vim-patch:8.1.1114: confusing overloaded operator "." for string concatenation

Problem:    Confusing overloaded operator "." for string concatenation.
Solution:   Add ".." for string concatenation.  Also "let a ..= b".
0f248b006c

runtime/doc/eval.txt

@@ -610,10 +610,10 @@ Expression syntax summary, from least to most significant:
 	expr2 ? expr1 : expr1	if-then-else
 
 |expr2|	expr3
-	expr3 || expr3 ..	logical OR
+	expr3 || expr3 ...	logical OR
 
 |expr3|	expr4
-	expr4 && expr4 ..	logical AND
+	expr4 && expr4 ...	logical AND
 
 |expr4|	expr5
 	expr5 == expr5		equal
@@ -634,9 +634,10 @@ Expression syntax summary, from least to most significant:
 	expr5 isnot expr5	different |List| instance
 
 |expr5|	expr6
-	expr6 +	 expr6 ..	number addition or list concatenation
-	expr6 -	 expr6 ..	number subtraction
-	expr6 .	 expr6 ..	string concatenation
+	expr6 +	  expr6 ..	number addition or list concatenation
+	expr6 -	  expr6 ..	number subtraction
+	expr6 .	  expr6 ..	string concatenation
+	expr6 ..  expr6 ..	string concatenation
 
 |expr6|	expr7
 	expr7 *	 expr7 ..	number multiplication
@@ -670,7 +671,7 @@ Expression syntax summary, from least to most significant:
 	{args -> expr1}		lambda expression
 
 
-".." indicates that the operations in this level can be concatenated.
+"..." indicates that the operations in this level can be concatenated.
 Example: >
 	&nu || &list && &shell == "csh"
 
@@ -850,10 +851,14 @@ expr5 and expr6						*expr5* *expr6*
 expr6 +	 expr6 ..	Number addition or |List| concatenation	*expr-+*
 expr6 -	 expr6 ..	Number subtraction			*expr--*
 expr6 .	 expr6 ..	String concatenation			*expr-.*
+expr6 ..	 expr6 ..	String concatenation			*expr-..*
 
 For |Lists| only "+" is possible and then both expr6 must be a list.  The
 result is a new list with the two lists Concatenated.
 
+For String concatenation ".." is preferred, since "." is ambiguous, it is also
+used for |Dict| member access and floating point numbers.
+
 expr7 *	 expr7 ..	Number multiplication			*expr-star*
 expr7 /	 expr7 ..	Number division				*expr-/*
 expr7 %	 expr7 ..	Number modulo				*expr-%*

src/nvim/eval.c

@@ -1446,6 +1446,7 @@ int eval_foldexpr(char_u *arg, int *cp)
  * ":let var /= expr"		assignment command.
  * ":let var %= expr"		assignment command.
  * ":let var .= expr"		assignment command.
+ * ":let var ..= expr"		assignment command.
  * ":let [var1, var2] = expr"	unpack list.
  */
 void ex_let(exarg_T *eap)
@@ -1468,8 +1469,8 @@ void ex_let(exarg_T *eap)
     argend--;
   }
   expr = skipwhite(argend);
-  if (*expr != '=' && !(vim_strchr((char_u *)"+-*/%.", *expr) != NULL
-                        && expr[1] == '=')) {
+  if (*expr != '=' && !((vim_strchr((char_u *)"+-*/%.", *expr) != NULL
+                         && expr[1] == '=') || STRNCMP(expr, "..=", 3) == 0)) {
     // ":let" without "=": list variables
     if (*arg == '[') {
       EMSG(_(e_invarg));
@@ -1493,6 +1494,9 @@ void ex_let(exarg_T *eap)
     if (*expr != '=') {
       if (vim_strchr((char_u *)"+-*/%.", *expr) != NULL) {
         op[0] = *expr;  // +=, -=, *=, /=, %= or .=
+        if (expr[0] == '.' && expr[1] == '.') {  // ..=
+          expr++;
+        }
       }
       expr = skipwhite(expr + 2);
     } else {
@@ -3789,6 +3793,7 @@ static int eval4(char_u **arg, typval_T *rettv, int evaluate)
  *	+	number addition
  *	-	number subtraction
  *	.	string concatenation
+ *	..	string concatenation
  *
  * "arg" must point to the first non-white of the expression.
  * "arg" is advanced to the next non-white after the recognized expression.
@@ -3836,6 +3841,9 @@ static int eval5(char_u **arg, typval_T *rettv, int evaluate)
     /*
      * Get the second variable.
      */
+    if (op == '.' && *(*arg + 1) == '.') {  // ..string concatenation
+      (*arg)++;
+    }
     *arg = skipwhite(*arg + 1);
     if (eval6(arg, &var2, evaluate, op == '.') == FAIL) {
       tv_clear(rettv);

src/nvim/testdir/test_eval_stuff.vim

@@ -49,3 +49,32 @@ func Test_line_continuation()
 	"\ and some more
   call assert_equal([5, 6], array)
 endfunc
+
+func Test_string_concatenation()
+  call assert_equal('ab', 'a'.'b')
+  call assert_equal('ab', 'a' .'b')
+  call assert_equal('ab', 'a'. 'b')
+  call assert_equal('ab', 'a' . 'b')
+
+  call assert_equal('ab', 'a'..'b')
+  call assert_equal('ab', 'a' ..'b')
+  call assert_equal('ab', 'a'.. 'b')
+  call assert_equal('ab', 'a' .. 'b')
+
+  let a = 'a'
+  let b = 'b'
+  let a .= b
+  call assert_equal('ab', a)
+
+  let a = 'a'
+  let a.=b
+  call assert_equal('ab', a)
+
+  let a = 'a'
+  let a ..= b
+  call assert_equal('ab', a)
+
+  let a = 'a'
+  let a..=b
+  call assert_equal('ab', a)
+endfunc