[clang-tidy] Initialize DiagnosticEngine in ExpandModularHeaders

Fix issue preventing suppression of compiler warnings with
-Wno-<warning> under C++20 and above. Add call to
ProcessWarningOptions and propagate DiagnosticOpts more properly.

Fixes: #56709, #61969

Reviewed By: carlosgalvezp

Differential Revision: https://reviews.llvm.org/D156056

clang-tools-extra/clang-tidy/ExpandModularHeadersPPCallbacks.cpp

@@ -79,6 +79,9 @@ ExpandModularHeadersPPCallbacks::ExpandModularHeadersPPCallbacks(
   OverlayFS->pushOverlay(InMemoryFs);
 
   Diags.setSourceManager(&Sources);
+  // FIXME: Investigate whatever is there better way to initialize DiagEngine
+  // or whatever DiagEngine can be shared by multiple preprocessors
+  ProcessWarningOptions(Diags, Compiler.getDiagnosticOpts());
 
   LangOpts.Modules = false;
 

clang-tools-extra/docs/ReleaseNotes.rst

@@ -111,6 +111,9 @@ Improvements to clang-tidy
   be promoted to errors. For custom error promotion, use `-Werror=<warning>`
   on the compiler command-line, irrespective of `Checks` (`--checks=`) settings.
 
+- Fixed an issue where compiler warnings couldn't be suppressed using
+  `-Wno-<warning>` under C++20 and above.
+
 New checks
 ^^^^^^^^^^
 

clang-tools-extra/test/clang-tidy/infrastructure/diagnostic.cpp

@@ -23,6 +23,8 @@
 // RUN: clang-tidy -checks='-*,modernize-use-override,clang-diagnostic-macro-redefined' %T/diagnostics/input.cpp -- -DMACRO_FROM_COMMAND_LINE 2>&1 | FileCheck -check-prefix=CHECK4 -implicit-check-not='{{warning:|error:}}' %s
 // RUN: not clang-tidy -checks='-*,clang-diagnostic-*,google-explicit-constructor' %T/diagnostics/input.cpp 2>&1 | FileCheck -check-prefix=CHECK5 -implicit-check-not='{{warning:|error:}}' %s
 // RUN: not clang-tidy -checks='-*,modernize-use-override' %T/diagnostics/input.cpp -- -DCOMPILATION_ERROR 2>&1 | FileCheck -check-prefix=CHECK6 -implicit-check-not='{{warning:|error:}}' %s
+// RUN: clang-tidy -checks='-*,modernize-use-override,clang-diagnostic-macro-redefined' %s -- -DMACRO_FROM_COMMAND_LINE -std=c++20 | FileCheck -check-prefix=CHECK4 -implicit-check-not='{{warning:|error:}}' %s
+// RUN: clang-tidy -checks='-*,modernize-use-override,clang-diagnostic-macro-redefined,clang-diagnostic-literal-conversion' %s -- -DMACRO_FROM_COMMAND_LINE -std=c++20 -Wno-macro-redefined | FileCheck --check-prefix=CHECK7 -implicit-check-not='{{warning:|error:}}' %s
 
 // CHECK1: error: no input files [clang-diagnostic-error]
 // CHECK1: error: no such file or directory: '{{.*}}nonexistent.cpp' [clang-diagnostic-error]
@@ -31,6 +33,7 @@
 // CHECK3: error: unknown argument: '-fan-unknown-option' [clang-diagnostic-error]
 // CHECK5: error: unknown argument: '-fan-option-from-compilation-database' [clang-diagnostic-error]
 
+// CHECK7: :[[@LINE+4]]:9: warning: implicit conversion from 'double' to 'int' changes value from 1.5 to 1 [clang-diagnostic-literal-conversion]
 // CHECK2: :[[@LINE+3]]:9: warning: implicit conversion from 'double' to 'int' changes value from 1.5 to 1 [clang-diagnostic-literal-conversion]
 // CHECK3: :[[@LINE+2]]:9: warning: implicit conversion from 'double' to 'int' changes value
 // CHECK5: :[[@LINE+1]]:9: warning: implicit conversion from 'double' to 'int' changes value
@@ -43,6 +46,7 @@ class A { A(int) {} };
 
 #define MACRO_FROM_COMMAND_LINE
 // CHECK4: :[[@LINE-1]]:9: warning: 'MACRO_FROM_COMMAND_LINE' macro redefined
+// CHECK7-NOT: :[[@LINE-2]]:9: warning: 'MACRO_FROM_COMMAND_LINE' macro redefined
 
 #ifdef COMPILATION_ERROR
 void f(int a) {