[Bug c++/105290] New: "Purely syntactic" disambiguation but GCC applies semantic rules for constant expressions

[hstong at ca dot ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105290

            Bug ID: 105290
           Summary: "Purely syntactic" disambiguation but GCC applies
                    semantic rules for constant expressions
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: accepts-invalid, rejects-valid, wrong-code
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hstong at ca dot ibm.com
  Target Milestone: ---

As far as parsing is concerned, a _constant-expression_ is just a
_conditional-expression_. There is no statement that a _constant-expression_
has to be semantically a constant expression to be parsed as one.

Every use of _constant-expression_ in the grammar is associated with a
case-specific (semantic) statement that applies the semantic constant
expression requirements.

It looks like GCC applies the semantic rules for constant expressions while
performing vexing parse disambiguation, which the standard says is "purely
syntactic".

Compiler Explorer link: https://godbolt.org/z/Ms38PPsqh

### SOURCE (`<stdin>`):
typedef int *Ptr;
int idx(unsigned *) { return 0; }
constexpr int idx(short *) { return 1; }
void zip(void **p) {
  int val(Ptr (*p)[idx((short *)0)]); // constant expression in array
declarator
  val(nullptr);
}
void zap(void **p) {
  int val(Ptr(*p)[idx((unsigned *)0)]); // non-constant expression in subscript
expression?!
  val(nullptr);
}


### ACTUAL OUTPUT:
<stdin>: In function 'void zap(void**)':
<stdin>:10:6: error: 'val' cannot be used as a function


### EXPECTED OUTPUT:
(Clean compile)


### COMPILER INVOCATION:
g++ -fsyntax-only -Wno-vexing-parse -xc++ -


### COMPILER VERSION INFO (`g++ -v`):
Using built-in specs.
COLLECT_GCC=/opt/wandbox/gcc-head/bin/g++
COLLECT_LTO_WRAPPER=/opt/wandbox/gcc-head/libexec/gcc/x86_64-pc-linux-gnu/12.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../source/configure --prefix=/opt/wandbox/gcc-head
--enable-languages=c,c++ --disable-multilib --without-ppl --without-cloog-ppl
--enable-checking=release --disable-nls --enable-lto
LDFLAGS=-Wl,-rpath,/opt/wandbox/gcc-head/lib,-rpath,/opt/wandbox/gcc-head/lib64,-rpath,/opt/wandbox/gcc-head/lib32
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 12.0.1 20220415 (experimental) (GCC)