[Clang] Fix P2564 handling of variable initializers (#89565)

The following program produces a diagnostic in Clang and EDG, but
compiles correctly in GCC and MSVC:
```cpp
#include <vector>

consteval std::vector<int> fn() { return {1,2,3}; }
constexpr int a = fn()[1];
```

Clang's diagnostic is as follows:
```cpp
<source>:6:19: error: call to consteval function 'fn' is not a constant expression
    6 | constexpr int a = fn()[1];
      |                   ^
<source>:6:19: note: pointer to subobject of heap-allocated object is not a constant expression
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/14.0.1/../../../../include/c++/14.0.1/bits/allocator.h:193:31: note: heap allocation performed here
  193 |             return static_cast<_Tp*>(::operator new(__n));
      |                                      ^
1 error generated.
Compiler returned: 1
```

Based on my understanding of
[`[dcl.constexpr]/6`](https://eel.is/c++draft/dcl.constexpr#6):
> In any constexpr variable declaration, the full-expression of the
initialization shall be a constant expression

It seems to me that GCC and MSVC are correct: the initializer `fn()[1]`
does not evaluate to an lvalue referencing a heap-allocated value within
the `vector` returned by `fn()`; it evaluates to an lvalue-to-rvalue
conversion _from_ that heap-allocated value.

This PR turns out to be a bug fix on the implementation of
[P2564R3](https://wg21.link/p2564r3); as such, it only applies to C++23
and later. The core problem is that the definition of a
constant-initialized variable
([`[expr.const/2]`](https://eel.is/c++draft/expr.const#2)) is contingent
on whether the initializer can be evaluated as a constant expression:

> A variable or temporary object o is _constant-initialized_ if [...]
the full-expression of its initialization is a constant expression when
interpreted as a _constant-expression_, [...]

That can't be known until we've finished parsing the initializer, by
which time we've already added immediate invocations and consteval
references to the current expression evaluation context. This will have
the effect of evaluating said invocations as full expressions when the
context is popped, even if they're subexpressions of a larger constant
expression initializer. If, however, the variable _is_
constant-initialized, then its initializer is [manifestly
constant-evaluated](https://eel.is/c++draft/expr.const#20):

> An expression or conversion is _manifestly constant-evaluated_ if it
is [...] **the initializer of a variable that is usable in constant
expressions or has constant initialization** [...]

which in turn means that any subexpressions naming an immediate function
are in an [immediate function
context](https://eel.is/c++draft/expr.const#16):

> An expression or conversion is in an immediate function context if it
is potentially evaluated and either [...] it is a **subexpression of a
manifestly constant-evaluated expression** or conversion

and therefore _are not to be considered [immediate
invocations](https://eel.is/c++draft/expr.const#16) or
[immediate-escalating
expressions](https://eel.is/c++draft/expr.const#17) in the first place_:

> An invocation is an _immediate invocation_ if it is a
potentially-evaluated explicit or implicit invocation of an immediate
function and **is not in an immediate function context**.

> An expression or conversion is _immediate-escalating_ if **it is not
initially in an immediate function context** and [...]


The approach that I'm therefore proposing is:
1. Create a new expression evaluation context for _every_ variable
initializer (rather than only nonlocal ones).
2. Attach initializers to `VarDecl`s _prior_ to popping the expression
evaluation context / scope / etc. This sequences the determination of
whether the initializer is in an immediate function context _before_ any
contained immediate invocations are evaluated.
3. When popping an expression evaluation context, elide all evaluations
of constant invocations, and all checks for consteval references, if the
context is an immediate function context. Note that if it could be
ascertained that this was an immediate function context at parse-time,
we [would never have
registered](https://github.com/llvm/llvm-project/blob/760910ddb918d77e7632be1678f69909384d69ae/clang/lib/Sema/SemaExpr.cpp#L17799)
these immediate invocations or consteval references in the first place.

Most of the test changes previously made for this PR are now reverted
and passing as-is. The only test updates needed are now as follows:
- A few diagnostics in `consteval-cxx2a.cpp` are updated to reflect that
it is the `consteval tester::tester` constructor, not the more narrow
`make_name` function call, which fails to be evaluated as a constant
expression.
- The reclassification of `warn_impcast_integer_precision_constant` as a
compile-time diagnostic adds a (somewhat duplicative) warning when
attempting to define an enum constant using a narrowing conversion. It
also, however, retains the existing diagnostics which @erichkeane
(rightly) objected to being lost from an earlier revision of this PR.

---------

Co-authored-by: cor3ntin <corentinjabot@gmail.com>

1 files changed, 10 insertions, 11 deletions

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 4e4b05b..2c11ae6 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -2587,25 +2587,30 @@ Decl *Parser::ParseDeclarationAfterDeclaratorAndAttributes(
     Parser &P;
     Declarator &D;
     Decl *ThisDecl;
+    bool Entered;
 
     InitializerScopeRAII(Parser &P, Declarator &D, Decl *ThisDecl)
-        : P(P), D(D), ThisDecl(ThisDecl) {
+        : P(P), D(D), ThisDecl(ThisDecl), Entered(false) {
       if (ThisDecl && P.getLangOpts().CPlusPlus) {
         Scope *S = nullptr;
         if (D.getCXXScopeSpec().isSet()) {
           P.EnterScope(0);
           S = P.getCurScope();
         }
-        P.Actions.ActOnCXXEnterDeclInitializer(S, ThisDecl);
+        if (ThisDecl && !ThisDecl->isInvalidDecl()) {
+          P.Actions.ActOnCXXEnterDeclInitializer(S, ThisDecl);
+          Entered = true;
+        }
       }
     }
-    ~InitializerScopeRAII() { pop(); }
-    void pop() {
+    ~InitializerScopeRAII() {
       if (ThisDecl && P.getLangOpts().CPlusPlus) {
         Scope *S = nullptr;
         if (D.getCXXScopeSpec().isSet())
           S = P.getCurScope();
-        P.Actions.ActOnCXXExitDeclInitializer(S, ThisDecl);
+
+        if (Entered)
+          P.Actions.ActOnCXXExitDeclInitializer(S, ThisDecl);
         if (S)
           P.ExitScope();
       }
@@ -2736,8 +2741,6 @@ Decl *Parser::ParseDeclarationAfterDeclaratorAndAttributes(
         FRI->RangeExpr = Init;
       }
 
-      InitScope.pop();
-
       if (Init.isInvalid()) {
         SmallVector<tok::TokenKind, 2> StopTokens;
         StopTokens.push_back(tok::comma);
@@ -2785,8 +2788,6 @@ Decl *Parser::ParseDeclarationAfterDeclaratorAndAttributes(
 
     bool SawError = ParseExpressionList(Exprs, ExpressionStarts);
 
-    InitScope.pop();
-
     if (SawError) {
       if (ThisVarDecl && PP.isCodeCompletionReached() && !CalledSignatureHelp) {
         Actions.ProduceConstructorSignatureHelp(
@@ -2818,8 +2819,6 @@ Decl *Parser::ParseDeclarationAfterDeclaratorAndAttributes(
     PreferredType.enterVariableInit(Tok.getLocation(), ThisDecl);
     ExprResult Init(ParseBraceInitializer());
 
-    InitScope.pop();
-
     if (Init.isInvalid()) {
       Actions.ActOnInitializerError(ThisDecl);
     } else