Fix nested look-ahead parsing.

When parsing nested vectors all using look-ahead, we need to return
control back to upper level when an inner look-ahead isn't found.

This may change the error message for "normal" look-ahead parsing (see
test baseline), but the new one seems fine and potentially even
better.

Closes #1844.

(cherry picked from commit 2223f18)

spicy/toolchain/src/compiler/codegen/parser-builder.cc

@@ -1787,18 +1787,15 @@ struct ProductionVisitor : public production::Visitor {
         }
 
         auto builder_default = switch_.addDefault();
-        pushBuilder(builder_default);
-        pb->parseError("no expected look-ahead token found", p.location());
-        popBuilder();
 
         if ( p.condition() )
             popBuilder();
 
-        return std::make_pair(builder_alt1, builder_alt2);
+        return std::make_tuple(builder_alt1, builder_alt2, builder_default);
     }
 
     void operator()(const production::LookAhead* p) final {
-        auto [builder_alt1, builder_alt2] = parseLookAhead(*p);
+        auto [builder_alt1, builder_alt2, builder_default] = parseLookAhead(*p);
 
         pushBuilder(builder_alt1);
         parseProduction(*p->alternatives().first);
@@ -1807,6 +1804,8 @@ struct ProductionVisitor : public production::Visitor {
         pushBuilder(builder_alt2);
         parseProduction(*p->alternatives().second);
         popBuilder();
+
+        pushBuilder(builder_default, [&]() { pb->parseError("no expected look-ahead token found", p->location()); });
     }
 
     void operator()(const production::Sequence* p) final {
@@ -1897,7 +1896,10 @@ struct ProductionVisitor : public production::Visitor {
 
                 std::shared_ptr<Builder> builder_alt1;
                 std::shared_ptr<Builder> builder_alt2;
-                auto parse = [&]() { std::tie(builder_alt1, builder_alt2) = parseLookAhead(*lah_prod); };
+                std::shared_ptr<Builder> builder_default;
+                auto parse = [&]() {
+                    std::tie(builder_alt1, builder_alt2, builder_default) = parseLookAhead(*lah_prod);
+                };
 
                 // If the list field generating this While is a synchronization point, set up a try/catch block
                 // for internal list synchronization (failure to parse a list element tries to synchronize at
@@ -1920,10 +1922,19 @@ struct ProductionVisitor : public production::Visitor {
 
                                     syncProductionNext(*p);
                                 });
+
+                    pushBuilder(builder_default,
+                                [&]() { pb->parseError("no expected look-ahead token found", p->location()); });
                 }
-                else
+                else {
                     parse();
 
+                    pushBuilder(builder_default, [&]() {
+                        // Terminate loop.
+                        builder()->addBreak();
+                    });
+                }
+
                 pushBuilder(builder_alt1, [&]() {
                     // Terminate loop.
                     builder()->addBreak();

tests/Baseline/spicy.types.vector.parse-lahead-int/output

@@ -1,4 +1,4 @@
 ### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
 [$a=[[$x=1, $y=2571], [$x=1, $y=3085]], $b=2, $c=3599]
 [$a=[], $b=2, $c=3599]
-[error] terminating with uncaught exception of type spicy::rt::ParseError: no expected look-ahead token found (<...>/parse-lahead-int.spicy:15:5-15:14)
+[error] terminating with uncaught exception of type spicy::rt::ParseError: expecting uint8(2) (<...>/parse-lahead-int.spicy:16:8-16:18)

tests/Baseline/spicy.types.vector.parse-lahead-nested/output

@@ -0,0 +1,29 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+Test::X {
+  y: [
+    Test::Y {
+      z: [
+        Test::Z {
+          zb: 1
+        }
+        Test::Z {
+          zb: 2
+        }
+        Test::Z {
+          zb: 3
+        }
+      ]
+    }
+    Test::Y {
+      z: []
+    }
+    Test::Y {
+      z: [
+        Test::Z {
+          zb: 1
+        }
+      ]
+    }
+  ]
+  x: x
+}

tests/spicy/types/vector/parse-lahead-nested.spicy

@@ -0,0 +1,21 @@
+# @TEST-EXEC: printf 'yz1z2z3yyz1x' | spicy-dump  %INPUT >output
+# @TEST-EXEC: btest-diff output
+#
+# @TEST-DOC: Parse two nested vectors each using look-ahead; regression test for #1844.
+
+module Test;
+
+type Z = unit {
+    : /z/;
+    zb: bytes &size=1;
+};
+
+type Y = unit {
+    : /y/;
+    z: Z[];
+};
+
+public type X = unit {
+    y: Y[];
+    x: /x/;
+};