Merging r372178:

------------------------------------------------------------------------
r372178 | rnk | 2019-09-17 13:29:10 -0700 (Tue, 17 Sep 2019) | 15 lines

Ignore exception specifier mismatch when merging redeclarations

Exception specifiers are now part of the function type in C++17.
Normally, it is illegal to redeclare the same function or specialize a
template with a different exception specifier, but under
-fms-compatibility, we accept it with a warning. Without this change,
the function types would not match due to the exception specifier, and
clang would claim that the types were "incompatible". Now we emit the
warning and merge the redeclaration as we would in C++14 and earlier.

Fixes PR42842, which is about compiling _com_ptr_t in C++17.

Based on a patch by Alex Fusco <alexfusco@google.com>!

Differential Revision: https://reviews.llvm.org/D67590
------------------------------------------------------------------------

Author: rnk

clang/lib/Sema/SemaDecl.cpp

@@ -3475,7 +3475,12 @@ bool Sema::MergeFunctionDecl(FunctionDecl *New, NamedDecl *&OldD,
       }
     }
 
-    if (OldQTypeForComparison == NewQType)
+    // If the function types are compatible, merge the declarations. Ignore the
+    // exception specifier because it was already checked above in
+    // CheckEquivalentExceptionSpec, and we don't want follow-on diagnostics
+    // about incompatible types under -fms-compatibility.
+    if (Context.hasSameFunctionTypeIgnoringExceptionSpec(OldQTypeForComparison,
+                                                         NewQType))
       return MergeCompatibleFunctionDecls(New, Old, S, MergeTypeWithOld);
 
     // If the types are imprecise (due to dependent constructs in friends or

clang/test/SemaCXX/ms-exception-spec.cpp

@@ -1,9 +1,36 @@
-// RUN: %clang_cc1 %s -fsyntax-only -verify -fms-extensions -fexceptions -fcxx-exceptions
+// RUN: %clang_cc1 -std=c++11 %s -fsyntax-only -verify -fms-extensions -fexceptions -fcxx-exceptions
+// RUN: %clang_cc1 -std=c++17 %s -fsyntax-only -verify -fms-extensions -fexceptions -fcxx-exceptions
 
+// FIXME: Should -fms-compatibility soften these errors into warnings to match
+// MSVC? In practice, MSVC never implemented dynamic exception specifiers, so
+// there isn't much Windows code in the wild that uses them.
+#if __cplusplus >= 201703L
+// expected-error@+3 {{ISO C++17 does not allow dynamic exception specifications}}
+// expected-note@+2 {{use 'noexcept(false)' instead}}
+#endif
 void f() throw(...) { }
 
 namespace PR28080 {
 struct S;           // expected-note {{forward declaration}}
+#if __cplusplus >= 201703L
+// expected-error@+3 {{ISO C++17 does not allow dynamic exception specifications}}
+// expected-note@+2 {{use 'noexcept(false)' instead}}
+#endif
 void fn() throw(S); // expected-warning {{incomplete type}} expected-note{{previous declaration}}
 void fn() throw();  // expected-warning {{does not match previous declaration}}
 }
+
+template <typename T> struct FooPtr {
+  template <typename U> FooPtr(U *p) : m_pT(nullptr) {}
+
+  template <>
+      // FIXME: It would be better if this note pointed at the primary template
+      // above.
+      // expected-note@+1 {{previous declaration is here}}
+  FooPtr(T *pInterface) throw() // expected-warning {{exception specification in declaration does not match previous declaration}}
+      : m_pT(pInterface) {}
+
+  T *m_pT;
+};
+struct Bar {};
+template struct FooPtr<Bar>; // expected-note {{requested here}}