Diagnose attempts to build a preprocessed module that defines an unavailable submodule.

The errors we would otherwise get are incomprehensible, as we would enter the
module but not make its contents visible to itself.

llvm-svn: 304190

clang/include/clang/Basic/DiagnosticLexKinds.td

@@ -525,6 +525,8 @@ def err_pp_module_begin_without_module_end : Error<
 def err_pp_module_end_without_module_begin : Error<
   "no matching '#pragma clang module begin' for this "
   "'#pragma clang module end'">;
+def note_pp_module_begin_here : Note<
+  "entering module '%0' due to this pragma">;
 
 def err_defined_macro_name : Error<"'defined' cannot be used as a macro name">;
 def err_paste_at_start : Error<

clang/lib/Lex/Pragma.cpp

@@ -1407,6 +1407,24 @@ struct PragmaModuleBeginHandler : public PragmaHandler {
       M = NewM;
     }
 
+    // If the module isn't available, it doesn't make sense to enter it.
+    if (!M->isAvailable()) {
+      Module::Requirement Requirement;
+      Module::UnresolvedHeaderDirective MissingHeader;
+      (void)M->isAvailable(PP.getLangOpts(), PP.getTargetInfo(),
+                           Requirement, MissingHeader);
+      if (MissingHeader.FileNameLoc.isValid()) {
+        PP.Diag(MissingHeader.FileNameLoc, diag::err_module_header_missing)
+          << MissingHeader.IsUmbrella << MissingHeader.FileName;
+      } else {
+        PP.Diag(M->DefinitionLoc, diag::err_module_unavailable)
+          << M->getFullModuleName() << Requirement.second << Requirement.first;
+      }
+      PP.Diag(BeginLoc, diag::note_pp_module_begin_here)
+        << M->getTopLevelModuleName();
+      return;
+    }
+
     // Enter the scope of the submodule.
     PP.EnterSubmodule(M, BeginLoc, /*ForPragma*/true);
     PP.EnterAnnotationToken(SourceRange(BeginLoc, ModuleName.back().second),

clang/test/Modules/preprocess-unavailable.cpp

@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -x c++-module-map %s -fmodule-name=a -verify
+module a {
+  module b {
+    requires cplusplus11
+  }
+}
+#pragma clang module contents
+// expected-error@3 {{module 'a.b' requires feature 'cplusplus11'}}
+#pragma clang module begin a.b // expected-note {{entering module 'a' due to this pragma}}
+int f();
+int g() { f(); }
+#pragma clang module end // expected-error {{no matching '#pragma clang module begin'}}