[clang] [Clang] Defer the instantiation of explicit-specifier until constraint checking completes (PR #70548)

[Erich Keane via cfe-commits]

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@@ -3553,6 +3553,47 @@ static unsigned getPackIndexForParam(Sema &S,
   llvm_unreachable("parameter index would not be produced from template");
 }
 
+// if `Specialization` is a `CXXConstructorDecl` or `CXXConversionDecl`,
+// we'll try to instantiate and update its explicit specifier after constraint
+// checking.
+static Sema::TemplateDeductionResult instantiateExplicitSpecifierDeferred(
+    Sema &S, FunctionDecl *Specialization,
+    const MultiLevelTemplateArgumentList &SubstArgs,
+    TemplateDeductionInfo &Info, FunctionTemplateDecl *FunctionTemplate,
+    ArrayRef<TemplateArgument> DeducedArgs) {
+  auto GetExplicitSpecifier = [](FunctionDecl *D) {
+    return isa<CXXConstructorDecl>(D)
+               ? cast<CXXConstructorDecl>(D)->getExplicitSpecifier()
+               : cast<CXXConversionDecl>(D)->getExplicitSpecifier();
+  };
+  auto SetExplicitSpecifier = [](FunctionDecl *D, ExplicitSpecifier ES) {
+    isa<CXXConstructorDecl>(D)
+        ? cast<CXXConstructorDecl>(D)->setExplicitSpecifier(ES)
+        : cast<CXXConversionDecl>(D)->setExplicitSpecifier(ES);
+  };
+
+  ExplicitSpecifier ES = GetExplicitSpecifier(Specialization);
+  Expr *const Expr = ES.getExpr();
+  if (!Expr)
+    return Sema::TDK_Success;
+  if (!Expr->isValueDependent())
+    return Sema::TDK_Success;
+
+  Sema::InstantiatingTemplate Inst(
+      S, Info.getLocation(), FunctionTemplate, DeducedArgs,
+      Sema::CodeSynthesisContext::DeducedTemplateArgumentSubstitution, Info);
+  if (Inst.isInvalid())
+    return Sema::TDK_InstantiationDepth;
+  Sema::SFINAETrap Trap(S);
+  const auto Instantiated = S.instantiateExplicitSpecifier(SubstArgs, ES);
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erichkeane wrote:

I think this is a case where we don't permit `auto`, though this is an argument that has happened before.

https://github.com/llvm/llvm-project/pull/70548