[AST] Factor out GenericSignature::forInvalid

Factor out the common logic from `getPlaceholderGenericSignature`.

include/swift/AST/GenericSignature.h

@@ -120,6 +120,10 @@ public:
                               ArrayRef<Requirement> requirements,
                               bool isKnownCanonical = false);
 
+  /// Create a new placeholder generic signature from a set of generic
+  /// parameters. This is necessary for recovery in invalid cases.
+  static GenericSignature forInvalid(ArrayRef<GenericTypeParamType *> params);
+
   /// Produce a new generic signature which drops all of the marker
   /// protocol conformance requirements associated with this one.
   GenericSignature withoutMarkerProtocols() const;

lib/AST/ASTContext.cpp

@@ -6007,6 +6007,25 @@ GenericSignature::get(ArrayRef<GenericTypeParamType *> params,
   return newSig;
 }
 
+GenericSignature
+GenericSignature::forInvalid(ArrayRef<GenericTypeParamType *> params) {
+  ASSERT(!params.empty());
+  auto &ctx = params.front()->getASTContext();
+
+  SmallVector<Requirement, 2> requirements;
+  for (auto *param : params) {
+    if (param->isValue())
+      continue;
+
+    for (auto ip : InvertibleProtocolSet::allKnown()) {
+      auto *proto = ctx.getProtocol(getKnownProtocolKind(ip));
+      requirements.emplace_back(RequirementKind::Conformance, param,
+                                proto->getDeclaredInterfaceType());
+    }
+  }
+  return GenericSignature::get(params, requirements);
+}
+
 GenericEnvironment *GenericEnvironment::forPrimary(GenericSignature signature) {
   auto &ctx = signature->getASTContext();
 

lib/AST/Decl.cpp

@@ -1660,22 +1660,13 @@ static GenericSignature getPlaceholderGenericSignature(
     gpLists[i]->setDepth(i);
 
   SmallVector<GenericTypeParamType *, 2> genericParams;
-  SmallVector<Requirement, 2> requirements;
-
   for (auto *gpList : gpLists) {
     for (auto *genericParam : *gpList) {
       auto type = genericParam->getDeclaredInterfaceType();
       genericParams.push_back(type->castTo<GenericTypeParamType>());
-
-      for (auto ip : InvertibleProtocolSet::allKnown()) {
-        auto proto = ctx.getProtocol(getKnownProtocolKind(ip));
-        requirements.emplace_back(RequirementKind::Conformance, type,
-                                  proto->getDeclaredInterfaceType());
-      }
     }
   }
-
-  return GenericSignature::get(genericParams, requirements);
+  return GenericSignature::forInvalid(genericParams);
 }
 
 GenericSignature GenericContext::getGenericSignature() const {

lib/AST/RequirementMachine/RequirementMachineRequests.cpp

@@ -750,26 +750,6 @@ AbstractGenericSignatureRequest::evaluate(
   }
 }
 
-/// If completion fails, build a dummy generic signature where everything is
-/// Copyable and Escapable, to avoid spurious downstream diagnostics
-/// concerning move-only types.
-static GenericSignature getPlaceholderGenericSignature(
-    ASTContext &ctx, ArrayRef<GenericTypeParamType *> genericParams) {
-  SmallVector<Requirement, 2> requirements;
-  for (auto param : genericParams) {
-    if (param->isValue())
-      continue;
-
-    for (auto ip : InvertibleProtocolSet::allKnown()) {
-      auto proto = ctx.getProtocol(getKnownProtocolKind(ip));
-      requirements.emplace_back(RequirementKind::Conformance, param,
-                                proto->getDeclaredInterfaceType());
-    }
-  }
-
-  return GenericSignature::get(genericParams, requirements);
-}
-
 GenericSignatureWithError
 InferredGenericSignatureRequest::evaluate(
         Evaluator &evaluator,
@@ -996,7 +976,7 @@ InferredGenericSignatureRequest::evaluate(
                          diag::requirement_machine_completion_rule,
                          rule);
 
-      auto result = getPlaceholderGenericSignature(ctx, genericParams);
+      auto result = GenericSignature::forInvalid(genericParams);
 
       if (rewriteCtx.getDebugOptions().contains(DebugFlags::Timers)) {
         rewriteCtx.endTimer("InferredGenericSignatureRequest");