-Wshadow: don't warn on ctor parameters with the same name as a field name

[alexfh]

Summary

-Wshadow: don't warn on ctor parameters with the same name as a field
name. This fixes a broad class of false positives resulting from a widely used
pattern:

struct A {
  int q;
  A(int q) : q(q) {}
};

Fixes http://llvm.org/PR16088.

As a data point: in a large code base this removes more than half of hits of -Wshadow, which all look useless (based on a manual inspection of a random sample of ~100 hits).

[alexfh]

Hm, didn't notice Reid's http://reviews.llvm.org/D18271, which seems to cover this in a better way.

[dblaikie]

Reid sent out a different patch for this warning improvement. Have you
checked that one out? Is it abandoned?

I'm still a bit concerned about whitelisting all these cases & not catching
cases where the parameter is then used inside the function in some
problematic way (the classic being a unique_ptr parameter, moved into a
member, then the parameter (instead of the member) is referenced in the
body of the function).

[dblaikie]

Ah, now I see your follow-up email. Soryr I missed it...

lib/Sema/SemaDecl.cpp

if (shadowedVar->isExternC()) {
       // declaration, not a locally scoped extern declaration.
       for (auto I : shadowedVar->redecls())
         if (I->isFileVarDecl()) {
           ShadowedDecl = I;
           break;
         }
     }
 
+  // Don't warn on constructor parameters with the same name as a field name.
+  if (isa<FieldDecl>(ShadowedDecl) && isa<CXXConstructorDecl>(NewDC) &&
+      isa<ParmVarDecl>(D))
+    return;
+
   DeclContext *OldDC = ShadowedDecl->getDeclContext();
 
   // Only warn about certain kinds of shadowing for class members.
   if (NewDC && NewDC->isRecord()) {
     // In particular, don't warn about shadowing non-class members.
     if (!OldDC->isRecord())
       return;
 

test/SemaCXX/warn-shadow.cpp

struct S {
   static void m() {
     if (1) {
       V v(0);
     }
   }
 };
 }
 
+// http://llvm.org/PR16088
+namespace PR16088 {
+struct S {
+  int i;
+  S(int i) : i(i) {}
+};
+}
+
 extern int bob; // expected-note {{previous declaration is here}}
 
 // rdar://8883302
 void rdar8883302() {
   extern int bob; // don't warn for shadowing.
 }
 
 void test8() {
   int bob; // expected-warning {{declaration shadows a variable in the global namespace}}
 }