[clang] CTAD alias: Emit a more descriptive diagnostic message when is_deducible constraint is evaluated to false.

[hokein]

Fixes #92225

[llvmbot]

@llvm/pr-subscribers-clang

Author: Haojian Wu (hokein)

Changes

Fixes #92225

---

Full diff: https://github.com/llvm/llvm-project/pull/92389.diff

3 Files Affected:

• (modified) clang/include/clang/Basic/DiagnosticSemaKinds.td (+2)
• (modified) clang/lib/Sema/SemaConcept.cpp (+7)
• (modified) clang/test/SemaCXX/cxx20-ctad-type-alias.cpp (+3-3)

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index e648b503ac034..04bf6ed2c52f5 100644
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -3003,6 +3003,8 @@ def note_single_arg_concept_specialization_constraint_evaluated_to_false : Note<
   "%select{and|because}0 %1 does not satisfy %2">;
 def note_atomic_constraint_evaluated_to_false_elaborated : Note<
   "%select{and|because}0 '%1' (%2 %3 %4) evaluated to false">;
+def note_is_deducible_constraint_evaluated_to_false : Note<
+  "cannot deduce template arguments of %0 from %1">;
 def err_constrained_virtual_method : Error<
   "virtual function cannot have a requires clause">;
 def err_trailing_requires_clause_on_deduction_guide : Error<
diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 7bfec4e11f7aa..202dd86c67f62 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -1162,6 +1162,13 @@ static void diagnoseWellFormedUnsatisfiedConstraintExpr(Sema &S,
         break;
       }
     return;
+  } else if (auto *TTE = dyn_cast<TypeTraitExpr>(SubstExpr);
+             TTE && TTE->getTrait() == clang::TypeTrait::BTT_IsDeducible) {
+    assert(TTE->getNumArgs() == 2);
+    S.Diag(SubstExpr->getSourceRange().getBegin(),
+           diag::note_is_deducible_constraint_evaluated_to_false)
+        << TTE->getArg(0)->getType() << TTE->getArg(1)->getType();
+    return;
   }
 
   S.Diag(SubstExpr->getSourceRange().getBegin(),
diff --git a/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp b/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
index 7c186dc379c7b..f7367d01bc2eb 100644
--- a/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
+++ b/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
@@ -111,7 +111,7 @@ struct Foo {
 template <typename X, int Y>
 using Bar = Foo<X, sizeof(X)>; // expected-note {{candidate template ignored: couldn't infer template argument 'X'}} \
                                // expected-note {{candidate template ignored: constraints not satisfied [with X = int]}} \
-                               // expected-note {{because '__is_deducible}}
+                               // expected-note {{can not deduce template arguments for 'Bar' from the type 'Foo<int, 4UL>'}}
 
 
 Bar s = {{1}}; // expected-error {{no viable constructor or deduction guide }}
@@ -138,7 +138,7 @@ template<class T> struct Foo { T c; };
 template<class X, class Y=A>
 using AFoo = Foo<Y>; // expected-note {{candidate template ignored: could not match 'Foo<type-parameter-0-0>' against 'int'}} \
                     // expected-note {{candidate template ignored: constraints not satisfied [with Y = int]}} \
-                    // expected-note {{because '__is_deducible(AFoo, Foo<int>)' evaluated to false}} \
+                    // expected-note {{cannot deduce template arguments for 'AFoo' from 'Foo<int>'}} \
                     // expected-note {{candidate function template not viable: requires 0 arguments, but 1 was provided}}
 
 AFoo s = {1}; // expected-error {{no viable constructor or deduction guide for deduction of template arguments of 'AFoo'}}
@@ -199,7 +199,7 @@ template<typename> concept False = false;
 // FIXME: emit a more descriptive diagnostic for "__is_deducible" constraint failure.
 template<False W>
 using BFoo = AFoo<W>; // expected-note {{candidate template ignored: constraints not satisfied [with V = int]}} \
-                      // expected-note {{because '__is_deducible(BFoo, Foo<int *>)' evaluated to false}} \
+                      // expected-note {{cannot deduce template arguments for 'BFoo' from 'Foo<int *>'}} \
                       // expected-note {{candidate template ignored: could not match 'Foo<type-parameter-0-0 *>' against 'int *'}}
 int i = 0;
 AFoo a1(&i); // OK, deduce Foo<int *>

[cor3ntin]

LGTM

[zeroomega]

Hi @hokein , your change probably broke the test "Clang :: SemaCXX/cxx20-ctad-type-alias.cpp" on Windows. Could you take a look please?

We are seeing test failures on this with error message:

Exit Code: 1

Command Output (stdout):
--
# RUN: at line 1
c:\b\s\w\ir\x\w\llvm_build\bin\clang.exe -cc1 -internal-isystem C:\b\s\w\ir\x\w\llvm_build\lib\clang\19\include -nostdsysteminc -fsyntax-only -Wno-c++11-narrowing -Wno-literal-conversion -std=c++20 -verify C:\b\s\w\ir\x\w\llvm-llvm-project\clang\test\SemaCXX\cxx20-ctad-type-alias.cpp
# executed command: 'c:\b\s\w\ir\x\w\llvm_build\bin\clang.exe' -cc1 -internal-isystem 'C:\b\s\w\ir\x\w\llvm_build\lib\clang\19\include' -nostdsysteminc -fsyntax-only -Wno-c++11-narrowing -Wno-literal-conversion -std=c++20 -verify 'C:\b\s\w\ir\x\w\llvm-llvm-project\clang\test\SemaCXX\cxx20-ctad-type-alias.cpp'
# .---command stderr------------
# | error: 'expected-note' diagnostics expected but not seen: 
# |   File C:\b\s\w\ir\x\w\llvm-llvm-project\clang\test\SemaCXX\cxx20-ctad-type-alias.cpp Line 112: cannot deduce template arguments for 'Bar' from 'Foo<int, 4UL>'
# | error: 'expected-note' diagnostics seen but not expected: 
# |   File C:\b\s\w\ir\x\w\llvm-llvm-project\clang\test\SemaCXX\cxx20-ctad-type-alias.cpp Line 112: cannot deduce template arguments for 'Bar' from 'Foo<int, 4ULL>'
# | 2 errors generated.
# `-----------------------------
# error: command failed with exit status: 1

--

This happens on our Windows builder. Link to the task: https://ci.chromium.org/ui/p/fuchsia/builders/toolchain.ci/clang-windows-x64/b8747755569988931969/overview

[mizvekov]

I already fixed this. We just had to constrain the triple, as there are differences in size_t underlying type which affect diagnostics.

[hokein]

Thanks @mizvekov for the quick fix.