Fix nested look-ahead switches.

Closes #1810.

spicy/toolchain/src/compiler/parser/parser.yy

@@ -903,6 +903,7 @@ unit_switch_case
               | exprs ARROW unit_item            { $$ = builder->typeUnitItemSwitchCase($1, {$3}, __loc__); }
               | '*'   ARROW unit_item            { $$ = builder->typeUnitItemSwitchCase(type::unit::Items{$3}, __loc__); }
               | ARROW unit_field                 { $$ = builder->typeUnitItemSwitchCase($2, __loc__); }
+              | ARROW unit_switch                { $$ = builder->typeUnitItemSwitchCase($2, __loc__); }
 
 /* --- End of Spicy units --- */
 

tests/Baseline/spicy.types.unit.switch-nested-lahead/output

@@ -0,0 +1,11 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+foo::X {
+  a: a
+}
+foo::X {
+  b: b
+}
+foo::X {
+  c: c
+}
+[fatal error] terminating with uncaught exception of type spicy::rt::ParseError: no expected look-ahead token found (<...>/switch-nested-lahead.spicy:18:10-18:20)

tests/spicy/types/unit/switch-nested-lahead.spicy

@@ -0,0 +1,20 @@
+# @TEST-EXEC: spicyc %INPUT -j -o %INPUT.hlto
+# @TEST-EXEC: ${SCRIPTS}/printf a | spicy-dump %INPUT.hlto >>output 2>&1
+# @TEST-EXEC: ${SCRIPTS}/printf b | spicy-dump %INPUT.hlto >>output 2>&1
+# @TEST-EXEC: ${SCRIPTS}/printf c | spicy-dump %INPUT.hlto >>output 2>&1
+# @TEST-EXEC-FAIL: ${SCRIPTS}/printf d | spicy-dump %INPUT.hlto >>output 2>&1
+# @TEST-EXEC: btest-diff output
+#
+# @TEST-DOC: Check nested switch constructs both using look-ahead.
+
+module foo;
+
+public type X = unit {
+     switch {
+         -> switch {
+             -> a: b"a";
+             -> b: b"b";
+         };
+         -> c: b"c";
+     };
+ };