[C11] Generic selection expressions and qualified rvalues

clang/docs/ReleaseNotes.rst

@@ -740,6 +740,10 @@ Bug Fixes in This Version
   negatives where the analysis failed to detect unchecked access to guarded
   data.
 
+- When passing a qualified rvalue as the controlling expression of a
+  ``_Generic`` selection expression, Clang now properly strips the qualifiers.
+  Fixes #GH96713
+
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

clang/lib/Sema/SemaExpr.cpp

@@ -1828,18 +1828,26 @@ ExprResult Sema::CreateGenericSelectionExpr(
   // Look at the canonical type of the controlling expression in case it was a
   // deduced type like __auto_type. However, when issuing diagnostics, use the
   // type the user wrote in source rather than the canonical one.
+  //
+  // Note: there is an edge case for this in C where you can get a
+  // qualified rvalue and the qualifiers are not stripped by the lvalue
+  // conversion applied above. e.g.,
+  //
+  //   struct { const int i; } foo();
+  //   _Generic(foo().i, ...);
+  //
+  // Therefore, in C, we will still ask for the non-atomic, unqualified
+  // type despite those qualifiers generally being stripped above.
+  QualType ControllingTypeToUse =
+      ControllingExpr ? ControllingExpr->getType() : ControllingType->getType();
+  ControllingTypeToUse = ControllingTypeToUse.getCanonicalType();
+  if (!getLangOpts().CPlusPlus && ControllingExpr)
+    ControllingTypeToUse = ControllingTypeToUse.getAtomicUnqualifiedType();
   for (unsigned i = 0; i < NumAssocs; ++i) {
     if (!Types[i])
       DefaultIndex = i;
-    else if (ControllingExpr &&
-             Context.typesAreCompatible(
-                 ControllingExpr->getType().getCanonicalType(),
-                 Types[i]->getType()))
-      CompatIndices.push_back(i);
-    else if (ControllingType &&
-             Context.typesAreCompatible(
-                 ControllingType->getType().getCanonicalType(),
-                 Types[i]->getType()))
+    else if (Context.typesAreCompatible(ControllingTypeToUse,
+                                        Types[i]->getType()))
       CompatIndices.push_back(i);
   }
 

clang/test/Sema/generic-selection.c

@@ -86,3 +86,13 @@ void GH55562(void) {
   struct S s = { 0 };
   int i = s.a;
 }
+
+struct { const int i; } GH96713_1(void);
+void GH96713_2(void) {
+  _Static_assert(           // ext-warning {{'_Static_assert' is a C11 extension}}
+    _Generic(GH96713_1().i, // ext-warning {{'_Generic' is a C11 extension}}
+      int : 1,
+      const int : 0         // expected-warning {{due to lvalue conversion of the controlling expression, association of type 'const int' will never be selected because it is qualified}}
+    ), ""
+  );
+}