[clang] handle extended integer constant expressions in _Static_assert (PR #57687)

[msebor]

Summary

The discussion in PR #57687 turned up an inconsistency in where extended integer constant expressions are accepted: in most contexts Clang accepts boolean expressions involving string literals, it rejects them as the first argument to the _Static_assert expression. This change is my attempt to make _Static_assert consistent with the other contexts and with C++. I looked at how some of the other contexts are handled, hoping to follow their example, but couldn't find a simple way to do it. They all seem quite different from _Static_assert.

[msebor]

Update to the latest diff that didn't get picked up in the first submission.

[tbaeder]

clang/lib/Sema/SemaDeclCXX.cpp

16733

There is Expr::ignoreParenImpCasts() or Expr::ImpCasts() that should do this loop for you.

[aaron.ballman]

Thank you for working on this! It looks pretty close to good, with just a few comments. Can you also add a release note for the changes as well (to clang/docs/ReleaseNotes.rst)?

clang/lib/Sema/SemaDeclCXX.cpp

16733

+1, I would use Expr::IgnoreImpCasts() instead of this manual loop.

clang/test/Sema/static-assert.c

79

This makes it a bit easier to see there are two diagnostics expected on that line instead of just one.

[msebor]

Changes from previous version:

• Replace loop with Expr::IgnoreImpCasts().
• Use a multiline comment in a test to improve readability.

clang/lib/Sema/SemaDeclCXX.cpp

16733

Sure. Thank you both for the suggestion!

[aaron.ballman]

LGTM, though please add the release note when landing.

clang/docs/ReleaseNotes.rst

 infrastructure are described first, followed by language-specific
 sections with improvements to Clang's support for those languages.
 
 Major New Features
 ------------------
 
 Bug Fixes
 ---------
+- Correct ``_Static_assert`` to accept the same set of extended integer
+  constant expressions as is accpted in other contexts that accept them.
+  This fixes `Issue 57687 <https://github.com/llvm/llvm-project/issues/57687>`_.
 - Fixes an accepts-invalid bug in C when using a ``_Noreturn`` function
   specifier on something other than a function declaration. This fixes
   `Issue 56800 <https://github.com/llvm/llvm-project/issues/56800>`_.
 - Fix `#56772 <https://github.com/llvm/llvm-project/issues/56772>`_ - invalid
   destructor names were incorrectly accepted on template classes.
 - Improve compile-times with large dynamic array allocations with trivial
   constructors. This fixes
   `Issue 56774 <https://github.com/llvm/llvm-project/issues/56774>`_.

clang/lib/Sema/SemaDeclCXX.cpp

if (!AssertExpr->isTypeDependent() && !AssertExpr->isValueDependent() &&
     ExprResult Converted = PerformContextuallyConvertToBool(AssertExpr);
     if (Converted.isInvalid())
       Failed = true;
 
     ExprResult FullAssertExpr =
         ActOnFinishFullExpr(Converted.get(), StaticAssertLoc,
                             /*DiscardedValue*/ false,
                             /*IsConstexpr*/ true);
     if (FullAssertExpr.isInvalid())
       Failed = true;
     else
       AssertExpr = FullAssertExpr.get();
 
     llvm::APSInt Cond;
+    Expr *BaseExpr = AssertExpr;
+    AllowFoldKind FoldKind = NoFold;
+
+    if (!getLangOpts().CPlusPlus) {
+      // In C mode only allow folding and strip the implicit conversion
+      // to the type of the first _Static_assert argument that would
+      // otherwise suppress diagnostics for arguments that convert to int.
+      FoldKind = AllowFold;
+      BaseExpr = BaseExpr->IgnoreImpCasts();
+    }
+
     if (!Failed && VerifyIntegerConstantExpression(
-                       AssertExpr, &Cond,
-                       diag::err_static_assert_expression_is_not_constant)
-                       .isInvalid())
+                       BaseExpr, &Cond,
+                       diag::err_static_assert_expression_is_not_constant,
+                       FoldKind).isInvalid())
       Failed = true;
 
     if (!Failed && !Cond) {
       SmallString<256> MsgBuffer;
       llvm::raw_svector_ostream Msg(MsgBuffer);
       if (AssertMessage) {
         const auto *MsgStr = cast<StringLiteral>(AssertMessage);
         if (MsgStr->isOrdinary())

clang/test/Sema/static-assert.c

 
 // But we should still warn if the user did something bonkers.
 #undef static_assert
 static_assert(1, "1 is nonzero"); // ms-warning {{use of 'static_assert' without}}
 #endif
 
 _Static_assert(1 , "") // expected-error {{expected ';' after '_Static_assert'}} \
                       // ext-warning {{'_Static_assert' is a C11 extension}}
+
+static int static_var;
+_Static_assert(&static_var != 0, "");  // ext-warning {{'_Static_assert' is a C11 extension}} \
+                                       // expected-warning {{comparison of address of 'static_var' not equal to a null pointer is always true}}
+_Static_assert("" != 0, "");           // ext-warning {{'_Static_assert' is a C11 extension}}
+_Static_assert(("" != 0), "");         // ext-warning {{'_Static_assert' is a C11 extension}}
+_Static_assert(*"1", "");              // ext-warning {{'_Static_assert' is a C11 extension}}
+_Static_assert("1"[0], "");            // ext-warning {{'_Static_assert' is a C11 extension}}
+_Static_assert(1.0 != 0, "");          // ext-warning {{'_Static_assert' is a C11 extension}}
+_Static_assert(__builtin_strlen("1"), "");  // ext-warning {{'_Static_assert' is a C11 extension}}
+#ifndef __cplusplus
+// ext-warning@-9 {{expression is not an integer constant expression; folding it to a constant is a GNU extension}}
+// ext-warning@-8 {{expression is not an integer constant expression; folding it to a constant is a GNU extension}}
+// ext-warning@-8 {{expression is not an integer constant expression; folding it to a constant is a GNU extension}}
+// ext-warning@-8 {{expression is not an integer constant expression; folding it to a constant is a GNU extension}}
+// ext-warning@-8 {{expression is not an integer constant expression; folding it to a constant is a GNU extension}}
+// ext-warning@-8 {{expression is not an integer constant expression; folding it to a constant is a GNU extension}}
+// __builtin_strlen(literal) is considered an integer constant expression
+// and doesn't cause a pedantic warning
+#endif