missing_type()
+  let lines =<< trim END
+    vim9script
+    func GetValue()
+      return 'this'
+    endfunc
+    let val = GetValue() 
+  END
+  writefile(lines, 'Xfinished')
+  assert_fails('source Xfinished', 'E1091:')
+
+  delete('Xfinished')
+enddef
+
 " Keep this last, it messes up highlighting.
 def Test_substitute_cmd()
   new
diff --git a/src/userfunc.c b/src/userfunc.c
index 5a64ccb..b3c4f90 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -239,7 +239,7 @@
 		whitep = p;
 		p = skipwhite(p);
 		expr = p;
-		if (eval1(&p, &rettv, FALSE) != FAIL)
+		if (eval1(&p, &rettv, 0) != FAIL)
 		{
 		    if (ga_grow(default_args, 1) == FAIL)
 			goto err_ret;
@@ -572,7 +572,8 @@
 	argp = skipwhite(argp + 1);	    // skip the '(' or ','
 	if (*argp == ')' || *argp == ',' || *argp == NUL)
 	    break;
-	if (eval1(&argp, &argvars[argcount], funcexe->evaluate) == FAIL)
+	if (eval1(&argp, &argvars[argcount],
+				funcexe->evaluate ? EVAL_EVALUATE : 0) == FAIL)
 	{
 	    ret = FAIL;
 	    break;
@@ -1223,7 +1224,7 @@
 
 		default_expr = ((char_u **)(fp->uf_def_args.ga_data))
 						 [ai + fp->uf_def_args.ga_len];
-		if (eval1(&default_expr, &def_rettv, TRUE) == FAIL)
+		if (eval1(&default_expr, &def_rettv, EVAL_EVALUATE) == FAIL)
 		{
 		    default_arg_err = 1;
 		    break;
@@ -1368,7 +1369,7 @@
 	// A Lambda always has the command "return {expr}".  It is much faster
 	// to evaluate {expr} directly.
 	++ex_nesting_level;
-	(void)eval1(&p, rettv, TRUE);
+	(void)eval1(&p, rettv, EVAL_EVALUATE);
 	--ex_nesting_level;
     }
     else
@@ -3623,7 +3624,8 @@
 
     eap->nextcmd = NULL;
     if ((*arg != NUL && *arg != '|' && *arg != '\n')
-	    && eval0(arg, &rettv, &eap->nextcmd, !eap->skip) != FAIL)
+	    && eval0(arg, &rettv, &eap->nextcmd, eap->skip ? 0 : EVAL_EVALUATE)
+								       != FAIL)
     {
 	if (!eap->skip)
 	    returning = do_return(eap, FALSE, TRUE, &rettv);
@@ -3680,7 +3682,7 @@
 	// instead to skip to any following command, e.g. for:
 	//   :if 0 | call dict.foo().bar() | endif
 	++emsg_skip;
-	if (eval0(eap->arg, &rettv, &eap->nextcmd, FALSE) != FAIL)
+	if (eval0(eap->arg, &rettv, &eap->nextcmd, 0) != FAIL)
 	    clear_tv(&rettv);
 	--emsg_skip;
 	return;
@@ -3768,8 +3770,8 @@
 	    dbg_check_breakpoint(eap);
 
 	// Handle a function returning a Funcref, Dictionary or List.
-	if (handle_subscript(&arg, &rettv, !eap->skip, TRUE,
-							  name, &name) == FAIL)
+	if (handle_subscript(&arg, &rettv, eap->skip ? 0 : EVAL_EVALUATE,
+						    TRUE, name, &name) == FAIL)
 	{
 	    failed = TRUE;
 	    break;
diff --git a/src/version.c b/src/version.c
index 3f67db4..ad56b7a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    753,
+/**/
     752,
 /**/
     751,
diff --git a/src/vim.h b/src/vim.h
index 9e4c61d..667e290 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -2133,7 +2133,7 @@
 // Flags for assignment functions.
 #define LET_IS_CONST	1   // ":const"
 #define LET_NO_COMMAND	2   // "var = expr" without ":let" or ":const"
-#define LET_REDEFINE	4   // variable can be redefined later
+#define LET_DISCOVERY	4   // discovery phase: variable can be redefined later
 
 #include "ex_cmds.h"	    // Ex command defines
 #include "spell.h"	    // spell checking stuff
@@ -2662,4 +2662,8 @@
 #define REPTERM_SPECIAL		4
 #define REPTERM_NO_SIMPLIFY	8
 
+// Flags for expression evaluation.
+#define EVAL_EVALUATE	    1	    // when missing don't actually evaluate
+#define EVAL_CONSTANT	    2	    // when not a constant return FAIL
+
 #endif // VIM__H
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 99246e0..4b294df 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -2869,14 +2869,14 @@
     {
 
 	// Can be "[1, 2, 3]->Func()".
-	if (get_list_tv(&p, &rettv, FALSE, FALSE) == FAIL)
+	if (get_list_tv(&p, &rettv, 0, FALSE) == FAIL)
 	    p = arg;
     }
     else if (p == arg && *arg == '#' && arg[1] == '{')
     {
 	// Can be "#{a: 1}->Func()".
 	++p;
-	if (eval_dict(&p, &rettv, FALSE, TRUE) == FAIL)
+	if (eval_dict(&p, &rettv, 0, TRUE) == FAIL)
 	    p = arg;
     }
     else if (p == arg && *arg == '{')
@@ -2886,7 +2886,7 @@
 	// Can be "{x -> ret}()".
 	// Can be "{'a': 1}->Func()".
 	if (ret == NOTDONE)
-	    ret = eval_dict(&p, &rettv, FALSE, FALSE);
+	    ret = eval_dict(&p, &rettv, 0, FALSE);
 	if (ret != OK)
 	    p = arg;
     }