round out tuple unpacking assignment, support underscores (#22537)

* round out tuple unpacking assignment, support underscores

fixes #18710

* fix test messages

* use discard instead of continue

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

---------

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>
(cherry picked from commit 53d43e9671)

tests/arc/topt_no_cursor.nim

@@ -39,13 +39,13 @@ var
   lresult
   lvalue
   lnext
-  _
+  tmpTupleAsgn
 lresult = @[123]
-_ = (
+tmpTupleAsgn = (
   let blitTmp = lresult
   blitTmp, ";")
-lvalue = _[0]
-lnext = _[1]
+lvalue = tmpTupleAsgn[0]
+lnext = tmpTupleAsgn[1]
 `=sink`(result.value, move lvalue)
 `=destroy`(lnext)
 `=destroy_1`(lvalue)

tests/errmsgs/tassignunpack.nim

@@ -1,3 +1,3 @@
 var a, b = 0
 (a, b) = 1 #[tt.Error
-         ^ type mismatch: got <int literal(1)> but expected 'tuple']#
+         ^ 'tuple' expected]#

tests/tuples/ttuples_various.nim

@@ -197,3 +197,15 @@ block: # bug #22054
 
   var v = A(field: (a: 1314))
   doAssert get(v)[0] == 1314
+
+block: # tuple unpacking assignment with underscore
+  var
+    a = 1
+    b = 2
+  doAssert (a, b) == (1, 2)
+  (a, _) = (3, 4)
+  doAssert (a, b) == (3, 2)
+  (_, a) = (5, 6)
+  doAssert (a, b) == (6, 2)
+  (b, _) = (7, 8)
+  doAssert (a, b) == (6, 7)

tests/types/tassignemptytuple.nim

@@ -1,5 +1,5 @@
 discard """
-  errormsg: "invalid type: 'empty' in this context: '(seq[empty], (seq[empty], set[empty]))' for let"
+  errormsg: "cannot infer the type of the tuple"
   file: "tassignemptytuple.nim"
   line: 11
 """