[PATCH] D134311: [clang] handle extended integer constant expressions in _Static_assert (PR #57687)

[Martin Sebor via Phabricator via cfe-commits]

msebor created this revision.
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The discussion in PR #57687 <https://github.com/llvm/llvm-project/issues/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`.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D134311

Files:
  clang/lib/Sema/SemaDeclCXX.cpp
  clang/test/Sema/static-assert.c


Index: clang/test/Sema/static-assert.c
===================================================================
--- clang/test/Sema/static-assert.c
+++ clang/test/Sema/static-assert.c
@@ -5,7 +5,7 @@
 
 _Static_assert("foo", "string is nonzero"); // ext-warning {{'_Static_assert' is a C11 extension}}
 #ifndef __cplusplus
-// expected-error at -2 {{static assertion expression is not an integral constant expression}}
+// ext-warning at -2 {{expression is not an integer constant}}
 #endif
 
 _Static_assert(1, "1 is nonzero"); // ext-warning {{'_Static_assert' is a C11 extension}}
Index: clang/lib/Sema/SemaDeclCXX.cpp
===================================================================
--- clang/lib/Sema/SemaDeclCXX.cpp
+++ clang/lib/Sema/SemaDeclCXX.cpp
@@ -16721,10 +16721,22 @@
       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;
+      while (auto *BaseCast = dyn_cast<ImplicitCastExpr>(BaseExpr))
+        BaseExpr = BaseCast->getSubExpr();
+    }
+
     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) {


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