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

[Younan Zhang via cfe-commits]

================
@@ -3553,6 +3553,56 @@ static unsigned getPackIndexForParam(Sema &S,
   llvm_unreachable("parameter index would not be produced from template");
 }
 
+// if `Specialization` is a `CXXConstructorDecl` or `CXXConversionDecl`
+// we try to instantiate and update its explicit specifier after constraint
+// checking.
+static Sema::TemplateDeductionResult
+tryInstantiateExplicitSpecifier(Sema &S, FunctionDecl *Specialization,
+                                const MultiLevelTemplateArgumentList &SubstArgs,
+                                TemplateDeductionInfo &Info,
+                                FunctionTemplateDecl *FunctionTemplate,
+                                ArrayRef<TemplateArgument> DeducedArgs) {
+
+  const auto TryInstantiateExplicitSpecifierForSingleDecl =
+      [&](auto *ExplicitDecl) {
+        ExplicitSpecifier ExplicitSpecifier =
+            ExplicitDecl->getExplicitSpecifier();
+        Expr *const Expr = ExplicitSpecifier.getExpr();
+        if (!Expr) {
+          return Sema::TDK_Success;
+        }
+        if (!Expr->isValueDependent()) {
+          return Sema::TDK_Success;
+        }
+        // TemplateDeclInstantiator::InitFunctionInstantiation set the
+        // ActiveInstType to TemplateInstantiation, but we need
+        // to enable SFINAE when instantiating explicit specifier.
----------------
zyn0217 wrote:

```suggestion
        // TemplateDeclInstantiator::InitFunctionInstantiation sets the
        // ActiveInstType to TemplateInstantiation, but we need
        // to enable SFINAE when instantiating an explicit specifier.
```

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