[Clang] Fix assert in Sema::LookupTemplateName so that it does not attempt an unconditional cast to TagType

[shafik]

Summary

In Sema::LookupTemplateName(...) seeks to assert that the ObjectType is complete or being defined. If the type is incomplete it will attempt to unconditionally cast it to a TagType and not all incomplete types are a TagType. For example the type could be void or it could be an IncompleteArray.

This change adds an additional check to confirm it is a TagType before attempting to check if it is incomplete or being defined.

[shafik]

Apply clang-format

[aaron.ballman]

Good catch! LGTM, but please add a release note for the fix.

[erichkeane]

Looking through isCompleteType I note that arrays of an elements of incomplete types would have also hit this crash, as would member-pointers in MicrosoftABI (in some weird cases?), and some ObjC types.

Anyway, other than the nit above, LGTM.

clang/lib/Sema/SemaTemplate.cpp

401–402

Slight preference for making the getAs happen after the completeness check, since that is in the 'order of costliness'.

Also, not sure the assert message here makes any sense, but I don't know of anything better here.

[shafik]

• Change to order of operations in assert to put off more costly check
• Add release note

[erichkeane]

LGTM.

clang/docs/ReleaseNotes.rst

 - Fix `#57151 <https://github.com/llvm/llvm-project/issues/57151>`_.
   ``-Wcomma`` is emitted for void returning functions.
 - ``-Wtautological-compare`` missed warnings for tautological comparisons
   involving a negative integer literal. This fixes
   `Issue 42918 <https://github.com/llvm/llvm-project/issues/42918>`_.
 - Fix a crash when generating code coverage information for an
   ``if consteval`` statement. This fixes
   `Issue 57377 <https://github.com/llvm/llvm-project/issues/57377>`_.
+- Fix assert that triggers a crash during template name lookup when a type was
+  incomplete but was not also a TagType. This fixes
+  `Issue 57387 <https://github.com/llvm/llvm-project/issues/57387>`_.
 
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Clang will now correctly diagnose as ill-formed a constant expression where an
   enum without a fixed underlying type is set to a value outside the range of
   the enumeration's values. Due to the extended period of time this bug was
   present in major C++ implementations (including Clang), this error has the

clang/lib/Sema/SemaTemplate.cpp

bool Sema::LookupTemplateName(LookupResult &Found,
   DeclContext *LookupCtx = nullptr;
   bool IsDependent = false;
   if (!ObjectType.isNull()) {
     // This nested-name-specifier occurs in a member access expression, e.g.,
     // x->B::f, and we are looking into the type of the object.
     assert(SS.isEmpty() && "ObjectType and scope specifier cannot coexist");
     LookupCtx = computeDeclContext(ObjectType);
     IsDependent = !LookupCtx && ObjectType->isDependentType();
     assert((IsDependent || !ObjectType->isIncompleteType() ||
+            !ObjectType->getAs<TagType>() ||
             ObjectType->castAs<TagType>()->isBeingDefined()) &&
            "Caller should have completed object type");
 
     // Template names cannot appear inside an Objective-C class or object type
     // or a vector type.
     //
     // FIXME: This is wrong. For example:
     //

clang/test/SemaCXX/member-expr.cpp

namespace pr16676 {
   struct S { int i; };
   struct T { S* get_s(); };
   int f(S* s) {
     T t;
     return t.get_s  // expected-error {{reference to non-static member function must be called; did you mean to call it with no arguments?}}
         .i;  // expected-error {{member reference type 'S *' is a pointer; did you mean to use '->'}}
   }
 }
+
+namespace LookupTemplateNameAssert {
+void f() {}
+
+typedef int at[];
+const at& f2(){}
+
+void g() {
+  f().junk<int>(); // expected-error {{member reference base type 'void' is not a structure or union}}
+// expected-error@-1 {{expected '(' for function-style cast or type construction}}
+// expected-error@-2 {{expected expression}}
+  f2().junk<int>(); // expected-error {{member reference base type 'const at' (aka 'const int[]') is not a structure or union}}
+// expected-error@-1 {{expected '(' for function-style cast or type construction}}
+// expected-error@-2 {{expected expression}}
+}
+}