[clang] Backport: stop error recovery in SFINAE for narrowing in converted constant expressions

[mizvekov]

A narrowing conversion in a converted constant expression should produce an invalid expression so that [temp.deduct.general]p7 is satisfied, by stopping substitution at this point.

Backport from #183614

Fixes #167709

[llvmbot]

@llvm/pr-subscribers-clang

Author: Matheus Izvekov (mizvekov)

Changes

A narrowing conversion in a converted constant expression should produce an invalid expression so that [temp.deduct.general]p7 is satisfied, by stopping substitution at this point.

Fixes #167709

---

Full diff: https://github.com/llvm/llvm-project/pull/183819.diff

2 Files Affected:

• (modified) clang/lib/Sema/SemaOverload.cpp (+8)
• (modified) clang/test/SemaTemplate/temp_arg_nontype_cxx11.cpp (+10-1)

diff --git a/clang/lib/Sema/SemaOverload.cpp b/clang/lib/Sema/SemaOverload.cpp
index 7115b8b7d446a..0b6296e9b2efa 100644
--- a/clang/lib/Sema/SemaOverload.cpp
+++ b/clang/lib/Sema/SemaOverload.cpp
@@ -6470,6 +6470,12 @@ static ExprResult BuildConvertedConstantExpression(Sema &S, Expr *From,
     S.Diag(From->getBeginLoc(), diag::ext_cce_narrowing)
         << CCE << /*Constant*/ 1
         << PreNarrowingValue.getAsString(S.Context, PreNarrowingType) << T;
+    // If this is an SFINAE Context, treat the result as invalid so it stops
+    // substitution at this point, respecting C++26 [temp.deduct.general]p7.
+    // FIXME: Should do this whenever the above diagnostic is an error, but
+    // without further changes this would degrade some other diagnostics.
+    if (S.isSFINAEContext())
+      return ExprError();
     break;
 
   case NK_Dependent_Narrowing:
@@ -6485,6 +6491,8 @@ static ExprResult BuildConvertedConstantExpression(Sema &S, Expr *From,
     // constant expression.
     S.Diag(From->getBeginLoc(), diag::ext_cce_narrowing)
         << CCE << /*Constant*/ 0 << From->getType() << T;
+    if (S.isSFINAEContext())
+      return ExprError();
     break;
   }
   if (!ReturnPreNarrowingValue)
diff --git a/clang/test/SemaTemplate/temp_arg_nontype_cxx11.cpp b/clang/test/SemaTemplate/temp_arg_nontype_cxx11.cpp
index 45bdb4c623dfe..0b785700ee238 100644
--- a/clang/test/SemaTemplate/temp_arg_nontype_cxx11.cpp
+++ b/clang/test/SemaTemplate/temp_arg_nontype_cxx11.cpp
@@ -43,7 +43,7 @@ void TempFunc() {}
 
 void Useage() {
   //expected-error@+2 {{no matching function}}
-  //expected-note@-4 {{candidate template ignored: substitution failure [with a = 1, b = 4294967295, c = 1]: non-type template argument evaluates to -1, which cannot be narrowed to type 'unsigned int'}}
+  //expected-note@-4 {{candidate template ignored: invalid explicitly-specified argument for template parameter 'b'}}
   TempFunc<1, -1, 1>();
 }
 }
@@ -114,3 +114,12 @@ void lookup() {
   Kolumn<&container::a>().ls();   // expected-error {{<&container::a}}
   Kolumn<nullptr>().ls();         // expected-error {{<nullptr}}
 }
+
+namespace GH167709 {
+  template <unsigned I> struct A {
+    static_assert(false, "shouldn't instantiate this");
+  };
+  template <int> void f() {}
+  template <int I> typename A<I>::type f() = delete;
+  template void f<-1>();
+} // namespace GH167709