[Sema] Fix assertion error in Sema::FindInstantiatedDecl

[alejandro-alvarez-sonarsource]

When looking for a template instantiation with a non-type parameter of unknown type and with a default value.

This can happen when a template non-type parameter has a broken expression that gets replaced by a RecoveryExpr.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Alejandro Álvarez Ayllón (alejandro-alvarez-sonarsource)

Changes

When looking for a template instantiation with a non-type parameter of unknown type and with a default value.

This can happen when a template non-type parameter has a broken expression that gets replaced by a RecoveryExpr.

---

Full diff: https://github.com/llvm/llvm-project/pull/96509.diff

2 Files Affected:

• (modified) clang/lib/Sema/SemaTemplateInstantiateDecl.cpp (+1-1)
• (added) clang/test/SemaCXX/instantiate-template-broken-nontype-default.cpp (+12)

diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index 0681520764d9a..999fec0ebb259 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -6300,7 +6300,7 @@ NamedDecl *Sema::FindInstantiatedDecl(SourceLocation Loc, NamedDecl *D,
                   getTrivialTemplateArgumentLoc(UnpackedArg, QualType(), Loc));
           }
           QualType T = CheckTemplateIdType(TemplateName(TD), Loc, Args);
-          if (T.isNull())
+          if (T.isNull() || T->containsErrors())
             return nullptr;
           CXXRecordDecl *SubstRecord = T->getAsCXXRecordDecl();
 
diff --git a/clang/test/SemaCXX/instantiate-template-broken-nontype-default.cpp b/clang/test/SemaCXX/instantiate-template-broken-nontype-default.cpp
new file mode 100644
index 0000000000000..a644cc3d057cb
--- /dev/null
+++ b/clang/test/SemaCXX/instantiate-template-broken-nontype-default.cpp
@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
+
+constexpr Missing a = 0; // expected-error {{unknown type name 'Missing'}}
+
+template < typename T, Missing b = a> // expected-error {{unknown type name 'Missing'}}
+class Klass { // expected-note {{candidate template ignored: could not match 'Klass<T, b>' against 'int'}}
+  Klass(T);   // expected-note {{candidate template ignored: substitution failure [with T = int, b = <recovery-expr>()]}}
+};
+
+Klass foo{5}; // no-crash \
+                 expected-error {{no viable constructor or deduction guide for deduction of template arguments of 'Klass'}}
+

[alejandro-alvarez-sonarsource]

Ping (@cor3ntin, @Endilll ?)

[Endilll]

Makes sense to me, but wait for other reviewers.
You published this PR on the first day of WG21 meeting, so it didn't get the usual amount of attention.

[cor3ntin]

Thanks for the fix; I think this makes sense. Can you provide a release note?

[alejandro-alvarez-sonarsource]

Thanks for the fix; I think this makes sense. Can you provide a release note?

Added (and fixed the conflict)

[cor3ntin]

LGTM.
Do you need me to merge that for you?

[alejandro-alvarez-sonarsource]

LGTM. Do you need me to merge that for you?

Yes, thanks! (P.S And another conflict resolution done)