[Sema] Rework DiscardAssignmentExpr checking wrt SequenceExpr folding

Now that we're checking DiscardAssignmentExprs in PreCheckExpr, we have to be careful to make sure we don't diagnose anything until after SequenceExprs have been folded, since the relevant AssignExprs won't even have a "right hand side" and "left hand side" until after folding.

lib/Sema/PreCheckExpr.cpp

@@ -814,6 +814,9 @@ namespace {
     /// Keep track of acceptable DiscardAssignmentExpr's.
     llvm::SmallPtrSet<DiscardAssignmentExpr*, 2> CorrectDiscardAssignmentExprs;
 
+    /// The current number of nested \c SequenceExprs that we're within.
+    unsigned SequenceExprDepth = 0;
+
     /// Simplify expressions which are type sugar productions that got parsed
     /// as expressions due to the parser not knowing which identifiers are
     /// type names.
@@ -1145,6 +1148,9 @@ namespace {
       if (auto *assignment = dyn_cast<AssignExpr>(expr))
         markAcceptableDiscardExprs(assignment->getDest());
 
+      if (isa<SequenceExpr>(expr))
+        SequenceExprDepth++;
+
       return finish(true, expr);
     }
 
@@ -1160,6 +1166,7 @@ namespace {
       // Fold sequence expressions.
       if (auto *seqExpr = dyn_cast<SequenceExpr>(expr)) {
         auto result = TypeChecker::foldSequence(seqExpr, DC);
+        SequenceExprDepth--;
         return result->walk(*this);
       }
 
@@ -1280,6 +1287,19 @@ namespace {
       if (auto *simplified = simplifyTypeExpr(expr))
         return simplified;
 
+      // Diagnose a '_' that isn't on the immediate LHS of an assignment. We
+      // skip diagnostics if we've explicitly marked the expression as valid,
+      // or if we're inside a SequenceExpr (since the whole tree will be
+      // re-checked when we finish folding anyway).
+      if (auto *DAE = dyn_cast<DiscardAssignmentExpr>(expr)) {
+        if (!CorrectDiscardAssignmentExprs.count(DAE) &&
+            SequenceExprDepth == 0) {
+          ctx.Diags.diagnose(expr->getLoc(),
+                             diag::discard_expr_outside_of_assignment);
+          return nullptr;
+        }
+      }
+
       if (auto KPE = dyn_cast<KeyPathExpr>(expr)) {
         resolveKeyPathExpr(KPE);
         return KPE;