[Sema] Avoid setting original type for ErrorType in transformDependentMemberType

`substBase` here can contain type variables

Author: hamishknight

lib/AST/TypeSubstitution.cpp

@@ -445,10 +445,9 @@ Type TypeSubstituter::transformDependentMemberType(DependentMemberType *dependen
   auto conformance = IFS.lookupConformance(origBase, proto, level);
 
   auto result = conformance.getTypeWitness(assocType, IFS.getOptions());
-  if (result->is<ErrorType>()) {
-    auto substBase = origBase.subst(IFS);
-    return DependentMemberType::get(ErrorType::get(substBase), assocType);
-  }
+  if (result->is<ErrorType>())
+    return DependentMemberType::get(ErrorType::get(ctx), assocType);
+
   return result;
 }
 
@@ -1242,4 +1241,4 @@ ProtocolConformanceRef ReplaceExistentialArchetypesWithConcreteTypes::operator()
 
   return subs.lookupConformance(
       getInterfaceType(existentialArchetype)->getCanonicalType(), proto);
-}
+}

validation-test/IDE/crashers_2_fixed/pr-82147.swift

@@ -0,0 +1,20 @@
+// RUN: %target-swift-ide-test -code-completion -code-completion-token=COMPLETE -source-filename=%s
+
+// https://github.com/swiftlang/swift/pull/82147
+
+protocol P {
+  associatedtype X
+}
+
+struct S<T> {
+  init<U, V>() where T == (U, V) {}
+}
+extension S : P where T : P {
+  typealias X = T.X
+}
+
+func foo<T: P, U>(_: () -> T) where U == T.X {}
+
+foo {
+  S(#^COMPLETE^#)
+}