release/21.x: [clang] Fix potential constant expression checking with constexpr-unknown.

[efriedma-quic]

Manual backport of 6a60f18

[efriedma-quic]

It looks like I triggered the wrong set of bots by opening the pull request with the wrong base branch. I'm going to close/reopen.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Eli Friedman (efriedma-quic)

Changes

Manual backport of 6a60f18

---

Full diff: https://github.com/llvm/llvm-project/pull/149402.diff

3 Files Affected:

• (modified) clang/lib/AST/ExprConstant.cpp (+7-4)
• (modified) clang/test/SemaCXX/constant-expression-p2280r4.cpp (+26)
• (modified) clang/test/SemaCXX/constexpr-never-constant.cpp (+7)

diff --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 1b33b6706e204..d7b1173283c57 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -4441,7 +4441,8 @@ static CompleteObject findCompleteObject(EvalInfo &Info, const Expr *E,
         }
       } else if (!IsAccess) {
         return CompleteObject(LVal.getLValueBase(), nullptr, BaseType);
-      } else if (IsConstant && Info.checkingPotentialConstantExpression() &&
+      } else if ((IsConstant || BaseType->isReferenceType()) &&
+                 Info.checkingPotentialConstantExpression() &&
                  BaseType->isLiteralType(Info.Ctx) && !VD->hasDefinition()) {
         // This variable might end up being constexpr. Don't diagnose it yet.
       } else if (IsConstant) {
@@ -4478,9 +4479,11 @@ static CompleteObject findCompleteObject(EvalInfo &Info, const Expr *E,
     // a null BaseVal. Any constexpr-unknown variable seen here is an error:
     // we can't access a constexpr-unknown object.
     if (!BaseVal) {
-      Info.FFDiag(E, diag::note_constexpr_access_unknown_variable, 1)
-          << AK << VD;
-      Info.Note(VD->getLocation(), diag::note_declared_at);
+      if (!Info.checkingPotentialConstantExpression()) {
+        Info.FFDiag(E, diag::note_constexpr_access_unknown_variable, 1)
+            << AK << VD;
+        Info.Note(VD->getLocation(), diag::note_declared_at);
+      }
       return CompleteObject();
     }
   } else if (DynamicAllocLValue DA = LVal.Base.dyn_cast<DynamicAllocLValue>()) {
diff --git a/clang/test/SemaCXX/constant-expression-p2280r4.cpp b/clang/test/SemaCXX/constant-expression-p2280r4.cpp
index 03fea91169787..16f5f823d26c1 100644
--- a/clang/test/SemaCXX/constant-expression-p2280r4.cpp
+++ b/clang/test/SemaCXX/constant-expression-p2280r4.cpp
@@ -357,3 +357,29 @@ namespace pointer_comparisons {
   static_assert(!f4()); // expected-error {{static assertion expression is not an integral constant expression}} \
                         // expected-note {{in call to 'f4()'}}
 }
+
+namespace GH149188 {
+namespace enable_if_1 {
+  template <__SIZE_TYPE__ N>
+  constexpr void foo(const char (&Str)[N])
+  __attribute((enable_if(__builtin_strlen(Str), ""))) {}
+
+  void x() {
+      foo("1234");
+  }
+}
+
+namespace enable_if_2 {
+  constexpr const char (&f())[];
+  extern const char (&Str)[];
+  constexpr int foo()
+  __attribute((enable_if(__builtin_strlen(Str), "")))
+  {return __builtin_strlen(Str);}
+
+  constexpr const char (&f())[] {return "a";}
+  constexpr const char (&Str)[] = f();
+  void x() {
+      constexpr int x = foo();
+  }
+}
+}
diff --git a/clang/test/SemaCXX/constexpr-never-constant.cpp b/clang/test/SemaCXX/constexpr-never-constant.cpp
index 307810ee263dd..5756bb647ce88 100644
--- a/clang/test/SemaCXX/constexpr-never-constant.cpp
+++ b/clang/test/SemaCXX/constexpr-never-constant.cpp
@@ -24,3 +24,10 @@ constexpr void other_func() {
 
   throw 12;
 }
+
+namespace GH149041 {
+  // Make sure these don't trigger the diagnostic.
+  extern const bool& b;
+  constexpr bool fun1() { return b; }
+  constexpr bool fun2(const bool& b) { return b; }
+}