[Parser] Hoist up 'try' exprs out from sequence exprs in the parser.

If the left-most sequence expr is a 'try', hoist it up to turn
'(try x) + y' into 'try (x + y)'. This is necessary to do in the
parser because 'try' nodes are represented in the ASTScope tree
to look up catch nodes. The scope tree must be syntactic because
it's constructed before sequence folding happens during preCheckExpr.
Otherwise, catch node lookup would find the incorrect catch node for
'try x + y' at the source location for 'y'.

'try' has restrictions for where it can appear within a sequence
expr. This is still diagnosed in TypeChecker::foldSequence.

Author: hborla

lib/Parse/ParseExpr.cpp

@@ -372,6 +372,23 @@ ParserResult<Expr> Parser::parseExprSequence(Diag<> Message,
   if (SequencedExprs.size() == 1)
     return makeParserResult(SequenceStatus, SequencedExprs[0]);
 
+  // If the left-most sequence expr is a 'try', hoist it up to turn
+  // '(try x) + y' into 'try (x + y)'. This is necessary to do in the
+  // parser because 'try' nodes are represented in the ASTScope tree
+  // to look up catch nodes. The scope tree must be syntactic because
+  // it's constructed before sequence folding happens during preCheckExpr.
+  // Otherwise, catch node lookup would find the incorrect catch node for
+  // 'try x + y' at the source location for 'y'.
+  //
+  // 'try' has restrictions for where it can appear within a sequence
+  // expr. This is still diagnosed in TypeChecker::foldSequence.
+  if (auto *tryEval = dyn_cast<AnyTryExpr>(SequencedExprs[0])) {
+    SequencedExprs[0] = tryEval->getSubExpr();
+    auto *sequence = SequenceExpr::create(Context, SequencedExprs);
+    tryEval->setSubExpr(sequence);
+    return makeParserResult(SequenceStatus, tryEval);
+  }
+
   return makeParserResult(SequenceStatus,
                           SequenceExpr::create(Context, SequencedExprs));
 }