diff --git a/clang/lib/Driver/Driver.cpp b/clang/lib/Driver/Driver.cpp
--- a/clang/lib/Driver/Driver.cpp
+++ b/clang/lib/Driver/Driver.cpp
@@ -2432,7 +2432,9 @@
             types::ID OldTy = Ty;
             Ty = types::lookupCXXTypeForCType(Ty);
 
-            if (Ty != OldTy)
+            // Do not complain about foo.h, when we are known to be processing
+            // it as a C++20 header unit.
+            if (Ty != OldTy && !(OldTy == types::TY_CHeader && hasHeaderMode()))
               Diag(clang::diag::warn_drv_treating_input_as_cxx)
                   << getTypeName(OldTy) << getTypeName(Ty);
           }
@@ -2457,8 +2459,11 @@
         }
 
         // Disambiguate headers that are meant to be header units from those
-        // intended to be PCH.
-        if (Ty == types::TY_CXXHeader && hasHeaderMode())
+        // intended to be PCH.  Avoid missing '.h' cases that are counted as
+        // C headers by default - we know we are in C++ mode and we do not
+        // want to issue a complaint about compiling things in the wrong mode.
+        if ((Ty == types::TY_CXXHeader || Ty == types::TY_CHeader) &&
+            hasHeaderMode())
           Ty = CXXHeaderUnitType(CXX20HeaderType);
       } else {
         assert(InputTypeArg && "InputType set w/o InputTypeArg");
diff --git a/clang/test/Driver/cxx20-header-units-02.cpp b/clang/test/Driver/cxx20-header-units-02.cpp
--- a/clang/test/Driver/cxx20-header-units-02.cpp
+++ b/clang/test/Driver/cxx20-header-units-02.cpp
@@ -3,7 +3,10 @@
 // RUN: %clang -### -std=c++20 -fmodule-header=user foo.hh  2>&1 | \
 // RUN:   FileCheck -check-prefix=CHECK-USER %s
 
-// RUN: %clang -### -std=c++20 -fmodule-header=system foo.hh 2>&1 | \
+// RUN: %clang -### -std=c++20 -fmodule-header=user foo.h  2>&1 | \
+// RUN:   FileCheck -check-prefix=CHECK-USER1 %s
+
+// RUN: %clang++ -### -std=c++20 -fmodule-header=system foo.hh 2>&1 | \
 // RUN:   FileCheck -check-prefix=CHECK-SYS1 %s
 
 // RUN: %clang -### -std=c++20 -fmodule-header=system \
@@ -19,6 +22,10 @@
 // CHECK-USER-SAME: "-o" "foo.pcm"
 // CHECK-USER-SAME: "-x" "c++-user-header" "foo.hh"
 
+// CHECK-USER1: "-emit-header-unit"
+// CHECK-USER1-SAME: "-o" "foo.pcm"
+// CHECK-USER1-SAME: "-x" "c++-user-header" "foo.h"
+
 // CHECK-SYS1: "-emit-header-unit"
 // CHECK-SYS1-SAME: "-o" "foo.pcm"
 // CHECK-SYS1-SAME: "-x" "c++-system-header" "foo.hh"