[Sema] Avoid solver-allocated original ErrorTypes in transformDependentMemberType

`substBase` here can contain type variables or placeholders, avoid
using them as the original ErrorTypes since ErrorTypes cannot be
solver-allocated currently. This only affects type printing so
shouldn't matter much.

Author: hamishknight

lib/AST/TypeSubstitution.cpp

@@ -446,8 +446,13 @@ Type TypeSubstituter::transformDependentMemberType(DependentMemberType *dependen
 
   auto result = conformance.getTypeWitness(assocType, IFS.getOptions());
   if (result->is<ErrorType>()) {
+    // Substitute the base type for the original ErrorType for type printing.
+    // Avoid doing this if the substitutions introduce type variables or
+    // placeholders since ErrorTypes can't be solver-allocated currently (and
+    // type variables aren't helpful when printing anyway).
     auto substBase = origBase.subst(IFS);
-    return DependentMemberType::get(ErrorType::get(substBase), assocType);
+    if (!substBase->hasTypeVariableOrPlaceholder())
+      return DependentMemberType::get(ErrorType::get(substBase), assocType);
   }
   return result;
 }
@@ -1242,4 +1247,4 @@ 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^#)
+}