Fix interaction of `constinit` and `weak`.

We previously took a shortcut and said that weak variables never have
constant initializers (because those initializers are never correct to
use outside the variable). We now say that weak variables can have
constant initializers, but are never usable in constant expressions.

clang/lib/AST/Decl.cpp

@@ -2287,6 +2287,10 @@ bool VarDecl::mightBeUsableInConstantExpressions(ASTContext &C) const {
   if (isa<ParmVarDecl>(this))
     return false;
 
+  // The values of weak variables are never usable in constant expressions.
+  if (isWeak())
+    return false;
+
   // In C++11, any variable of reference type can be used in a constant
   // expression if it is initialized by a constant expression.
   if (Lang.CPlusPlus11 && getType()->isReferenceType())
@@ -2414,10 +2418,6 @@ bool VarDecl::isInitICE() const {
 }
 
 bool VarDecl::checkInitIsICE() const {
-  // Initializers of weak variables are never ICEs.
-  if (isWeak())
-    return false;
-
   EvaluatedStmt *Eval = ensureEvaluatedStmt();
   if (Eval->CheckedICE)
     // We have already checked whether this subexpression is an

clang/lib/AST/ExprConstant.cpp

@@ -14816,7 +14816,7 @@ static ICEDiag CheckICE(const Expr* E, const ASTContext &Ctx) {
         const VarDecl *VD;
         // Look for a declaration of this variable that has an initializer, and
         // check whether it is an ICE.
-        if (Dcl->getAnyInitializer(VD) && VD->checkInitIsICE())
+        if (Dcl->getAnyInitializer(VD) && !VD->isWeak() && VD->checkInitIsICE())
           return NoDiag();
         else
           return ICEDiag(IK_NotICE, cast<DeclRefExpr>(E)->getLocation());

clang/lib/Sema/SemaDeclCXX.cpp

@@ -11112,7 +11112,7 @@ QualType Sema::CheckComparisonCategoryType(ComparisonCategoryType Kind,
     // might be foobar, including it failing to be a constant expression.
     // TODO Handle more ways the lookup or result can be invalid.
     if (!VD->isStaticDataMember() || !VD->isConstexpr() || !VD->hasInit() ||
-        !VD->checkInitIsICE())
+        VD->isWeak() || !VD->checkInitIsICE())
       return UnsupportedSTLError(USS_InvalidMember, MemName, VD);
 
     // Attempt to evaluate the var decl as a constant expression and extract

clang/test/SemaCXX/cxx20-constinit.cpp

@@ -0,0 +1,4 @@
+// RUN: %clang_cc1 %s -std=c++20 -verify
+// expected-no-diagnostics
+
+constinit int a __attribute__((weak)) = 0;