`performance-unnecessary-value-param` not detected for STL container when accessed via index operator

[firewave]

#include <string>

void f(std::string type)
{
    const char Type = type[0];
}

https://godbolt.org/z/ae3efnTvs

[llvmbot]

@llvm/issue-subscribers-clang-tidy

Author: Oliver Stöneberg (firewave)

```cpp #include <string>

void f(std::string type)
{
const char Type = type[0];
}

https://godbolt.org/z/ae3efnTvs
</details>

[firewave]

Also not detected with other STL containers:

#include <vector>

void cb(const int *);

void f(std::vector<const int*> v)
{
    if (v.size() != 2)
        return;
    cb(v[0]);
}

https://godbolt.org/z/bcsr8hs3c

[neoncube2]

I did some digging into this :)

The code results in this AST:

CompoundStmt 0x166f9b9c8b8 <D:\llvm\bugs\UnnecessaryValueParam\Example.cpp:4:1, line:6:1>
`-DeclStmt 0x166f9b9c8a0 <line:5:5, col:30>
  `-VarDecl 0x166f9b9c6d0 <col:5, col:29> col:16 Type 'const char' cinit
    `-ImplicitCastExpr 0x166f9b9c840 <col:23, col:29> 'value_type':'char' <LValueToRValue>
      `-CXXOperatorCallExpr 0x166f9b9c7d0 <col:23, col:29> 'value_type':'char' lvalue '[]'
        |-ImplicitCastExpr 0x166f9b9c7b8 <col:27, col:29> 'reference (*)(const size_type) noexcept' <FunctionToPointerDecay>
        | `-DeclRefExpr 0x166f9b9c798 <col:27, col:29> 'reference (const size_type) noexcept' lvalue CXXMethod 0x166fa443d58 'operator[]' 'reference (const size_type) noexcept'
        |-DeclRefExpr 0x166f9b9c738 <col:23> 'std::string':'std::basic_string<char>' lvalue ParmVar 0x166f9b9c4f8 'type' 'std::string':'std::basic_string<char>'
        `-ImplicitCastExpr 0x166f9b9c780 <col:28> 'size_type':'unsigned long long' <IntegralCast>
          `-IntegerLiteral 0x166f9b9c758 <col:28> 'int' 0

The CXXOperatorCallExpr is causing performance-unnecessary-value-param not to fire.

There are two different methods in std::basic_string<CharT,Traits,Allocator>::operator[], one const and one non-const:

• reference operator[]( size_type pos );
• const_reference operator[]( size_type pos ) const;

For some reason, it looks like clang is picking up the non-const method, and performance-unnecessary-value-param only works if all of the method calls are const.

[firewave]

Related/duplicate: #59750.