Clang importer: collapse redefined macros.

In C, macros can be redefined so long as the redefinitions are
tokenwise equivalent. Provide the Clang importer with the same ability by
determining when tokenwise equivalent macros would be imported as
different Swift declarations, and collapse them into a single
declaration.

Author: DougGregor

lib/ClangImporter/ImportMacro.cpp

@@ -393,10 +393,25 @@ ValueDecl *ClangImporter::Implementation::importMacro(Identifier name,
   if (!macro)
     return nullptr;
 
-  // Look for the value for an already-imported macro.
-  auto known = ImportedMacros.find(macro);
+  // Look for macros imported with the same name.
+  auto known = ImportedMacros.find(name);
   if (known != ImportedMacros.end()) {
-    return known->second;
+    // Check whether this macro has already been imported.
+    for (const auto &entry : known->second) {
+      if (entry.first == macro) return entry.second;
+    }
+
+    // Otherwise, check whether this macro is identical to a macro that has
+    // already been imported.
+    auto &clangPP = getClangPreprocessor();
+    for (const auto &entry : known->second) {
+      // If the macro is equal to an existing macro, map down to the same
+      // declaration.
+      if (macro->isIdenticalTo(*entry.first, clangPP, true)) {
+        known->second.push_back({macro, entry.second});
+        return entry.second;
+      }
+    }
   }
 
   ImportingEntityRAII ImportingEntity(*this);
@@ -408,6 +423,6 @@ ValueDecl *ClangImporter::Implementation::importMacro(Identifier name,
     return nullptr;
 
   auto valueDecl = ::importMacro(*this, DC, name, macro, macro);
-  ImportedMacros[macro] = valueDecl;
+  ImportedMacros[name].push_back({macro, valueDecl});
   return valueDecl;
 }

lib/ClangImporter/ImporterImpl.h

@@ -402,10 +402,16 @@ class LLVM_LIBRARY_VISIBILITY ClangImporter::Implementation
   llvm::DenseMap<std::pair<ClangDeclAndFlag, DeclContext *>, Decl *>
     ImportedProtocolDecls;
 
-  /// \brief Mapping of already-imported macros.
-  llvm::DenseMap<clang::MacroInfo *, ValueDecl *> ImportedMacros;
+  /// Mapping from identifiers to the set of macros that have that name along
+  /// with their corresponding Swift declaration.
+  ///
+  /// Multiple macro definitions can map to the same declaration if the
+  /// macros are identically defined.
+  llvm::DenseMap<Identifier,
+                 SmallVector<std::pair<clang::MacroInfo *, ValueDecl *>, 2>>
+    ImportedMacros;
 
-  /// Keeps track of active selector-basde lookups, so that we don't infinitely
+  /// Keeps track of active selector-based lookups, so that we don't infinitely
   /// recurse when checking whether a method with a given selector has already
   /// been imported.
   llvm::DenseMap<std::pair<ObjCSelector, char>, unsigned>

test/ClangModules/Inputs/custom-modules/MacrosRedefA.h

@@ -0,0 +1,2 @@
+#define REDEF_1 "hello"
+#define REDEF_2 "world"

test/ClangModules/Inputs/custom-modules/MacrosRedefB.h

@@ -0,0 +1,2 @@
+#define REDEF_1 "hello"
+#define REDEF_2 "Swift"

test/ClangModules/Inputs/custom-modules/module.map

@@ -128,3 +128,11 @@ module Requires {
 module SwiftName {
   header "SwiftName.h"
 }
+
+module MacrosRedefA {
+  header "MacrosRedefA.h"
+}
+
+module MacrosRedefB {
+  header "MacrosRedefB.h"
+}

test/ClangModules/macros_redef.swift

@@ -0,0 +1,11 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -I %S/Inputs/custom-modules -parse -verify %s
+
+import MacrosRedefA
+import MacrosRedefB
+
+func testMacroRedef() {
+  var s: String
+  s = REDEF_1
+  s = REDEF_2 // expected-error{{ambiguous use of 'REDEF_2'}}
+}
+