[CSDiag] Mark all sub-expressions which are about to be re-typechecked

Currently that has been limited only to expressions which aren't
flagged as `TCC_ForceRecheck`, but it should be extended to all
sub-expressions, otherwise we might produce invalid diagnostics
in `PreCheckExpression`.

Author: xedin

lib/Sema/CSDiag.cpp

@@ -1753,10 +1753,11 @@ Expr *FailureDiagnosis::typeCheckChildIndependently(
       CS.cacheExprTypes(savedExpr);
       return savedExpr;
     }
-
-    CS.TC.addExprForDiagnosis(subExpr, subExpr);
   }
 
+  // Mark current expression as about to be diagnosed.
+  CS.TC.addExprForDiagnosis(subExpr, subExpr);
+
   // Validate contextual type before trying to use it.
   std::tie(convertType, convertTypePurpose) =
       validateContextualType(convertType, convertTypePurpose);
@@ -5651,6 +5652,15 @@ bool FailureDiagnosis::visitAssignExpr(AssignExpr *assignExpr) {
     return true;
   }
 
+  // Situation like `var foo = &bar` didn't get diagnosed early
+  // because originally its parent is a `SequenceExpr` which hasn't
+  // been folded yet, and could represent an operator which accepts
+  // `inout` arguments.
+  if (auto *AddrOf = dyn_cast<InOutExpr>(assignExpr->getSrc())) {
+    diagnose(AddrOf->getLoc(), diag::extraneous_address_of);
+    return true;
+  }
+
   if (CS.TC.diagnoseSelfAssignment(assignExpr))
     return true;