[WIP][Sema] Improve static_assert diagnostics for type traits.

Summary:
In our codebase, `static_assert(std::some_type_trait<Ts...>::value, "msg")`
(where `some_type_trait` is an std type_trait and `Ts...` is the
appropriate template parameters) account for 11.2% of the `static_assert`s.

In these cases, the `Ts` are typically not spelled out explicitly, e.g.
`static_assert(std::is_same<SomeT::TypeT, typename SomeDependentT::value_type>::value, "message");`

The diagnostic when the assert fails is typically not very useful, e.g.
`static_assert failed due to requirement 'std::is_same<SomeT::TypeT, typename SomeDependentT::value_type>::value' "message"`

This change makes the diagnostic spell out the types explicitly , e.g.
`static_assert failed due to requirement 'std::is_same<int, float>::value' "message"`

See tests for more examples.

After this is submitted, I intend to handle
`static_assert(!std::some_type_trait<Ts...>::value, "msg")`,
which is another 6.6% of static_asserts.

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D54903

llvm-svn: 348239

1 files changed, 23 insertions, 1 deletions

diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index a4da62b6cb4..56302d62484 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/clang/lib/Sema/SemaTemplate.cpp
@@ -3052,6 +3052,28 @@ static Expr *lookThroughRangesV3Condition(Preprocessor &PP, Expr *Cond) {
   return Cond;
 }
 
+// Print a diagnostic for the failing static_assert expression. Defaults to
+// pretty-printing the expression.
+static void prettyPrintFailedBooleanCondition(llvm::raw_string_ostream &OS,
+                                              const Expr *FailedCond,
+                                              const PrintingPolicy &Policy) {
+  const auto *DR = dyn_cast<DeclRefExpr>(FailedCond);
+  if (DR && DR->getQualifier()) {
+    // If this is a qualified name, expand the template arguments in nested
+    // qualifiers.
+    DR->getQualifier()->print(OS, Policy, true);
+    // Then print the decl itself.
+    const ValueDecl *VD = DR->getDecl();
+    OS << VD->getName();
+    if (const auto *IV = dyn_cast<VarTemplateSpecializationDecl>(VD)) {
+      // This is a template variable, print the expanded template arguments.
+      printTemplateArgumentList(OS, IV->getTemplateArgs().asArray(), Policy);
+    }
+    return;
+  }
+  FailedCond->printPretty(OS, nullptr, Policy);
+}
+
 std::pair<Expr *, std::string>
 Sema::findFailedBooleanCondition(Expr *Cond, bool AllowTopLevelCond) {
   Cond = lookThroughRangesV3Condition(PP, Cond);
@@ -3093,7 +3115,7 @@ Sema::findFailedBooleanCondition(Expr *Cond, bool AllowTopLevelCond) {
   std::string Description;
   {
     llvm::raw_string_ostream Out(Description);
-    FailedCond->printPretty(Out, nullptr, getPrintingPolicy());
+    prettyPrintFailedBooleanCondition(Out, FailedCond, getPrintingPolicy());
   }
   return { FailedCond, Description };
 }