[clang] [Clang] prevent assertion failure from an invalid template instantiation pattern when adding instantiated params to the scope in friend functions with defaulted params (PR #113777)

[Younan Zhang via cfe-commits]

================
@@ -3437,10 +3437,10 @@ bool Sema::SubstDefaultArgument(
       //   template<typename T> void f(T a, int = decltype(a)());
       //   void g() { f(0); }
       LIS = std::make_unique<LocalInstantiationScope>(*this);
-      FunctionDecl *PatternFD = FD->getTemplateInstantiationPattern(
-          /*ForDefinition*/ false);
-      if (addInstantiatedParametersToScope(FD, PatternFD, *LIS, TemplateArgs))
-        return true;
+      if (FunctionDecl *PatternFD =
+              FD->getTemplateInstantiationPattern(/*ForDefinition*/ false))
+        if (addInstantiatedParametersToScope(FD, PatternFD, *LIS, 
TemplateArgs))
+          return true;
----------------
zyn0217 wrote:

OTOH I'm wondering if we could avoid substituting into default arguments if we 
already know the friend declaration is not a definition. The function 
declaration instantiation seems to intentionally avoid adding connections to 
the primary template if the friend declaration is not a definition, which is 
why `getTemplateInstantiationPattern()` returns nullptr here.

https://github.com/llvm/llvm-project/pull/113777
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