[clang] [Clang] Fix deduction from constant TP of reference type. (PR #223645)

[Corentin Jabot via cfe-commits]

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@@ -261,6 +261,20 @@ getDeducedNTTParameterFromExpr(TemplateDeductionInfo &Info, Expr *E) {
   return getDeducedNTTParameterFromExpr(E, Info.getDeducedDepth());
 }
 
+/// C++26 [temp.deduct.type]p13:
+///   When the value of the argument corresponding to a constant template
+///   parameter P that is declared with a dependent type is deduced from an
+///   expression, the template parameters in the type of P are deduced from the
+///   type of the value.
+static QualType getTypeOfTemplateArgumentValue(TemplateDeductionInfo &Info,
+                                               const TemplateArgument &A) {
+  const Expr *E = unwrapExpressionForDeduction(A.getAsExpr());
+  if (NonTypeOrVarTemplateParmDecl NTTP =
+          getDeducedNTTParameterFromExpr(E, Info.getDeducedDepth()))
+    return NTTP.getType();
+  return E->getType();
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cor3ntin wrote:

We used to use `IgnoreImplicitAsWritten` - however, it's slightly different from `unwrapExpressionForDeduction`, and i could not convince myself `IgnoreImplicitAsWritten` was correct

https://github.com/llvm/llvm-project/pull/223645