[flang] Fix crashes due to failure to find a subprogram

[klausler]

Summary

In error recovery situations, the mappings from source locations
to scopes were failing in a way that tripped some asserts.
Specifically, FindPureProcedureContaining() wasn't coping well
when starting at the global scope. (And since the global scope
no longer has a source range, clean up the Semantics constructor
to avoid confusion.)

[PeteSteinfeld]

All builds, tests, and looks good.

[awarzynski]

LGTM, thanks. Before merging, could you please fix the clang-tidy warning? No need to upload another version here.

flang/lib/Semantics/tools.cpp

85

As pointed out by clang-tidy, you can skip else here without changing the semantics: https://llvm.org/docs/CodingStandards.html#don-t-use-else-after-a-return.

[klausler]

flang/lib/Semantics/tools.cpp

85

We don't always follow that LLVM rule in the more functional style used in f18. This whole function is essentially one big expression that would be a single return statement if C++ could handle it.

flang/include/flang/Semantics/semantics.h

 class Semantics {
 public:
   explicit Semantics(SemanticsContext &context, parser::Program &program,
-      parser::CharBlock charBlock, bool debugModuleWriter = false)
+      bool debugModuleWriter = false)
       : context_{context}, program_{program} {
     context.set_debugModuleWriter(debugModuleWriter);
-    context.globalScope().AddSourceRange(charBlock);
   }
 
   SemanticsContext &context() const { return context_; }

flang/lib/Frontend/FrontendActions.cpp

   // Prepare semantics
   setSemantics(std::make_unique<Fortran::semantics::Semantics>(
       ci.invocation().semanticsContext(), parseTree,
-      ci.parsing().cooked().AsCharBlock(), ci.invocation().debugModuleDir()));
+      ci.invocation().debugModuleDir()));
   auto &semantics = this->semantics();
 
   // Run semantic checks

flang/lib/Semantics/check-io.cpp

 
 void IoChecker::CheckForPureSubprogram() const { // C1597
   CHECK(context_.location());
-  if (FindPureProcedureContaining(context_.FindScope(*context_.location()))) {
-    context_.Say("External I/O is not allowed in a pure subprogram"_err_en_US);
+  if (const Scope *
+      scope{context_.globalScope().FindScope(*context_.location())}) {
+    if (FindPureProcedureContaining(*scope)) {
+      context_.Say(
+          "External I/O is not allowed in a pure subprogram"_err_en_US);
+    }
   }
 }
 

flang/lib/Semantics/scope.cpp

 }
 
 void Scope::AddSourceRange(const parser::CharBlock &source) {
-  for (auto *scope = this; !scope->IsGlobal(); scope = &scope->parent()) {
+  for (auto *scope{this}; !scope->IsGlobal(); scope = &scope->parent()) {
     scope->sourceRange_.ExtendToCover(source);
   }
 }

flang/lib/Semantics/tools.cpp

   // N.B. We only need to examine the innermost containing program unit
   // because an internal subprogram of a pure subprogram must also
   // be pure (C1592).
-  const Scope &scope{GetProgramUnitContaining(start)};
-  return IsPureProcedure(scope) ? &scope : nullptr;
+  if (start.IsGlobal()) {
+    return nullptr;
+  } else {
+    const Scope &scope{GetProgramUnitContaining(start)};
+    return IsPureProcedure(scope) ? &scope : nullptr;
+  }
 }
 
 static bool MightHaveCompatibleDerivedtypes(

flang/tools/f18/f18.cpp

   if (!driver.debugNoSemantics || driver.dumpSymbols ||
       driver.dumpUnparseWithSymbols || driver.getDefinition ||
       driver.getSymbolsSources) {
-    Fortran::semantics::Semantics semantics{semanticsContext, parseTree,
-        parsing.cooked().AsCharBlock(), driver.debugModuleWriter};
+    Fortran::semantics::Semantics semantics{
+        semanticsContext, parseTree, driver.debugModuleWriter};
     semantics.Perform();
     Fortran::semantics::RuntimeDerivedTypeTables tables;
     if (!semantics.AnyFatalError()) {