[Clang] Diagnose invalid function types in dependent contexts

When forming an invalid function type, we were not diagnosing
it if the call was dependent.

However, we later rely on the function type to be sensible
during argument deduction.

We now diagnose anything that is not a potential function type,
to avoid constructing bogus call expressions.

Fixes #138657
Fixes #115725
Fixes #68852

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -655,6 +655,7 @@ Bug Fixes to C++ Support
 - Fixed an assertion when trying to constant-fold various builtins when the argument
   referred to a reference to an incomplete type. (#GH129397)
 - Fixed a crash when a cast involved a parenthesized aggregate initialization in dependent context. (#GH72880)
+- Fixed a crash when forming an invalid function type in a dependent context. (#GH138657) (#GH115725) (#GH68852)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaExpr.cpp

@@ -6541,6 +6541,15 @@ ExprResult Sema::ActOnCallExpr(Scope *Scope, Expr *Fn, SourceLocation LParenLoc,
   return Call;
 }
 
+// Any type that could be used to form a callable expression
+static bool MayBeFunctionType(const ASTContext &Context, QualType T) {
+  return T == Context.BoundMemberTy || T == Context.UnknownAnyTy ||
+         T == Context.BuiltinFnTy || T == Context.OverloadTy ||
+         T->isFunctionType() || T->isFunctionReferenceType() ||
+         T->isMemberFunctionPointerType() || T->isFunctionPointerType() ||
+         T->isBlockPointerType() || T->isRecordType();
+}
+
 ExprResult Sema::BuildCallExpr(Scope *Scope, Expr *Fn, SourceLocation LParenLoc,
                                MultiExprArg ArgExprs, SourceLocation RParenLoc,
                                Expr *ExecConfig, bool IsExecConfig,
@@ -6594,6 +6603,16 @@ ExprResult Sema::BuildCallExpr(Scope *Scope, Expr *Fn, SourceLocation LParenLoc,
             *this, dyn_cast<UnresolvedMemberExpr>(Fn->IgnoreParens()),
             Fn->getBeginLoc());
 
+        if (!Fn->getType()->isDependentType()) {
+          // If the type of the function itself is not dependent
+          // check that it is a reasonable as a function, as type deduction
+          // later assume the CallExpr has a sensible TYPE.
+          if (!MayBeFunctionType(Context, Fn->getType()))
+            return ExprError(
+                Diag(LParenLoc, diag::err_typecheck_call_not_function)
+                << Fn->getType() << Fn->getSourceRange());
+        }
+
         return CallExpr::Create(Context, Fn, ArgExprs, Context.DependentTy,
                                 VK_PRValue, RParenLoc, CurFPFeatureOverrides());
       }

clang/test/SemaTemplate/fun-template-def.cpp

@@ -1,6 +1,7 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++98 %s
 // RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
 
 // Tests that dependent expressions are always allowed, whereas non-dependent
 // are checked as usual.
@@ -32,7 +33,7 @@ T f1(T t1, U u1, int i1, T** tpp)
   i1 = t1[u1];
   i1 *= t1;
 
-  i1(u1, t1); // error
+  i1(u1, t1); // expected-error {{called object type 'int' is not a function or function pointer}}
   u1(i1, t1);
 
   U u2 = (T)i1;
@@ -60,3 +61,51 @@ void f3() {
   f2<int*>(0);
   f2<int>(0); // expected-error {{no matching function for call to 'f2'}}
 }
+
+#if __cplusplus >= 202002L
+namespace GH138657 {
+template <auto V> // #gh138657-template-head
+class meta {};
+template<int N>
+class meta<N()> {}; // expected-error {{called object type 'int' is not a function or function point}}
+
+template<int N[1]>
+class meta<N()> {}; // expected-error {{called object type 'int *' is not a function or function point}}
+
+template<char* N>
+class meta<N()> {}; // expected-error {{called object type 'char *' is not a function or function point}}
+
+struct S {};
+template<S>
+class meta<S()> {}; // expected-error {{template argument for non-type template parameter is treated as function type 'S ()'}}
+                    // expected-note@#gh138657-template-head {{template parameter is declared here}}
+
+}
+
+namespace GH115725 {
+template<auto ...> struct X {};
+template<typename T, typename ...Ts> struct A {
+  template<Ts ...Ns, T *...Ps>
+  A(X<0(Ps)...>, Ts (*...qs)[Ns]);
+  // expected-error@-1{{called object type 'int' is not a function or function pointer}}
+
+};
+}
+
+namespace GH68852 {
+template <auto v>
+struct constexpr_value {
+  template <class... Ts>
+  constexpr constexpr_value<v(Ts::value...)> call(Ts...) {
+    //expected-error@-1 {{called object type 'int' is not a function or function pointer}}
+    return {};
+  }
+};
+
+template <auto v> constexpr static inline auto c_ = constexpr_value<v>{};
+// expected-note@-1 {{in instantiation of template}}
+auto k = c_<1>; // expected-note {{in instantiation of variable}}
+
+}
+
+#endif