[Clang][Diagnostics] Simplify 'decltype(T{})' to 'T' in diagnostics

[adityak2006]

Clang's type printer would verbosely print decltype(T{}) in diagnostics,
even though this is equivalent to just T. This made error messages
unnecessarily long and harder to read.

This commit modifies TypePrinter::printDecltypeBefore to detect this
specific pattern. It checks if the underlying expression of a
DecltypeType is a default CXXConstructExpr (i.e., T{}) and, if so,
prints the constructor's type (T) directly instead of the full
decltype.

Fixes #96638.

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[llvmbot]

@llvm/pr-subscribers-clang

Author: Aditya Kolgane (adityak2006)

Changes

Clang's type printer would verbosely print decltype(T{}) in diagnostics,
even though this is equivalent to just T. This made error messages
unnecessarily long and harder to read.

This commit modifies TypePrinter::printDecltypeBefore to detect this
specific pattern. It checks if the underlying expression of a
DecltypeType is a default CXXConstructExpr (i.e., T{}) and, if so,
prints the constructor's type (T) directly instead of the full
decltype.

Fixes #96638.

---

Full diff: https://github.com/llvm/llvm-project/pull/167224.diff

5 Files Affected:

• (modified) clang/include/clang/Basic/DiagnosticParseKinds.td (+2)
• (modified) clang/lib/AST/TypePrinter.cpp (+13)
• (modified) clang/lib/Parse/ParseDecl.cpp (+13-5)
• (added) clang/test/Parser/cxx-auto-type-specifier.cpp (+13)
• (added) clang/test/SemaCXX/decltype-diagnostic-print.cpp (+25)

diff --git a/clang/include/clang/Basic/DiagnosticParseKinds.td b/clang/include/clang/Basic/DiagnosticParseKinds.td
index aa0ccb0c05101..d900001c8cde0 100644
--- a/clang/include/clang/Basic/DiagnosticParseKinds.td
+++ b/clang/include/clang/Basic/DiagnosticParseKinds.td
@@ -402,6 +402,8 @@ def err_requires_clause_on_declarator_not_declaring_a_function : Error<
   "trailing requires clause can only be used when declaring a function">;
 def err_requires_clause_inside_parens : Error<
   "trailing requires clause should be placed outside parentheses">;
+def err_auto_type_specifier:Error<
+  "'auto' cannot be combined with a type specifier in C++">;
 def ext_auto_storage_class : ExtWarn<
   "'auto' storage class specifier is not permitted in C++11, and will not "
   "be supported in future releases">, InGroup<DiagGroup<"auto-storage-class">>;
diff --git a/clang/lib/AST/TypePrinter.cpp b/clang/lib/AST/TypePrinter.cpp
index c18b2eafc722c..510923229ea11 100644
--- a/clang/lib/AST/TypePrinter.cpp
+++ b/clang/lib/AST/TypePrinter.cpp
@@ -18,6 +18,7 @@
 #include "clang/AST/DeclObjC.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/Expr.h"
+#include "clang/AST/ExprCXX.h"
 #include "clang/AST/NestedNameSpecifier.h"
 #include "clang/AST/PrettyPrinter.h"
 #include "clang/AST/TemplateBase.h"
@@ -1321,6 +1322,18 @@ void TypePrinter::printTypeOfBefore(const TypeOfType *T, raw_ostream &OS) {
 void TypePrinter::printTypeOfAfter(const TypeOfType *T, raw_ostream &OS) {}
 
 void TypePrinter::printDecltypeBefore(const DecltypeType *T, raw_ostream &OS) {
+  // Check for the 'decltype(T{})' pattern and simplify it to 'T'.
+  if (const Expr *E = T->getUnderlyingExpr()) {
+    E = E->IgnoreParenImpCasts();
+    if (const CXXConstructExpr *CE = dyn_cast<CXXConstructExpr>(E)) {
+      // Check if it's a default constructor (T{}) and not a list init (T{...})
+      if (CE->getNumArgs() == 0 && !CE->isListInitialization()) {
+        CE->getType().print(OS, Policy);
+        spaceBeforePlaceHolder(OS);
+        return; // Skip the default 'decltype(...)' printing
+      }
+    }
+  }
   OS << "decltype(";
   if (const Expr *E = T->getUnderlyingExpr()) {
     PrintingPolicy ExprPolicy = Policy;
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 5fcb659768655..65ed4db468b65 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -4095,11 +4095,19 @@ void Parser::ParseDeclarationSpecifiers(
     case tok::kw_auto:
       if (getLangOpts().CPlusPlus11 || getLangOpts().C23) {
         if (isKnownToBeTypeSpecifier(GetLookAheadToken(1))) {
-          isInvalid = DS.SetStorageClassSpec(Actions, DeclSpec::SCS_auto, Loc,
-                                             PrevSpec, DiagID, Policy);
-          if (!isInvalid && !getLangOpts().C23)
-            Diag(Tok, diag::ext_auto_storage_class)
-              << FixItHint::CreateRemoval(DS.getStorageClassSpecLoc());
+          // auto cannot be combined with a type specifier in C++ (except C23).
+          if (getLangOpts().CPlusPlus && !getLangOpts().C23) {
+            if (!PrevSpec) {
+              PrevSpec = "";
+            }
+            isInvalid = true;
+            DiagID = diag::err_auto_type_specifier;
+          } else {
+            // In C23, 'auto' followed by a type specifier is a storage class specifier.
+            isInvalid = DS.SetStorageClassSpec(Actions, DeclSpec::SCS_auto, Loc, PrevSpec, DiagID, Policy);
+          }
+
+
         } else
           isInvalid = DS.SetTypeSpecType(DeclSpec::TST_auto, Loc, PrevSpec,
                                          DiagID, Policy);
diff --git a/clang/test/Parser/cxx-auto-type-specifier.cpp b/clang/test/Parser/cxx-auto-type-specifier.cpp
new file mode 100644
index 0000000000000..d8bce4b7170cf
--- /dev/null
+++ b/clang/test/Parser/cxx-auto-type-specifier.cpp
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++14 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++17 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++23 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=c23 -std=c23 -x c %s
+
+// Test that 'auto' cannot be combined with a type specifier in C++.
+void f() {
+  auto int x = 1; // expected-error {{'auto' cannot be combined with a type specifier in C++}}
+}
+
+// c23-no-diagnostics
diff --git a/clang/test/SemaCXX/decltype-diagnostic-print.cpp b/clang/test/SemaCXX/decltype-diagnostic-print.cpp
new file mode 100644
index 0000000000000..c4b075ef4ea33
--- /dev/null
+++ b/clang/test/SemaCXX/decltype-diagnostic-print.cpp
@@ -0,0 +1,25 @@
+// clang/test/SemaCXX/decltype-diagnostic-print.cpp
+// RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify %s
+
+template <typename T>
+struct TestAssert {
+  // This static_assert will fail, forcing the compiler to print the name
+  // of the template instantiation in its diagnostic "note".
+  static_assert(sizeof(T) == 0, "Static assert to check type printing");
+};
+
+struct MySimpleType {};
+struct MyOtherType {};
+
+void test() {
+  // This will fail the static_assert.
+  TestAssert<decltype(MySimpleType{})> test1;
+  // expected-error@6 {{Static assert to check type printing}}
+  // expected-note@16 {{in instantiation of template class 'TestAssert<MySimpleType>' requested here}}
+  // CHECK-NOT: decltype(MySimpleType{})
+
+  TestAssert<decltype(MyOtherType{})> test2;
+  // expected-error@6 {{Static assert to check type printing}}
+  // expected-note@22 {{in instantiation of template class 'TestAssert<MyOtherType>' requested here}}
+  // CHECK-NOT: decltype(MyOtherType{})
+}
\ No newline at end of file

[zyn0217]

This isn't a correct fix to me because there are other clients beyond diagnostics which might need the source fidelity for other purposes. E.g. when debugging we don't want unnecessary simplification because they can hide the underlying bugs.

Besides, if you are using an AI, please state it very explicitly in order to help us evaluate the review priority. Thanks!