patch 8.2.4934: string interpolation fails when not evaluating

Problem:    String interpolation fails when not evaluating.
Solution:   Skip the expression when not evaluating. (closes #10398)

src/evalvars.c

@@ -605,10 +605,11 @@ list_script_vars(int *first)
 /*
  * Evaluate one Vim expression {expr} in string "p" and append the
  * resulting string to "gap".  "p" points to the opening "{".
+ * When "evaluate" is FALSE only skip over the expression.
  * Return a pointer to the character after "}", NULL for an error.
  */
     char_u *
-eval_one_expr_in_str(char_u *p, garray_T *gap)
+eval_one_expr_in_str(char_u *p, garray_T *gap, int evaluate)
 {
     char_u	*block_start = skipwhite(p + 1);  // skip the opening {
     char_u	*block_end = block_start;
@@ -627,6 +628,8 @@ eval_one_expr_in_str(char_u *p, garray_T *gap)
 	semsg(_(e_missing_close_curly_str), p);
 	return NULL;
     }
+    if (evaluate)
+    {
 	*block_end = NUL;
 	expr_val = eval_to_string(block_start, TRUE);
 	*block_end = '}';
@@ -634,6 +637,7 @@ eval_one_expr_in_str(char_u *p, garray_T *gap)
 	    return NULL;
 	ga_concat(gap, expr_val);
 	vim_free(expr_val);
+    }
 
     return block_end + 1;
 }
@@ -691,7 +695,7 @@ eval_all_expr_in_str(char_u *str)
 	}
 
 	// Evaluate the expression and append the result.
-	p = eval_one_expr_in_str(p, &ga);
+	p = eval_one_expr_in_str(p, &ga, TRUE);
 	if (p == NULL)
 	{
 	    ga_clear(&ga);

src/proto/evalvars.pro

@@ -13,7 +13,7 @@ list_T *eval_spell_expr(char_u *badword, char_u *expr);
 int get_spellword(list_T *list, char_u **pp);
 void prepare_vimvar(int idx, typval_T *save_tv);
 void restore_vimvar(int idx, typval_T *save_tv);
-char_u *eval_one_expr_in_str(char_u *p, garray_T *gap);
+char_u *eval_one_expr_in_str(char_u *p, garray_T *gap, int evaluate);
 char_u *eval_all_expr_in_str(char_u *str);
 list_T *heredoc_get(exarg_T *eap, char_u *cmd, int script_get, int vim9compile);
 void ex_var(exarg_T *eap);

src/testdir/test_vim9_expr.vim

@@ -2156,6 +2156,13 @@ def Test_expr8_string()
               ->split($'x{x}x')
               ->map((_, v: string) => v =~ 'bar')
   assert_equal([false, true, false], vl)
+
+  # interpolated string in a lambda
+  lines =<< trim END
+      assert_equal(['gnome-256color', 'xterm-256color'], ['gnome', 'xterm']
+              ->map((_, term: string) => $'{term}-256color'))
+  END
+  v9.CheckDefAndScriptSuccess(lines)
 enddef
 
 def Test_expr8_vimvar()

src/typval.c

@@ -2363,7 +2363,7 @@ eval_interp_string(char_u **arg, typval_T *rettv, int evaluate)
 	    ++*arg;
 	    break;
 	}
-	p = eval_one_expr_in_str(*arg, &ga);
+	p = eval_one_expr_in_str(*arg, &ga, evaluate);
 	if (p == NULL)
 	{
 	    ret = FAIL;

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4934,
 /**/
     4933,
 /**/