Index: Makefile.config.in
===================================================================
--- Makefile.config.in
+++ Makefile.config.in
@@ -58,6 +58,10 @@
 PROJ_SRC_ROOT   := $(LLVM_SRC_ROOT)
 PROJ_SRC_DIR    := $(LLVM_SRC_ROOT)$(patsubst $(PROJ_OBJ_ROOT)%,%,$(PROJ_OBJ_DIR))
 
+ifeq ($(LLVM_SRC_ROOT), $(LLVM_OBJ_ROOT))
+$(error In-source builds are discouraged. Configure from a separate build directory)
+endif
+
 ifneq ($(CLANG_SRC_ROOT),)
   CLANG_SRC_ROOT:= $(call realpath, $(CLANG_SRC_ROOT))
   PROJ_SRC_DIR  := $(patsubst $(LLVM_SRC_ROOT)/tools/clang%,$(CLANG_SRC_ROOT)%,$(PROJ_SRC_DIR))