Fix #8450. Prevented internal compiler errors when assigning nested tuples.

test/libsolidity/semanticTests/types/tuple_in_tuple.sol

@@ -0,0 +1,24 @@
+contract test {
+    function f0() public returns(int, bool) {
+        int a;
+        bool b;
+        ((a, b)) = (2, true);
+        return (a, b);
+    }
+    function f1() public returns(int) {
+        int a;
+        (((a, ), )) = ((1, 2) ,3);
+        return a;
+    }
+    function f2() public returns(int) {
+        int a;
+        (((, a),)) = ((1, 2), 3);
+        return a;
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// f0() -> 2, true
+// f1() -> 1
+// f2() -> 2

test/libsolidity/syntaxTests/tupleAssignments/tuple_in_tuple_long.sol

@@ -0,0 +1,12 @@
+contract C {
+  function f() {
+    (((((((((((,2),)),)),),))=4)));
+  }
+}
+// ----
+// SyntaxError: (15-69): No visibility specified. Did you intend to add "public"?
+// TypeError: (46-47): Expression has to be an lvalue.
+// TypeError: (60-61): Type int_const 4 is not implicitly convertible to expected type tuple(tuple(tuple(tuple(tuple(,int_const 2),),),),).
+// TypeError: (37-61): Tuple component cannot be empty.
+// TypeError: (36-62): Tuple component cannot be empty.
+// TypeError: (35-63): Tuple component cannot be empty.

test/libsolidity/syntaxTests/tupleAssignments/tuple_in_tuple_short.sol

@@ -0,0 +1,6 @@
+contract C {
+  function f() public pure {
+    int a;
+    (((a,),)) = ((1,2),3);
+  }
+}