[ObjC] Name lookup in methods shouldn't allow shadowing types (#116683)

Arguably as a bug, Clang has previously not mixed up Objective-C
parameter names with types. This allows developers to write parameter
names that _should_ shadow type names, but don't. For instance:

    @interface Foo
    -(void)foo:(int)id bar:(id)name; // OK
    @end

Commit 9778808998 changed the way that
parameters are parsed to bring it more in line with how C parameters are
parsed, but it breaks the example above. Given an expectation that the
change wouldn't introduce source breaks, this is not something we can go
forward with.

9778808998 did this so that late-parsed
attributes could reference Objective-C parameters. This change buffers
Objective-C parameter info until after all parameters are parsed and
turns them into parameter declarations before realizing late-parsed
attributes instead.

Radar-ID: 139996306

clang/lib/Parse/ParseObjc.cpp

@@ -1454,7 +1454,7 @@ Decl *Parser::ParseObjCMethodDecl(SourceLocation mLoc,
 
   SmallVector<const IdentifierInfo *, 12> KeyIdents;
   SmallVector<SourceLocation, 12> KeyLocs;
-  SmallVector<ParmVarDecl *, 12> ObjCParamInfo;
+  SmallVector<SemaObjC::ObjCArgInfo, 12> ArgInfos;
   ParseScope PrototypeScope(this, Scope::FunctionPrototypeScope |
                             Scope::FunctionDeclarationScope | Scope::DeclScope);
 
@@ -1495,9 +1495,7 @@ Decl *Parser::ParseObjCMethodDecl(SourceLocation mLoc,
     ArgInfo.NameLoc = Tok.getLocation();
     ConsumeToken(); // Eat the identifier.
 
-    ParmVarDecl *Param = Actions.ObjC().ActOnMethodParmDeclaration(
-        getCurScope(), ArgInfo, ObjCParamInfo.size(), MethodDefinition);
-    ObjCParamInfo.push_back(Param);
+    ArgInfos.push_back(ArgInfo);
     KeyIdents.push_back(SelIdent);
     KeyLocs.push_back(selLoc);
 
@@ -1557,6 +1555,17 @@ Decl *Parser::ParseObjCMethodDecl(SourceLocation mLoc,
                                                     nullptr));
   }
 
+  // Turn ArgInfos into parameters. This must happen after parsing all
+  // parameters for bug compatibility with previous versions of Clang. (For
+  // instance, if a method declares a parameter called "id", that parameter must
+  // not shadow the "id" type.)
+  SmallVector<ParmVarDecl *, 12> ObjCParamInfo;
+  for (auto &ArgInfo : ArgInfos) {
+    ParmVarDecl *Param = Actions.ObjC().ActOnMethodParmDeclaration(
+        getCurScope(), ArgInfo, ObjCParamInfo.size(), MethodDefinition);
+    ObjCParamInfo.push_back(Param);
+  }
+
   // FIXME: Add support for optional parameter list...
   // If attributes exist after the method, parse them.
   MaybeParseAttributes(PAKM_CXX11 | (getLangOpts().ObjC ? PAKM_GNU : 0),

clang/test/SemaObjC/method-param-named-id.m

@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wno-objc-root-class %s
+
+
+@interface Foo
+-(void)paramNamedID:(int)id usesIDType:(id)notShadowed;
+-(void)paramNamedID:(int)id, id notShadowed; // expected-warning{{use of C-style parameters in Objective-C method declarations is deprecated}}
+@end