[Sema] Re-enable placeholder type parsing and folding

Author: Jumhyn

lib/Parse/ParsePattern.cpp

@@ -152,10 +152,6 @@ static ParserStatus parseDefaultArgument(
 /// Determine whether we are at the start of a parameter name when
 /// parsing a parameter.
 bool Parser::startsParameterName(bool isClosure) {
-  // '_' cannot be a type, so it must be a parameter name.
-  if (Tok.is(tok::kw__))
-    return true;
-
   // To have a parameter name here, we need a name.
   if (!Tok.canBeArgumentLabel())
     return false;

lib/Parse/ParseType.cpp

@@ -157,6 +157,7 @@ LayoutConstraint Parser::parseLayoutConstraint(Identifier LayoutConstraintID) {
 ///     type-simple '!'
 ///     type-collection
 ///     type-array
+///     '_'
 ParserResult<TypeRepr> Parser::parseTypeSimple(
     Diag<> MessageID, ParseTypeReason reason) {
   ParserResult<TypeRepr> ty;
@@ -189,6 +190,9 @@ ParserResult<TypeRepr> Parser::parseTypeSimple(
     ty = parseTypeCollection();
     break;
   }
+  case tok::kw__:
+    ty = makeParserResult(new (Context) PlaceholderTypeRepr(consumeToken()));
+    break;
   case tok::kw_protocol:
     if (startsWithLess(peekToken())) {
       ty = parseOldStyleProtocolComposition();
@@ -1469,6 +1473,9 @@ bool Parser::canParseType() {
     if (!consumeIf(tok::r_square))
       return false;
     break;
+  case tok::kw__:
+    consumeToken();
+    break;
 
 
   default:

lib/Sema/PreCheckExpr.cpp

@@ -1534,8 +1534,18 @@ TypeExpr *PreCheckExpression::simplifyTypeExpr(Expr *E) {
     return simplifyNestedTypeExpr(UDE);
   }
 
-  // TODO: Fold DiscardAssignmentExpr into a placeholder type here once parsing
-  // them is supported.
+  // Fold '_' into a placeholder type if it hasn't been marked as a "correct"
+  // discard assignment expr (on the LHS of an assignment). If we're inside a
+  // SequenceExpr, skip this step since we might have a correct
+  // DiscardAssignmentExpr that hasn't been marked as correct yet. The whole
+  // tree will be rechecked once SequenceExpr folding completes, anyway.
+  if (auto *DAE = dyn_cast<DiscardAssignmentExpr>(E)) {
+    if (!CorrectDiscardAssignmentExprs.count(DAE) && SequenceExprDepth == 0) {
+      auto *placeholderRepr =
+          new (getASTContext()) PlaceholderTypeRepr(DAE->getLoc());
+      return new (getASTContext()) TypeExpr(placeholderRepr);
+    }
+  }
 
   // Fold T? into an optional type when T is a TypeExpr.
   if (isa<OptionalEvaluationExpr>(E) || isa<BindOptionalExpr>(E)) {
@@ -1742,6 +1752,8 @@ TypeExpr *PreCheckExpression::simplifyTypeExpr(Expr *E) {
         return nullptr;
       if (auto *TyE = dyn_cast<TypeExpr>(E))
         return TyE->getTypeRepr();
+      if (auto *DAE = dyn_cast<DiscardAssignmentExpr>(E))
+        return new (getASTContext()) PlaceholderTypeRepr(DAE->getLoc());
       if (auto *TE = dyn_cast<TupleExpr>(E))
         if (TE->getNumElements() == 0)
           return TupleTypeRepr::createEmpty(ctx, TE->getSourceRange());