[clang] Don't create import decls without -fmodules

When modules are disabled, there's no loaded module for these import
decls to point at. This results in crashes when there are modulemap
files but no -fmodules flag (this configuration is used for layering
check violations).

This patch makes sure import declarations are introduced only when
modules are enabled, which makes this case similar to textual headers
(no import decls are created for #include of textual headers from a
modulemap).

Differential Revision: https://reviews.llvm.org/D152274

clang/lib/Sema/SemaModule.cpp

@@ -636,11 +636,9 @@ void Sema::BuildModuleInclude(SourceLocation DirectiveLoc, Module *Mod) {
       TUKind == TU_Module &&
       getSourceManager().isWrittenInMainFile(DirectiveLoc);
 
-  bool ShouldAddImport = !IsInModuleIncludes;
-
-  // If this module import was due to an inclusion directive, create an
-  // implicit import declaration to capture it in the AST.
-  if (ShouldAddImport) {
+  // If we are really importing a module (not just checking layering) due to an
+  // #include in the main file, synthesize an ImportDecl.
+  if (getLangOpts().Modules && !IsInModuleIncludes) {
     TranslationUnitDecl *TU = getASTContext().getTranslationUnitDecl();
     ImportDecl *ImportD = ImportDecl::CreateImplicit(getASTContext(), TU,
                                                      DirectiveLoc, Mod,

clang/test/Modules/Inputs/modulemaps-nomodules/module.modulemap

@@ -0,0 +1,3 @@
+module M {
+  private header "header.h"
+}

clang/test/Modules/getSourceDescriptor-crash.cpp

@@ -1,4 +1,5 @@
-// RUN: %clang_cc1 -I %S/Inputs/getSourceDescriptor-crash -S -emit-llvm -debug-info-kind=limited -debugger-tuning=lldb -fimplicit-module-maps %s -o - | FileCheck %s
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -I %S/Inputs/getSourceDescriptor-crash -S -emit-llvm -debug-info-kind=limited -debugger-tuning=lldb -fmodules -fmodules-cache-path=%t -fimplicit-module-maps %s -o - | FileCheck %s
 
 #include "h1.h"
 #include "h1.h"

clang/test/Modules/modulemaps-nomodules.cpp

@@ -0,0 +1,8 @@
+// Make sure we treat includes that are part of modulemaps the same as textual
+// headers when modules are not enabled (e.g do't generate import decls, but
+// still perform layering checks).
+// No need to pass -fno-modules explicitly, absence implies negation for cc1.
+// RUN: %clang_cc1 -I %S/Inputs/modulemaps-nomodules -fmodule-map-file=%S/Inputs/modulemaps-nomodules/module.modulemap %s -verify -ast-dump | FileCheck %s
+
+#include "header.h" // expected-error{{use of private header from outside its module: 'header.h'}}
+// CHECK-NOT: ImportDecl

clang/test/PCH/Inputs/modulemaps-nomodules/module.modulemap

@@ -0,0 +1,3 @@
+module M {
+  header "header.h"
+}

clang/test/PCH/modulemaps-nomodules.cpp

@@ -0,0 +1,6 @@
+// Make sure we don't crash when serializing a PCH with an include from a
+// modulemap file in nomodules mode.
+// No need to pass -fno-modules explicitly, absence implies negation for cc1.
+// RUN: %clang_cc1 -I %S/Inputs/modulemaps-nomodules -fmodule-map-file=%S/Inputs/modulemaps-nomodules/module.modulemap %s -emit-pch -o /dev/null
+
+#include "header.h"