[clang] [Concepts] Avoid substituting into constraints for invalid TemplateDecls (PR #75697)

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 created https://github.com/llvm/llvm-project/pull/75697

Fixes https://github.com/llvm/llvm-project/issues/73885.

Substituting into constraints for invalid TemplateDecls might still yield dependent expressions and end up crashing later while attempting evaluation.

>From a6e01586f5d406b51492d973cb693ba32cc63d99 Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Sat, 16 Dec 2023 18:49:59 +0800
Subject: [PATCH] [Concepts] Avoid substituting into constraints for invalid
 TemplateDecls

Fixes https://github.com/llvm/llvm-project/issues/73885.

Substituting into constraints for invalid TemplateDecls might still
yield dependent expressions and end up crashing later while
attempting evaluation.
---
 clang/docs/ReleaseNotes.rst                           |  3 +++
 clang/lib/Sema/SemaConcept.cpp                        |  4 ++++
 clang/test/SemaTemplate/instantiate-requires-expr.cpp | 10 ++++++++++
 3 files changed, 17 insertions(+)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index b4b5352a306c1c..83f98b97925250 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -831,6 +831,9 @@ Bug Fixes to C++ Support
 - Fix crash when parsing nested requirement. Fixes:
   (`#73112 <https://github.com/llvm/llvm-project/issues/73112>`_)
 
+- Fixed a crash when substituting into constraint expressions for invalid variable templates.
+  Fixes: (`#73885 <https://github.com/llvm/llvm-project/issues/73885>`_)
+
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed an import failure of recursive friend class template.
diff --git a/clang/lib/Sema/SemaConcept.cpp b/clang/lib/Sema/SemaConcept.cpp
index 719c6aab74e017..73683fee7c1487 100644
--- a/clang/lib/Sema/SemaConcept.cpp
+++ b/clang/lib/Sema/SemaConcept.cpp
@@ -353,6 +353,10 @@ static ExprResult calculateConstraintSatisfaction(
           if (Inst.isInvalid())
             return ExprError();
 
+          // An empty expression for substitution failure messages.
+          if (Template && Template->isInvalidDecl())
+            return ExprEmpty();
+
           llvm::FoldingSetNodeID ID;
           if (Template &&
               DiagRecursiveConstraintEval(S, ID, Template, AtomicExpr, MLTAL)) {
diff --git a/clang/test/SemaTemplate/instantiate-requires-expr.cpp b/clang/test/SemaTemplate/instantiate-requires-expr.cpp
index ba82fc1313fc95..fb4127182d1cb6 100644
--- a/clang/test/SemaTemplate/instantiate-requires-expr.cpp
+++ b/clang/test/SemaTemplate/instantiate-requires-expr.cpp
@@ -227,3 +227,13 @@ struct r6 {};
 
 using r6i = r6<int>;
 // expected-error at -1 {{constraints not satisfied for class template 'r6' [with T = int]}}
+
+namespace GH73885 {
+template <typename T> // expected-error {{extraneous template parameter list}}
+template <typename T> // expected-error {{'T' shadows template parameter}}
+                      // expected-note at -2 {{template parameter is declared here}}
+requires(T{})
+T e_v;
+double e = e_v<double>;
+// expected-error at -1 {{constraints not satisfied for variable template 'e_v' [with T = double]}}
+}