diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
--- a/llvm/cmake/modules/HandleLLVMOptions.cmake
+++ b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -77,7 +77,13 @@
 
 if(LLVM_ENABLE_EXPENSIVE_CHECKS)
   add_definitions(-DEXPENSIVE_CHECKS)
-  add_definitions(-D_GLIBCXX_DEBUG)
+
+  # With gcc 9's STL, std::min is not constexpr when _GLIBCXX_DEBUG is enabled.
+  if (NOT ((CMAKE_CXX_COMPILER_ID STREQUAL "GNU") AND
+           (CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 9) AND
+           (CMAKE_CXX_COMPILER_VERSION VERSION_LESS 10))) # optimistic assumption about gcc 10
+    add_definitions(-D_GLIBCXX_DEBUG)
+  endif()
 endif()
 
 string(TOUPPER "${LLVM_ABI_BREAKING_CHECKS}" uppercase_LLVM_ABI_BREAKING_CHECKS)