[Concepts] Correctly form initial parameter mapping for parameter packs, support substitution into SubstNonTypeTemplateParmExpr

We previously would not correctly for the initial parameter mapping for variadic template parameters in Concepts.
Testing this lead to the discovery that with the normalization process we would need to substitute into already-substituted-into
template arguments, which means we need to add NonTypeTemplateParmExpr support to TemplateInstantiator.
We do that by substituting into the replacement and the type separately, and then re-checking the expression against the NTTP
with the new type, in order to form any new required implicit casts (for cases where the type of the NTTP was dependent).

(cherry picked from commit ba1f3db4b0729ad932aa4f091e9578132d98a0c8)

1 files changed, 28 insertions, 0 deletions

diff --git a/clang/test/SemaTemplate/instantiate-template-argument.cpp b/clang/test/SemaTemplate/instantiate-template-argument.cpp
new file mode 100644
index 00000000000..43d5d00c8cb
--- /dev/null
+++ b/clang/test/SemaTemplate/instantiate-template-argument.cpp
@@ -0,0 +1,28 @@
+// RUN: %clang_cc1 -std=c++2a -x c++ %s -verify
+
+template<auto T, decltype(T) U>
+concept C1 = sizeof(U) >= 4;
+// sizeof(U) >= 4 [U = U (decltype(T))]
+
+template<typename Y, char V>
+concept C2 = C1<Y{}, V>;
+// sizeof(U) >= 4 [U = V (decltype(Y{}))]
+
+template<char W>
+constexpr int foo() requires C2<int, W> { return 1; }
+// sizeof(U) >= 4 [U = W (decltype(int{}))]
+
+template<char X>
+// expected-note@+1{{candidate function}}
+constexpr int foo() requires C1<1, X> && true { return 2; }
+// sizeof(U) >= 4 [U = X (decltype(1))]
+
+static_assert(foo<'a'>() == 2);
+
+template<char Z>
+// expected-note@+1{{candidate function}}
+constexpr int foo() requires C2<long long, Z> && true { return 3; }
+// sizeof(U) >= 4 [U = Z (decltype(long long{}))]
+
+static_assert(foo<'a'>() == 3);
+// expected-error@-1{{call to 'foo' is ambiguous}}
\ No newline at end of file