[Clang] Functions called in discarded statements should not be instantiated

Functions referenced in discarded statements could be treated as odr-used
because we did not properly set the correct evaluation context in some
places.

Fixes #140449

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -745,6 +745,7 @@ Bug Fixes to C++ Support
 - Fix an incorrect deduction when calling an explicit object member function template through an overload set address.
 - Fixed bug in constant evaluation that would allow using the value of a
   reference in its own initializer in C++23 mode (#GH131330).
+- Clang could incorrectly instantiate functions in discarded contexts (#GH140449)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Sema/Sema.h

@@ -6825,8 +6825,9 @@ class Sema final : public SemaBase {
 
     bool isDiscardedStatementContext() const {
       return Context == ExpressionEvaluationContext::DiscardedStatement ||
-             (Context ==
-                  ExpressionEvaluationContext::ImmediateFunctionContext &&
+             ((Context ==
+                   ExpressionEvaluationContext::ImmediateFunctionContext ||
+               isPotentiallyEvaluated()) &&
               InDiscardedStatement);
     }
   };

clang/lib/Sema/SemaCoroutine.cpp

@@ -699,6 +699,9 @@ bool Sema::ActOnCoroutineBodyStart(Scope *SC, SourceLocation KWLoc,
   // Ignore previous expr evaluation contexts.
   EnterExpressionEvaluationContext PotentiallyEvaluated(
       *this, Sema::ExpressionEvaluationContext::PotentiallyEvaluated);
+
+  ExprEvalContexts.back().InDiscardedStatement = false;
+
   if (!checkCoroutineContext(*this, KWLoc, Keyword))
     return false;
   auto *ScopeInfo = getCurFunction();

clang/lib/Sema/SemaDecl.cpp

@@ -15889,6 +15889,9 @@ Decl *Sema::ActOnStartOfFunctionDef(Scope *FnBodyScope, Decl *D,
   ExprEvalContexts.back().InImmediateEscalatingFunctionContext =
       getLangOpts().CPlusPlus20 && FD->isImmediateEscalating();
 
+  // A function that is not a lambda is never in a discarded statement
+  ExprEvalContexts.back().InDiscardedStatement = false;
+
   // Check for defining attributes before the check for redefinition.
   if (const auto *Attr = FD->getAttr<AliasAttr>()) {
     Diag(Attr->getLocation(), diag::err_alias_is_definition) << FD << 0;

clang/lib/Sema/SemaExpr.cpp

@@ -18360,35 +18360,23 @@ enum class OdrUseContext {
 /// Are we within a context in which references to resolved functions or to
 /// variables result in odr-use?
 static OdrUseContext isOdrUseContext(Sema &SemaRef) {
-  OdrUseContext Result;
+  const Sema::ExpressionEvaluationContextRecord &Context =
+      SemaRef.currentEvaluationContext();
 
-  switch (SemaRef.ExprEvalContexts.back().Context) {
-    case Sema::ExpressionEvaluationContext::Unevaluated:
-    case Sema::ExpressionEvaluationContext::UnevaluatedList:
-    case Sema::ExpressionEvaluationContext::UnevaluatedAbstract:
-      return OdrUseContext::None;
-
-    case Sema::ExpressionEvaluationContext::ConstantEvaluated:
-    case Sema::ExpressionEvaluationContext::ImmediateFunctionContext:
-    case Sema::ExpressionEvaluationContext::PotentiallyEvaluated:
-      Result = OdrUseContext::Used;
-      break;
-
-    case Sema::ExpressionEvaluationContext::DiscardedStatement:
-      Result = OdrUseContext::FormallyOdrUsed;
-      break;
-
-    case Sema::ExpressionEvaluationContext::PotentiallyEvaluatedIfUsed:
-      // A default argument formally results in odr-use, but doesn't actually
-      // result in a use in any real sense until it itself is used.
-      Result = OdrUseContext::FormallyOdrUsed;
-      break;
-  }
+  if (Context.isUnevaluated())
+    return OdrUseContext::None;
 
   if (SemaRef.CurContext->isDependentContext())
     return OdrUseContext::Dependent;
 
-  return Result;
+  if (Context.isDiscardedStatementContext())
+    return OdrUseContext::FormallyOdrUsed;
+
+  else if (Context.Context ==
+           Sema::ExpressionEvaluationContext::PotentiallyEvaluatedIfUsed)
+    return OdrUseContext::FormallyOdrUsed;
+
+  return OdrUseContext::Used;
 }
 
 static bool isImplicitlyDefinableConstexprFunction(FunctionDecl *Func) {
@@ -20355,9 +20343,11 @@ MarkExprReferenced(Sema &SemaRef, SourceLocation Loc, Decl *D, Expr *E,
 }
 
 void Sema::MarkDeclRefReferenced(DeclRefExpr *E, const Expr *Base) {
-  // TODO: update this with DR# once a defect report is filed.
-  // C++11 defect. The address of a pure member should not be an ODR use, even
-  // if it's a qualified reference.
+  // [basic.def.odr] (CWG 1614)
+  // A function is named by an expression or conversion [...]
+  // unless it is a pure virtual function and either the expression is not an
+  // id-expression naming the function with an explicitly qualified name or
+  // the expression forms a pointer to member
   bool OdrUse = true;
   if (const CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(E->getDecl()))
     if (Method->isVirtual() &&

clang/test/CXX/stmt.stmt/stmt.select/stmt.if/p2.cpp

@@ -177,4 +177,37 @@ void f() {
 }
 } // namespace deduced_return_type_in_discareded_statement
 
+namespace GH140449 {
+
+template <typename T>
+int f() {
+    T *ptr;
+    return 0;
+}
+
+template <typename T>
+constexpr int g() {
+    T *ptr; // expected-error{{'ptr' declared as a pointer to a reference of type 'int &'}}
+    return 0;
+}
+
+template <typename T>
+auto h() {
+    T *ptr; // expected-error{{'ptr' declared as a pointer to a reference of type 'int &'}}
+    return 0;
+}
+
+void test() {
+    if constexpr (false) {
+        int x = f<int &>();
+        constexpr int y = g<int &>();
+        // expected-error@-1 {{constexpr variable 'y' must be initialized by a constant expression}} \
+        // expected-note@-1{{in instantiation of function template specialization}}
+        int z = h<int &>();
+        // expected-note@-1{{in instantiation of function template specialization}}
+
+    }
+}
+}
+
 #endif