[clang] [Clang][Modules] Fix crash when redefining default template argument (PR #185237)

[Chuanqi Xu via cfe-commits]

================
@@ -0,0 +1,63 @@
+// RUN: %clang_cc1 -std=c++20 -verify %s
+
+// Global module fragment for named module 'a'.
+module;
+
+// Case 1: original crash – type parameter in GMF
+template <class = int> class FooGMF;  // expected-note {{previous default template argument defined here}}
+template <class = void> class FooGMF; // expected-error {{template parameter redefines default argument}}
+
+// Case 2: same-TU redefinition without explicit template name
+template<class T = int> class FooNonMod;   // expected-note {{previous default template argument defined here}}
+template<class T = void> class FooNonMod;  // expected-error {{template parameter redefines default argument}}
+
+// Case 3: non-type parameter in GMF
+template <int N = 5> class NTGMF;          // expected-note {{previous default template argument defined here}}
+template <int N = 7> class NTGMF;          // expected-error {{template parameter redefines default argument}}
+
+
+// Case 4: legal vs illegal redefinitions across declaration/definition
+template <class T = int> class Legal;
+template <class T> class Legal { T value; };
+
+template <class T = int> class Illegal;    // expected-note {{previous default template argument defined here}}
+template <class T = void> class Illegal { T value; }; // expected-error {{template parameter redefines default argument}}
+
+// Case 5: multiple redeclarations (type and non-type)
+// When the 3rd decl fires, Clang notes every prior decl that had a default.
+template <class = int> class Multi;        // expected-note {{previous default template argument defined here}}
+template <class = int> class Multi;        // expected-error {{template parameter redefines default argument}} expected-note {{previous default template argument defined here}}
+template <class = float> class Multi;      // expected-error {{template parameter redefines default argument}}
+
+template <int N = 1> class MultiNT;        // expected-note {{previous default template argument defined here}}
+template <int N = 1> class MultiNT;        // expected-error {{template parameter redefines default argument}} expected-note {{previous default template argument defined here}}
+template <int N = 2> class MultiNT;        // expected-error {{template parameter redefines default argument}}
+
+// Case 6: multiple parameters
+// For multi-param templates, each parameter gets its own error+note.
+// For a 3-decl chain, the 3rd decl notes all prior decls that had defaults (per param).
+template <class T = int, class U = double> class Pair; // expected-note 2 {{previous default template argument defined here}}
+template <class T = int, class U = double> class Pair; // expected-error 2 {{template parameter redefines default argument}} expected-note 2 {{previous default template argument defined here}}
+template <class T = void, class U = float> class Pair; // expected-error 2 {{template parameter redefines default argument}}
+
+template <int N = 5, char C = 'a'> class NTMulti;      // expected-note 2 {{previous default template argument defined here}}
+template <int N = 5, char C = 'a'> class NTMulti;      // expected-error 2 {{template parameter redefines default argument}} expected-note 2 {{previous default template argument defined here}}
+template <int N = 7, char C = 'b'> class NTMulti;      // expected-error 2 {{template parameter redefines default argument}}
+
+// Case 7: template-template parameter defaults
+// Same cascading note pattern for a 3-decl chain with 2 params.
+template <class> class A;
+template <class> class B;
+
+template <template <class> class TT = A, class T = int> class TmplT; // expected-note 2 {{previous default template argument defined here}}
+template <template <class> class TT = A, class T = int> class TmplT; // expected-error 2 {{template parameter redefines default argument}} expected-note 2 {{previous default template argument defined here}}
+template <template <class> class TT = B, class T = void> class TmplT; // expected-error 2 {{template parameter redefines default argument}}
+
+// Case 8: forward declaration pattern
+template <class T = int> class Forward;   // expected-note {{previous default template argument defined here}}
+template <class T> class Forward { T value; };
+template <class T = void> class Forward;  // expected-error {{template parameter redefines default argument}}
+
+export module a;
+
+int main() {} // expected-warning {{'main' never has module linkage}}
----------------
ChuanqiXu9 wrote:

nit: newline

https://github.com/llvm/llvm-project/pull/185237