[Type checker] When we sugar an extension type, infer requirements.

The optimization that avoided forming new generic signatures for extensions
of nominal types was kicking in too eagerly, preventing us from inferring
requirements from the typealias.

Author: DougGregor

lib/Sema/TypeCheckDecl.cpp

@@ -8279,7 +8279,8 @@ bool swift::isPassThroughTypealias(TypeAliasDecl *typealias) {
 /// extension's list of generic parameters.
 static Type formExtensionInterfaceType(TypeChecker &tc, ExtensionDecl *ext,
                                        Type type,
-                                       GenericParamList *genericParams) {
+                                       GenericParamList *genericParams,
+                                       bool &mustInferRequirements) {
   // Find the nominal type declaration and its parent type.
   Type parentType;
   GenericTypeDecl *genericDecl;
@@ -8301,7 +8302,8 @@ static Type formExtensionInterfaceType(TypeChecker &tc, ExtensionDecl *ext,
                                  ? genericParams->getOuterParameters()
                                  : genericParams;
     parentType =
-      formExtensionInterfaceType(tc, ext, parentType, parentGenericParams);
+      formExtensionInterfaceType(tc, ext, parentType, parentGenericParams,
+                                 mustInferRequirements);
   }
 
   // Find the nominal type.
@@ -8345,6 +8347,8 @@ static Type formExtensionInterfaceType(TypeChecker &tc, ExtensionDecl *ext,
 
       resultType = BoundNameAliasType::get(typealias, parentType,
                                            subMap, resultType);
+
+      mustInferRequirements = true;
     } else {
       resultType = typealias->getDeclaredInterfaceType();
     }
@@ -8372,8 +8376,10 @@ checkExtensionGenericParams(TypeChecker &tc, ExtensionDecl *ext, Type type,
   assert(!ext->getGenericEnvironment());
 
   // Form the interface type of the extension.
+  bool mustInferRequirements = false;
   Type extInterfaceType =
-    formExtensionInterfaceType(tc, ext, type, genericParams);
+    formExtensionInterfaceType(tc, ext, type, genericParams,
+                               mustInferRequirements);
 
   // Prepare all of the generic parameter lists for generic signature
   // validation.
@@ -8395,7 +8401,8 @@ checkExtensionGenericParams(TypeChecker &tc, ExtensionDecl *ext, Type type,
   auto *env = tc.checkGenericEnvironment(genericParams,
                                          ext->getDeclContext(), nullptr,
                                          /*allowConcreteGenericParams=*/true,
-                                         ext, inferExtendedTypeReqs);
+                                         ext, inferExtendedTypeReqs,
+                                         mustInferRequirements);
 
   // Validate the generic parameters for the last time, to splat down
   // actual archetypes.

lib/Sema/TypeCheckGeneric.cpp

@@ -1147,13 +1147,14 @@ GenericEnvironment *TypeChecker::checkGenericEnvironment(
                       bool allowConcreteGenericParams,
                       ExtensionDecl *ext,
                       llvm::function_ref<void(GenericSignatureBuilder &)>
-                        inferRequirements) {
+                        inferRequirements,
+                      bool mustInferRequirements) {
   assert(genericParams && "Missing generic parameters?");
   bool recursivelyVisitGenericParams =
     genericParams->getOuterParameters() && !parentSig;
 
   GenericSignature *sig;
-  if (!ext || ext->getTrailingWhereClause() ||
+  if (!ext || mustInferRequirements || ext->getTrailingWhereClause() ||
       getExtendedTypeGenericDepth(ext) != genericParams->getDepth()) {
     // Collect the generic parameters.
     SmallVector<GenericTypeParamType *, 4> allGenericParams;

lib/Sema/TypeChecker.h

@@ -1419,7 +1419,8 @@ class TypeChecker final : public LazyResolver {
                         bool allowConcreteGenericParams,
                         ExtensionDecl *ext,
                         llvm::function_ref<void(GenericSignatureBuilder &)>
-                          inferRequirements);
+                          inferRequirements,
+                        bool mustInferRequirements);
 
   /// Construct a new generic environment for the given declaration context.
   ///
@@ -1439,7 +1440,8 @@ class TypeChecker final : public LazyResolver {
                         ExtensionDecl *ext) {
     return checkGenericEnvironment(genericParams, dc, outerSignature,
                                    allowConcreteGenericParams, ext,
-                                   [&](GenericSignatureBuilder &) { });
+                                   [&](GenericSignatureBuilder &) { },
+                                   /*mustInferRequirements=*/false);
   }
 
   /// Validate the signature of a generic type.