error: invalid argument type 'T *' to unary expression

[dcci]

After #90500 -- some of our internal code stopped compiling.

$ ~/llvm-build-upstream/bin/clang /tmp/Out.cpp
/tmp/Out.cpp:3:9: warning: #pragma once in main file [-Wpragma-once-outside-header]
    3 | #pragma once
      |         ^
/tmp/Out.cpp:65:14: error: invalid argument type 'T *' to unary expression
   65 |     *item_ = -item_;
      |              ^~~~~~
1 warning and 1 error generated.

File attached.
repro.cpp.txt

cc: @erichkeane / @sdkrystian

I don't exclude that our code might be incorrect, but I wonder why this only applies to operator- (and not operator+)

[erichkeane]

This is a case where the source is wrong, unary minus on a pointer is ill formed, but unary plus is not.

See:
https://eel.is/c++draft/expr.unary#op-7

The operand of the unary + operator shall be a prvalue of arithmetic, unscoped enumeration, or pointer type and the result is the value of the argument.  Integral promotion is performed on integral or enumeration operands. The type of the result is the type of the promoted operand.

And:
https://eel.is/c++draft/expr.unary#op-8

The operand of the unary - operator shall be a prvalue of arithmetic or unscoped enumeration type and the result is the negative of its operand. Integral promotion is performed on integral or enumeration operands. The negative of an unsigned quantity is computed by subtracting its value from 2n, where n is the number of bits in the promoted operand.
The type of the result is the type of the promoted operand.

Note the unary+ permits arithmetic, unscoped enum, and pointer type, but unary- is arithmetic or unscoped enum only. Effectively, the semantics of a negation operator on a pointer is not defined, and has no real reasonable way to define it,
whereas the unary + operator has the effect of 'do nothing', which is reasonable.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Davide Italiano (dcci)

After https://github.com//pull/90500 -- some of our internal code stopped compiling.

$ ~/llvm-build-upstream/bin/clang /tmp/Out.cpp
/tmp/Out.cpp:3:9: warning: #pragma once in main file [-Wpragma-once-outside-header]
    3 | #pragma once
      |         ^
/tmp/Out.cpp:65:14: error: invalid argument type 'T *' to unary expression
   65 |     *item_ = -item_;
      |              ^~~~~~
1 warning and 1 error generated.

File attached.
repro.cpp.txt

cc: @erichkeane / @sdkrystian

I don't exclude that our code might be incorrect, but I wonder why this only applies to operator- (and not operator+)