diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -81,6 +81,9 @@
 - 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
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -399,6 +399,7 @@
     LookupCtx = computeDeclContext(ObjectType);
     IsDependent = !LookupCtx && ObjectType->isDependentType();
     assert((IsDependent || !ObjectType->isIncompleteType() ||
+            !ObjectType->getAs<TagType>() ||
             ObjectType->castAs<TagType>()->isBeingDefined()) &&
            "Caller should have completed object type");
 
diff --git a/clang/test/SemaCXX/member-expr.cpp b/clang/test/SemaCXX/member-expr.cpp
--- a/clang/test/SemaCXX/member-expr.cpp
+++ b/clang/test/SemaCXX/member-expr.cpp
@@ -228,3 +228,19 @@
         .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}}
+}
+}