[CodeCompletion] Allow preChecking an expression twice in code completion

lib/Sema/TypeCheckExpr.cpp

@@ -355,31 +355,51 @@ static Expr *makeBinOp(ASTContext &Ctx, Expr *Op, Expr *LHS, Expr *RHS,
 
   if (auto *ifExpr = dyn_cast<IfExpr>(Op)) {
     // Resolve the ternary expression.
-    assert(!ifExpr->isFolded() && "already folded if expr in sequence?!");
+    if (!Ctx.CompletionCallback) {
+      // In code completion we might call preCheckExpression twice - once for
+      // the first pass and once for the second pass. This is fine since
+      // preCheckExpression idempotent.
+      assert(!ifExpr->isFolded() && "already folded if expr in sequence?!");
+    }
     ifExpr->setCondExpr(LHS);
     ifExpr->setElseExpr(RHS);
     return ifExpr;
   }
 
   if (auto *assign = dyn_cast<AssignExpr>(Op)) {
     // Resolve the assignment expression.
-    assert(!assign->isFolded() && "already folded assign expr in sequence?!");
+    if (!Ctx.CompletionCallback) {
+      // In code completion we might call preCheckExpression twice - once for
+      // the first pass and once for the second pass. This is fine since
+      // preCheckExpression idempotent.
+      assert(!assign->isFolded() && "already folded assign expr in sequence?!");
+    }
     assign->setDest(LHS);
     assign->setSrc(RHS);
     return assign;
   }
 
   if (auto *as = dyn_cast<ExplicitCastExpr>(Op)) {
     // Resolve the 'as' or 'is' expression.
-    assert(!as->isFolded() && "already folded 'as' expr in sequence?!");
+    if (!Ctx.CompletionCallback) {
+      // In code completion we might call preCheckExpression twice - once for
+      // the first pass and once for the second pass. This is fine since
+      // preCheckExpression idempotent.
+      assert(!as->isFolded() && "already folded 'as' expr in sequence?!");
+    }
     assert(RHS == as && "'as' with non-type RHS?!");
     as->setSubExpr(LHS);    
     return as;
   }
 
   if (auto *arrow = dyn_cast<ArrowExpr>(Op)) {
     // Resolve the '->' expression.
-    assert(!arrow->isFolded() && "already folded '->' expr in sequence?!");
+    if (!Ctx.CompletionCallback) {
+      // In code completion we might call preCheckExpression twice - once for
+      // the first pass and once for the second pass. This is fine since
+      // preCheckExpression idempotent.
+      assert(!arrow->isFolded() && "already folded '->' expr in sequence?!");
+    }
     arrow->setArgsExpr(LHS);
     arrow->setResultExpr(RHS);
     return arrow;

validation-test/IDE/crashers_2_fixed/sr14755.swift

@@ -0,0 +1,3 @@
+// RUN: %swift-ide-test --code-completion --source-filename %s --code-completion-token=COMPLETE
+
+Foo.#^COMPLETE^#bar ?? baz(_) = baba