[Clang] Fix crash with source_location in lambda declarators. (#107411)

Parsing lambdas require pushing a declaration context for the lambda, so
that parameters can be attached to it, before its trailing type is
parsed. DAt that point, partially-parsed lambda don't have a name that
can be computed for then.
This would cause source_location::current() to crash when use in the
decltype of a lambda().
We work around this by producing a source_location for an enclosing
scope in that scenario.

Fixes #67134

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -369,6 +369,7 @@ Bug Fixes to C++ Support
 - Clang no longer tries to capture non-odr used default arguments of template parameters of generic lambdas (#GH107048)
 - Fixed a bug where defaulted comparison operators would remove ``const`` from base classes. (#GH102588)
 
+- Fix a crash when using ``source_location`` in the trailing return type of a lambda expression. (#GH67134)
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/AST/Expr.cpp

@@ -13,6 +13,7 @@
 #include "clang/AST/Expr.h"
 #include "clang/AST/APValue.h"
 #include "clang/AST/ASTContext.h"
+#include "clang/AST/ASTLambda.h"
 #include "clang/AST/Attr.h"
 #include "clang/AST/ComputeDependence.h"
 #include "clang/AST/DeclCXX.h"
@@ -2287,6 +2288,15 @@ APValue SourceLocExpr::EvaluateInContext(const ASTContext &Ctx,
     Context = getParentContext();
   }
 
+  // If we are currently parsing a lambda declarator, we might not have a fully
+  // formed call operator declaration yet, and we could not form a function name
+  // for it. Because we do not have access to Sema/function scopes here, we
+  // detect this case by relying on the fact such method doesn't yet have a
+  // type.
+  if (const auto *D = dyn_cast<CXXMethodDecl>(Context);
+      D && D->getFunctionTypeLoc().isNull() && isLambdaCallOperator(D))
+    Context = D->getParent()->getParent();
+
   PresumedLoc PLoc = Ctx.getSourceManager().getPresumedLoc(
       Ctx.getSourceManager().getExpansionRange(Loc).getEnd());
 

clang/test/SemaCXX/source_location.cpp

@@ -989,3 +989,26 @@ void Test() {
 }
 
 #endif
+
+
+namespace GH67134 {
+template <int loc = std::source_location::current().line()>
+constexpr auto f(std::source_location loc2 = std::source_location::current()) { return loc; }
+
+int g = []() -> decltype(f()) { return 0; }();
+
+int call() {
+#if __cplusplus >= 202002L
+  return []<decltype(f()) = 0>() -> decltype(f()) { return  0; }();
+#endif
+  return []() -> decltype(f()) { return  0; }();
+}
+
+#if __cplusplus >= 202002L
+template<typename T>
+int Var = requires { []() -> decltype(f()){}; };
+int h = Var<int>;
+#endif
+
+
+}