[Macros] Diagnose when an accessor macro produces an accessor that already exists

We used to crash; now we diagnose and... don't crash.

Fixes rdar://111588129.

Author: DougGregor

lib/AST/Decl.cpp

@@ -6478,7 +6478,8 @@ void AbstractStorageDecl::setAccessors(SourceLoc lbraceLoc,
   auto record = Accessors.getPointer();
   if (record) {
     for (auto accessor : accessors) {
-      (void) record->addOpaqueAccessor(accessor);
+      if (!record->getAccessor(accessor->getAccessorKind()))
+        (void) record->addOpaqueAccessor(accessor);
     }
   } else {
     record = AccessorRecord::create(getASTContext(),

lib/Parse/ParseDecl.cpp

@@ -7448,8 +7448,14 @@ void Parser::parseTopLevelAccessors(
 
   bool hadLBrace = consumeIf(tok::l_brace);
 
-  ParserStatus status;
+  // Prepopulate the field for any accessors that were already parsed parsed accessors
   ParsedAccessors accessors;
+#define ACCESSOR(ID)                                            \
+    if (auto accessor = storage->getAccessor(AccessorKind::ID)) \
+      accessors.ID = accessor;
+#include "swift/AST/AccessorKinds.def"
+
+  ParserStatus status;
   bool hasEffectfulGet = false;
   bool parsingLimitedSyntax = false;
   while (!Tok.isAny(tok::r_brace, tok::eof)) {

test/Macros/Inputs/syntax_macro_definitions.swift

@@ -422,8 +422,7 @@ extension PropertyWrapperMacro: AccessorMacro, Macro {
   ) throws -> [AccessorDeclSyntax] {
     guard let varDecl = declaration.as(VariableDeclSyntax.self),
       let binding = varDecl.bindings.first,
-      let identifier = binding.pattern.as(IdentifierPatternSyntax.self)?.identifier,
-      binding.accessor == nil
+      let identifier = binding.pattern.as(IdentifierPatternSyntax.self)?.identifier
     else {
       return []
     }

test/Macros/accessor_macros.swift

@@ -6,6 +6,10 @@
 // First check for no errors.
 // RUN: %target-typecheck-verify-swift -swift-version 5 -load-plugin-library %t/%target-library-name(MacroDefinition)
 
+// Check for expected errors.
+// RUN: not %target-swift-frontend -typecheck -swift-version 5 -load-plugin-library %t/%target-library-name(MacroDefinition) -DTEST_DIAGNOSTICS %s > %t/diags.txt 2>&1
+// RUN: %FileCheck -check-prefix=CHECK-DIAGS %s < %t/diags.txt
+
 // Check that the expansion buffer are as expected.
 // RUN: %target-swift-frontend -swift-version 5 -typecheck -load-plugin-library %t/%target-library-name(MacroDefinition) %s -dump-macro-expansions > %t/expansions-dump.txt 2>&1
 // RUN: %FileCheck -check-prefix=CHECK-DUMP %s < %t/expansions-dump.txt
@@ -72,3 +76,23 @@ _ = ms.name
 // CHECK-NEXT: Setting value World
 ms.name = "World"
 
+
+#if TEST_DIAGNOSTICS
+struct MyBrokenStruct {
+  var _birthDate: MyWrapperThingy<Date?> = .init(storage: nil)
+
+  @myPropertyWrapper
+  var birthDate: Date? {
+    // CHECK-DIAGS: variable already has a getter
+    // CHECK-DIAGS: in expansion of macro
+    // CHECK-DIAGS: previous definition of getter here
+    get { fatalError("Boom") }
+
+    // CHECK-DIAGS: variable already has a setter
+    // CHECK-DIAGS: in expansion of macro
+    // CHECK-DIAGS: previous definition of setter here
+    set { fatalError("Boom") }
+  }
+}
+
+#endif