[clang-tools-extra] [clang-tidy] performance-unnecessary-copy-initialization: Enhance the check for the scenario with MemberExpr initialization. (PR #151936)

[via cfe-commits]

================
@@ -273,6 +273,18 @@ void UnnecessaryCopyInitialization::registerMatchers(MatchFinder *Finder) {
   Finder->addMatcher(LocalVarCopiedFrom(declRefExpr(
                          to(varDecl(hasLocalStorage()).bind(OldVarDeclId)))),
                      this);
+
+  auto DeclRefToConstVar =
+      declRefExpr(to(varDecl(anyOf(hasType(isConstQualified()),
+                                   hasType(references(isConstQualified()))))
+                         .bind(OldVarDeclId)));
+  Finder->addMatcher(
+      LocalVarCopiedFrom(
+          memberExpr(hasObjectExpression(anyOf(hasDescendant(DeclRefToConstVar),
----------------
movie-travel-code wrote:

Good catch, vbvictor! However the current check results are reasonable, although they don't quite meet expectations🤔, pretty wierd!
```c++
  struct NestedStruct {
    Struct s;
    const Struct& getConstRefS() const { return s; }
    const Struct getConstS() const { return s; }
    Struct getS() const { return s; }
  };
  const auto m1 = ns.getConstRefS().Member; // warn
  const auto m2 = ns.getConstS().Member; // not warn
  const auto m3 = ns.getS().Member; // not warn
```
I suspect that another matcher is at work, but these checks are currently mixed together. I will analyze the reasons next week and try to give a reasonable explanation.
```
  Finder->addMatcher(
      LocalVarCopiedFrom(anyOf(
          isConstRefReturningFunctionCall(),
          isRefReturningMethodCallWithConstOverloads(ExcludedContainerTypes))),
      this);
```

btw, I added more tests to analyze these behaviors.

https://github.com/llvm/llvm-project/pull/151936