[Clang] Fix immediate escalation of template function specializations. (#124404)

We record whether an expression is immediate escalating in the
FunctionScope.
However, that only happen when parsing or transforming an expression.
This might not happen when transforming a non dependent expression.

This patch fixes that by considering a function immediate when
instantiated from an immediate function.

Fixes #123405

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -1003,6 +1003,7 @@ Bug Fixes to C++ Support
 - Fixed assertions or false compiler diagnostics in the case of C++ modules for
   lambda functions or inline friend functions defined inside templates (#GH122493).
 - Clang now rejects declaring an alias template with the same name as its template parameter. (#GH123423)
+- Fixed immediate escalation of non-dependent expressions. (#GH123405)
 - Fix type of expression when calling a template which returns an ``__array_rank`` querying a type depending on a
   template parameter. Now, such expression can be used with ``static_assert`` and ``constexpr``. (#GH123498)
 - Correctly determine the implicit constexprness of lambdas in dependent contexts. (#GH97958) (#GH114234)

clang/lib/AST/Decl.cpp

@@ -3314,6 +3314,10 @@ bool FunctionDecl::isImmediateFunction() const {
         .getConstructor()
         ->isImmediateFunction();
 
+  if (FunctionDecl *P = getTemplateInstantiationPattern();
+      P && P->isImmediateFunction())
+    return true;
+
   if (const auto *MD = dyn_cast<CXXMethodDecl>(this);
       MD && MD->isLambdaStaticInvoker())
     return MD->getParent()->getLambdaCallOperator()->isImmediateFunction();

clang/test/SemaCXX/cxx2b-consteval-propagate.cpp

@@ -528,3 +528,21 @@ D d(0); // expected-note {{in implicit initialization for inherited constructor
 // expected-error@-1 {{call to immediate function 'GH112677::D::SimpleCtor' is not a constant expression}}
 
 }
+
+namespace GH123405 {
+
+consteval void fn() {}
+
+template <typename>
+constexpr int tfn(int) {
+    auto p = &fn;  // expected-note {{'tfn<int>' is an immediate function because its body evaluates the address of a consteval function 'fn'}}
+    return int(p); // expected-error {{cast from pointer to smaller type 'int' loses information}}
+}
+
+int g() {
+   int a; // expected-note {{declared here}}
+   return tfn<int>(a); // expected-error {{call to immediate function 'GH123405::tfn<int>' is not a constant expression}}\
+                       // expected-note {{read of non-const variable 'a' is not allowed in a constant expression}}
+}
+
+}