[clang] format string checking for conpile-time evaluated str literal

This patch enhances clang's ability to check compile-time determinable
string literals as format strings, and can give FixIt hints at literals
(unlike gcc). Issue https://github.com/llvm/llvm-project/issues/55805
mentiond two compile-time string cases. And this patch partially fixes
one.

```
constexpr const char* foo() {
  return "%s %d";
}
int main() {
   printf(foo(), "abc", "def");
   return 0;
}
```

This patch enables clang check format string for this:

```
<source>:4:24: warning: format specifies type 'int' but the argument has type 'const char *' [-Wformat]
  printf(foo(), "abc", "def");
         ~~~~~         ^~~~~
<source>:2:42: note: format string is defined here
constexpr const char *foo() { return "%s %d"; }
                                         ^~
                                         %s
1 warning generated.
```

Reviewed By: aaron.ballman

Signed-off-by: YingChi Long <me@inclyc.cn>

Differential Revision: https://reviews.llvm.org/D130906

1 files changed, 22 insertions, 1 deletions

diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 6b60312..6428be1 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -8473,6 +8473,9 @@ static void CheckFormatString(
     llvm::SmallBitVector &CheckedVarArgs, UncoveredArgHandler &UncoveredArg,
     bool IgnoreStringsWithoutSpecifiers);
 
+static const Expr *maybeConstEvalStringLiteral(ASTContext &Context,
+                                               const Expr *E);
+
 // Determine if an expression is a string literal or constant string.
 // If this function returns false on the arguments to a function expecting a
 // format string, we will usually need to emit a warning.
@@ -8713,7 +8716,11 @@ tryAgain:
         }
       }
     }
-
+    if (const auto *SLE = maybeConstEvalStringLiteral(S.Context, E))
+      return checkFormatStringExpr(S, SLE, Args, APK, format_idx, firstDataArg,
+                                   Type, CallType, /*InFunctionCall*/ false,
+                                   CheckedVarArgs, UncoveredArg, Offset,
+                                   IgnoreStringsWithoutSpecifiers);
     return SLCT_NotALiteral;
   }
   case Stmt::ObjCMessageExprClass: {
@@ -8823,6 +8830,20 @@ tryAgain:
   }
 }
 
+// If this expression can be evaluated at compile-time,
+// check if the result is a StringLiteral and return it
+// otherwise return nullptr
+static const Expr *maybeConstEvalStringLiteral(ASTContext &Context,
+                                               const Expr *E) {
+  Expr::EvalResult Result;
+  if (E->EvaluateAsRValue(Result, Context) && Result.Val.isLValue()) {
+    const auto *LVE = Result.Val.getLValueBase().dyn_cast<const Expr *>();
+    if (isa_and_nonnull<StringLiteral>(LVE))
+      return LVE;
+  }
+  return nullptr;
+}
+
 Sema::FormatStringType Sema::GetFormatStringType(const FormatAttr *Format) {
   return llvm::StringSwitch<FormatStringType>(Format->getType()->getName())
       .Case("scanf", FST_Scanf)