[Module-scope lookup] Find arbitrary names in macro expansions.

Whenever we perform a name lookup, we need to make sure to expand
all macros that use `names: arbitrary`, because of course they can
produce... wait for it... arbitrary names, and we don't know what they
are until we instatiate them.

lib/AST/Module.cpp

@@ -174,6 +174,10 @@ class swift::SourceLookupCache {
 
   using AuxiliaryDeclMap = llvm::DenseMap<DeclName, TinyPtrVector<MissingDecl *>>;
   AuxiliaryDeclMap TopLevelAuxiliaryDecls;
+
+  /// Top-level macros that produce arbitrary names.
+  SmallVector<MissingDecl *, 4> TopLevelArbitraryMacros;
+
   SmallVector<Decl *, 4> MayHaveAuxiliaryDecls;
   void populateAuxiliaryDeclCache();
 
@@ -352,26 +356,46 @@ void SourceLookupCache::populateAuxiliaryDeclCache() {
     for (auto attrConst : decl->getAttrs().getAttributes<CustomAttr>()) {
       auto *attr = const_cast<CustomAttr *>(attrConst);
       UnresolvedMacroReference macroRef(attr);
+      bool introducesArbitraryNames = false;
       namelookup::forEachPotentialResolvedMacro(
           decl->getDeclContext()->getModuleScopeContext(),
           macroRef.getMacroName(), MacroRole::Peer,
           [&](MacroDecl *macro, const MacroRoleAttr *roleAttr) {
+            // First check for arbitrary names.
+            if (roleAttr->hasNameKind(MacroIntroducedDeclNameKind::Arbitrary)) {
+              introducesArbitraryNames = true;
+            }
+
             macro->getIntroducedNames(MacroRole::Peer,
                                       dyn_cast<ValueDecl>(decl),
                                       introducedNames[attr]);
           });
+
+      // Record this macro where appropriate.
+      if (introducesArbitraryNames)
+        TopLevelArbitraryMacros.push_back(MissingDecl::forUnexpandedMacro(attr, decl));
     }
 
     if (auto *med = dyn_cast<MacroExpansionDecl>(decl)) {
       UnresolvedMacroReference macroRef(med);
+      bool introducesArbitraryNames = false;
       namelookup::forEachPotentialResolvedMacro(
           decl->getDeclContext()->getModuleScopeContext(),
           macroRef.getMacroName(), MacroRole::Declaration,
           [&](MacroDecl *macro, const MacroRoleAttr *roleAttr) {
+            // First check for arbitrary names.
+            if (roleAttr->hasNameKind(MacroIntroducedDeclNameKind::Arbitrary)) {
+              introducesArbitraryNames = true;
+            }
+
             macro->getIntroducedNames(MacroRole::Declaration,
                                       /*attachedTo*/ nullptr,
                                       introducedNames[med]);
           });
+
+      // Record this macro where appropriate.
+      if (introducesArbitraryNames)
+        TopLevelArbitraryMacros.push_back(MissingDecl::forUnexpandedMacro(med, decl));
     }
 
     // Add macro-introduced names to the top-level auxiliary decl cache as
@@ -440,14 +464,29 @@ void SourceLookupCache::lookupValue(DeclName Name, NLKind LookupKind,
     ? UniqueMacroNamePlaceholder
     : Name;
   auto auxDecls = TopLevelAuxiliaryDecls.find(keyName);
-  if (auxDecls == TopLevelAuxiliaryDecls.end())
+
+  // Check macro expansions that could produce this name.
+  SmallVector<MissingDecl *, 4> unexpandedDecls;
+  if (auxDecls != TopLevelAuxiliaryDecls.end()) {
+    unexpandedDecls.insert(
+      unexpandedDecls.end(), auxDecls->second.begin(), auxDecls->second.end());
+  }
+
+  // Check macro expansions that can produce arbitrary names.
+  unexpandedDecls.insert(
+      unexpandedDecls.end(),
+      TopLevelArbitraryMacros.begin(), TopLevelArbitraryMacros.end());
+
+  if (unexpandedDecls.empty())
     return;
 
-  for (auto *unexpandedDecl : auxDecls->second) {
-    // Add expanded peers and freestanding declarations to the result.
+  // Add matching expanded peers and freestanding declarations to the results.
+  SmallPtrSet<ValueDecl *, 4> macroExpandedDecls;
+  for (auto *unexpandedDecl : unexpandedDecls) {
     unexpandedDecl->forEachMacroExpandedDecl(
         [&](ValueDecl *decl) {
           if (decl->getName().matchesRef(Name)) {
+            if (macroExpandedDecls.insert(decl).second)
               Result.push_back(decl);
           }
         });

lib/Sema/TypeCheckExpr.cpp

@@ -615,9 +615,11 @@ static Type lookupDefaultLiteralType(const DeclContext *dc,
                                      StringRef name) {
   auto &ctx = dc->getASTContext();
   DeclNameRef nameRef(ctx.getIdentifier(name));
-  auto lookup = TypeChecker::lookupUnqualified(dc->getModuleScopeContext(),
+  auto lookup = TypeChecker::lookupUnqualified(
+      dc->getModuleScopeContext(),
       nameRef, SourceLoc(),
-                                               defaultUnqualifiedLookupOptions);
+      defaultUnqualifiedLookupOptions | NameLookupFlags::ExcludeMacroExpansions
+  );
   TypeDecl *TD = lookup.getSingleTypeResult();
   if (!TD)
     return Type();

lib/Sema/TypeCheckNameLookup.cpp

@@ -217,6 +217,8 @@ convertToUnqualifiedLookupOptions(NameLookupOptions options) {
     newOptions |= UnqualifiedLookupFlags::IncludeOuterResults;
   if (options.contains(NameLookupFlags::IncludeUsableFromInline))
     newOptions |= UnqualifiedLookupFlags::IncludeUsableFromInline;
+  if (options.contains(NameLookupFlags::ExcludeMacroExpansions))
+    newOptions |= UnqualifiedLookupFlags::ExcludeMacroExpansions;
 
   return newOptions;
 }

lib/Sema/TypeChecker.h

@@ -160,6 +160,8 @@ enum class NameLookupFlags {
   IncludeOuterResults = 1 << 1,
   // Whether to include results that are marked @inlinable or @usableFromInline.
   IncludeUsableFromInline = 1 << 2,
+  /// This lookup should exclude any names introduced by macro expansions.
+  ExcludeMacroExpansions = 1 << 3,
 };
 
 /// A set of options that control name lookup.

test/Macros/top_level_freestanding.swift

@@ -54,3 +54,13 @@ struct HasInnerClosure {
   #freestandingWithClosure(0) { x in x }
   #freestandingWithClosure(1) { x in x }
 }
+
+// Arbitrary names at global scope
+
+@freestanding(declaration, names: arbitrary) macro bitwidthNumberedStructs(_ baseName: String) = #externalMacro(module: "MacroDefinition", type: "DefineBitwidthNumberedStructsMacro")
+
+#bitwidthNumberedStructs("MyIntGlobal")
+
+func testArbitraryAtGlobal() {
+  _ = MyIntGlobal16()
+}