Index: clang/lib/Sema/SemaExprCXX.cpp
===================================================================
--- clang/lib/Sema/SemaExprCXX.cpp
+++ clang/lib/Sema/SemaExprCXX.cpp
@@ -567,11 +567,14 @@
       //   polymorphic class type [...] [the] expression is an unevaluated
       //   operand. [...]
       if (RecordD->isPolymorphic() && E->isGLValue()) {
-        // The subexpression is potentially evaluated; switch the context
-        // and recheck the subexpression.
-        ExprResult Result = TransformToPotentiallyEvaluated(E);
-        if (Result.isInvalid()) return ExprError();
-        E = Result.get();
+        if (isUnevaluatedContext()) {
+          // The operand was processed in unevaluated context, switch the
+          // context and recheck the subexpression.
+          ExprResult Result = TransformToPotentiallyEvaluated(E);
+          if (Result.isInvalid())
+            return ExprError();
+          E = Result.get();
+        }
 
         // We require a vtable to query the type at run time.
         MarkVTableUsed(TypeidLoc, RecordD);
Index: clang/lib/Sema/TreeTransform.h
===================================================================
--- clang/lib/Sema/TreeTransform.h
+++ clang/lib/Sema/TreeTransform.h
@@ -11561,15 +11561,20 @@
                                              TInfo, E->getEndLoc());
   }
 
-  // We don't know whether the subexpression is potentially evaluated until
-  // after we perform semantic analysis.  We speculatively assume it is
-  // unevaluated; it will get fixed later if the subexpression is in fact
-  // potentially evaluated.
-  EnterExpressionEvaluationContext Unevaluated(
-      SemaRef, Sema::ExpressionEvaluationContext::Unevaluated,
-      Sema::ReuseLambdaContextDecl);
-
-  ExprResult SubExpr = getDerived().TransformExpr(E->getExprOperand());
+  // Typeid's operand is an unevaluated context, unless it's a polymorphic
+  // type.  We must not unilaterally enter unevaluated context here, as then
+  // semantic processing can re-transform an already transformed operand.
+  Expr *Op = E->getExprOperand();
+  auto EvalCtx = Sema::ExpressionEvaluationContext::Unevaluated;
+  if (E->isGLValue())
+    if (auto *RecordT = Op->getType()->getAs<RecordType>())
+      if (cast<CXXRecordDecl>(RecordT->getDecl())->isPolymorphic())
+        EvalCtx = SemaRef.ExprEvalContexts.back().Context;
+
+  EnterExpressionEvaluationContext Unevaluated(SemaRef, EvalCtx,
+                                               Sema::ReuseLambdaContextDecl);
+
+  ExprResult SubExpr = getDerived().TransformExpr(Op);
   if (SubExpr.isInvalid())
     return ExprError();
 
Index: clang/test/SemaCXX/pr50497-crash-typeid.cpp
===================================================================
--- /dev/null
+++ clang/test/SemaCXX/pr50497-crash-typeid.cpp
@@ -0,0 +1,26 @@
+// RUN: %clang_cc1 -verify %s -Wno-unevaluated-expression
+// Don't crash (PR50497).
+
+// expected-no-diagnostics
+namespace std {
+class type_info;
+}
+
+class Ex {
+  // polymorphic
+  virtual ~Ex();
+};
+void Frob(const std::type_info &type);
+
+void Foo(Ex *ex) {
+  // generic lambda
+  [=](auto rate) {
+    // typeid
+    Frob(typeid(*ex));
+  }(1);
+
+  [=](auto rate) {
+    // unevaluated nested typeid
+    Frob(typeid((typeid(*ex), ex)));
+  }(1);
+}