Don’t create a new generic signature builder for non-generic members.

When type-checking a function or subscript that itself does not have generic
parameters (but is within a generic context), we were creating a generic
signature builder which will always produce the same generic signature as
the enclosing context. Stop creating that generic signature builder.

Instead, teach the CompleteGenericTypeResolver to use the generic signature
+ the canonical generic signature builder for that signature to resolve
types, which also eliminates some extraneous re-type-checking.

Improves type-checking performance of the standard library by 36%.

Author: DougGregor

include/swift/AST/Decl.h

@@ -4591,7 +4591,7 @@ class ParamDecl : public VarDecl {
   /// Clone constructor, allocates a new ParamDecl identical to the first.
   /// Intentionally not defined as a typical copy constructor to avoid
   /// accidental copies.
-  ParamDecl(ParamDecl *PD);
+  ParamDecl(ParamDecl *PD, bool withTypes);
 
   /// Retrieve the argument (API) name for this function parameter.
   Identifier getArgumentName() const { return ArgumentName; }

include/swift/AST/ParameterList.h

@@ -128,9 +128,15 @@ class alignas(ParamDecl *) ParameterList final :
     Implicit = 0x01,
     /// The cloned pattern is for an inherited constructor; mark default
     /// arguments as inherited, and mark unnamed arguments as named.
-    Inherited = 0x02
+    Inherited = 0x02,
+    /// The cloned pattern will strip type information.
+    WithoutTypes = 0x04,
   };
-  
+
+  friend OptionSet<CloneFlags> operator|(CloneFlags flag1, CloneFlags flag2) {
+    return OptionSet<CloneFlags>(flag1) | flag2;
+  }
+
   /// Make a duplicate copy of this parameter list.  This allocates copies of
   /// the ParamDecls, so they can be reparented into a new DeclContext.
   ParameterList *clone(const ASTContext &C,

lib/AST/Decl.cpp

@@ -4217,18 +4217,22 @@ ParamDecl::ParamDecl(Specifier specifier,
 
 /// Clone constructor, allocates a new ParamDecl identical to the first.
 /// Intentionally not defined as a copy constructor to avoid accidental copies.
-ParamDecl::ParamDecl(ParamDecl *PD)
+ParamDecl::ParamDecl(ParamDecl *PD, bool withTypes)
   : VarDecl(DeclKind::Param, /*IsStatic*/false, PD->getSpecifier(),
             /*IsCaptureList*/false, PD->getNameLoc(), PD->getName(),
-            PD->hasType() ? PD->getType() : Type(), PD->getDeclContext()),
+            PD->hasType() && withTypes? PD->getType() : Type(),
+            PD->getDeclContext()),
     ArgumentName(PD->getArgumentName()),
     ArgumentNameLoc(PD->getArgumentNameLoc()),
     SpecifierLoc(PD->getSpecifierLoc()),
     DefaultValueAndIsVariadic(nullptr, PD->DefaultValueAndIsVariadic.getInt()),
     IsTypeLocImplicit(PD->IsTypeLocImplicit),
     defaultArgumentKind(PD->defaultArgumentKind) {
   typeLoc = PD->getTypeLoc().clone(PD->getASTContext());
-  if (PD->hasInterfaceType())
+  if (!withTypes && typeLoc.getTypeRepr())
+    typeLoc.setType(Type());
+
+  if (withTypes && PD->hasInterfaceType())
     setInterfaceType(PD->getInterfaceType());
 }
 

lib/AST/Parameter.cpp

@@ -88,11 +88,12 @@ ParameterList *ParameterList::clone(const ASTContext &C,
   SmallVector<ParamDecl*, 8> params(begin(), end());
 
   // Remap the ParamDecls inside of the ParameterList.
+  bool withTypes = !options.contains(ParameterList::WithoutTypes);
   for (auto &decl : params) {
     bool hadDefaultArgument =
         decl->getDefaultArgumentKind() == DefaultArgumentKind::Normal;
 
-    decl = new (C) ParamDecl(decl);
+    decl = new (C) ParamDecl(decl, withTypes);
     if (options & Implicit)
       decl->setImplicit();
 

lib/Sema/CodeSynthesis.cpp

@@ -104,7 +104,8 @@ buildIndexForwardingParamList(AbstractStorageDecl *storage,
 
   // Clone the parameter list over for a new decl, so we get new ParamDecls.
   auto indices = subscript->getIndices()->clone(context,
-                                                ParameterList::Implicit);
+                                                ParameterList::Implicit|
+                                                ParameterList::WithoutTypes);
   if (prefix.empty())
     return indices;
 

lib/Sema/GenericTypeResolver.h

@@ -130,14 +130,14 @@ class ProtocolRequirementTypeResolver : public GenericTypeResolver {
 /// to check the signature of a generic declaration and resolve (for example)
 /// all dependent member refers to archetype members.
 class CompleteGenericTypeResolver : public GenericTypeResolver {
-  TypeChecker &TC;
-  GenericSignatureBuilder &Builder;
-  ArrayRef<GenericTypeParamType *> GenericParams;
+  TypeChecker &tc;
+  GenericSignature *genericSig;
+  ModuleDecl &module;
+  GenericSignatureBuilder &builder;
 
 public:
-  CompleteGenericTypeResolver(TypeChecker &tc, GenericSignatureBuilder &builder,
-                              ArrayRef<GenericTypeParamType *> genericParams)
-    : TC(tc), Builder(builder), GenericParams(genericParams) { }
+  CompleteGenericTypeResolver(TypeChecker &tc, GenericSignature *genericSig,
+                              ModuleDecl &module);
 
   virtual Type mapTypeIntoContext(Type type);
 

lib/Sema/TypeCheckDecl.cpp

@@ -5233,10 +5233,18 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
       // Revert the types within the signature so it can be type-checked with
       // archetypes below.
       TC.revertGenericFuncSignature(FD);
-    } else if (FD->getDeclContext()->getGenericSignatureOfContext()) {
-      (void)TC.validateGenericFuncSignature(FD);
-      // Revert all of the types within the signature of the function.
-      TC.revertGenericFuncSignature(FD);
+    } else if (auto genericSig =
+                 FD->getDeclContext()->getGenericSignatureOfContext()) {
+      if (!FD->getAccessorStorageDecl()) {
+        (void)TC.validateGenericFuncSignature(FD);
+
+        // Revert all of the types within the signature of the function.
+        TC.revertGenericFuncSignature(FD);
+      } else {
+        // We've inherited all of the type information already.
+        TC.configureInterfaceType(FD, genericSig);
+      }
+
       FD->setGenericEnvironment(
           FD->getDeclContext()->getGenericEnvironmentOfContext());
     }