[clang] Set FP options in Sema when instantiating CompoundStmt

[sepavloff]

Summary

When an expression is instantiated, TreeTransform skips ImplicitCastExpr
nodes, assuming they are recreated when the instantiated expression is
built. It breaks functions that use non-default floating-point options,
because they are kept in these ImplicitCastExprs. In this case the
recreated ImplicitCastExpr takes FP options from the current Sema state
and not from AST node.

To fix this issue FP options in Sema object are set when a compound
statement is cloned in TreeTransform.

This change fixes https://github.com/llvm/llvm-project/issues/64605
([Regression 16 -> 17] Template instantiation ignores FENV_ACCESS being
ON for both definition and instantiation).

[hubert.reinterpretcast]

LGTM; thanks!

clang/lib/Sema/TreeTransform.h

TreeTransform<Derived>::TransformCompoundStmt(CompoundStmt *S) {
   return getDerived().TransformCompoundStmt(S, false);
 }
 
 template<typename Derived>
 StmtResult
 TreeTransform<Derived>::TransformCompoundStmt(CompoundStmt *S,
                                               bool IsStmtExpr) {
   Sema::CompoundScopeRAII CompoundScope(getSema());
+  Sema::FPFeaturesStateRAII FPSave(getSema());
+  if (S->hasStoredFPFeatures())
+    getSema().resetFPOptions(
+        S->getStoredFPFeatures().applyOverrides(getSema().getLangOpts()));
 
   const Stmt *ExprResult = S->getStmtExprResult();
   bool SubStmtInvalid = false;
   bool SubStmtChanged = false;
   SmallVector<Stmt*, 8> Statements;
   for (auto *B : S->body()) {
     StmtResult Result = getDerived().TransformStmt(
         B, IsStmtExpr && B == ExprResult ? SDK_StmtExprResult : SDK_Discarded);

clang/test/SemaCXX/template-64605.cpp

+// RUN: %clang_cc1 -ast-dump -ast-dump-filter=b_64605 %s | FileCheck %s
+
+// https://github.com/llvm/llvm-project/issues/64605
+
+#pragma STDC FENV_ACCESS ON
+template <typename>
+int b_64605() {
+  int x;
+  if ((float)0xFFFFFFFF != (float)0x100000000) {
+    x = 1;
+  }
+  return x;
+}
+int f() { return b_64605<void>(); }
+
+// CHECK:      ImplicitCastExpr {{.*}} 'float' <IntegralToFloating> RoundingMath=1 AllowFEnvAccess=1
+// CHECK-NEXT: IntegerLiteral {{.*}} 4294967295
+
+// CHECK:      FunctionDecl {{.*}} b_64605 'int ()' implicit_instantiation
+// CHECK-NEXT: TemplateArgument type 'void'
+
+// CHECK:      ImplicitCastExpr {{.*}} 'float' <IntegralToFloating> RoundingMath=1 AllowFEnvAccess=1
+// CHECK-NEXT: IntegerLiteral {{.*}} 4294967295