diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/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) {
diff --git a/clang/test/Sema/static-assert.c b/clang/test/Sema/static-assert.c
--- a/clang/test/Sema/static-assert.c
+++ b/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@-2 {{static assertion expression is not an integral constant expression}}
+// ext-warning@-2 {{expression is not an integer constant}}
 #endif
 
 _Static_assert(1, "1 is nonzero"); // ext-warning {{'_Static_assert' is a C11 extension}}