[Clang] Do not defer variable template instantiation for undeduced types (#141009)

The previous approach broke the instantiation convention for templated
substitutions, as we were attempting to instantiate the initializer
even when it was still dependent.

We deferred variable template instantiation until the end of the TU.
However, type deduction requires the initializer immediately,
similar to how constant evaluation does.

Fixes #140773
Fixes #135032
Fixes #134526

Reapplies #138122

Author: zyn0217

clang/docs/ReleaseNotes.rst

@@ -743,6 +743,7 @@ Bug Fixes to C++ Support
   in a ``constexpr`` function. (#GH131432)
 - Fixed an incorrect TreeTransform for calls to ``consteval`` functions if a conversion template is present. (#GH137885)
 - Clang now emits a warning when class template argument deduction for alias templates is used in C++17. (#GH133806)
+- Fixed a missed initializer instantiation bug for variable templates. (#GH134526), (#GH138122)
 - Fix a crash when checking the template template parameters of a dependent lambda appearing in an alias declaration.
   (#GH136432), (#GH137014), (#GH138018)
 - Fixed an assertion when trying to constant-fold various builtins when the argument

clang/lib/Sema/SemaExpr.cpp

@@ -20158,9 +20158,10 @@ static void DoMarkVarDeclReferenced(
           Var->setTemplateSpecializationKind(TSK, PointOfInstantiation);
       }
 
-      if (UsableInConstantExpr) {
+      if (UsableInConstantExpr || Var->getType()->isUndeducedType()) {
         // Do not defer instantiations of variables that could be used in a
         // constant expression.
+        // The type deduction also needs a complete initializer.
         SemaRef.runWithSufficientStackSpace(PointOfInstantiation, [&] {
           SemaRef.InstantiateVariableDefinition(PointOfInstantiation, Var);
         });

clang/test/CodeGenCXX/cxx1z-inline-variables.cpp

@@ -1,5 +1,19 @@
 // RUN: %clang_cc1 -std=c++1z %s -emit-llvm -o - -triple x86_64-linux-gnu | FileCheck %s
 
+template <typename T> struct InlineAuto {
+  template <typename G> inline static auto var = 5;
+};
+int inlineauto = InlineAuto<int>::var<int>;
+// CHECK: @_ZN10InlineAutoIiE3varIiEE = {{.*}}i32 5{{.*}}comdat
+
+template <typename> struct PartialInlineAuto {
+  template <typename, typename> inline static auto var = 6;
+  template <typename T> inline static auto var<int, T> = 7;
+};
+
+int partialinlineauto = PartialInlineAuto<int>::var<int, int>;
+// CHECK: @_ZN17PartialInlineAutoIiE3varIiiEE = {{.*}}i32 7{{.*}}comdat
+
 struct Q {
   // CHECK: @_ZN1Q1kE = linkonce_odr constant i32 5, comdat
   static constexpr int k = 5;

clang/test/SemaTemplate/cxx17-inline-variables.cpp

@@ -1,5 +1,5 @@
 // RUN: %clang_cc1 -std=c++17 -verify %s
-// expected-no-diagnostics
+
 template<bool> struct DominatorTreeBase {
   static constexpr bool IsPostDominator = true;
 };
@@ -27,3 +27,45 @@ template <typename T> constexpr int A<T>::n = sizeof(A) + sizeof(T);
 template <typename T> inline constexpr int A<T>::m = sizeof(A) + sizeof(T);
 static_assert(A<int>().f() == 5);
 static_assert(A<int>().g() == 5);
+
+namespace GH135032 {
+
+template <typename T> struct InlineAuto {
+  template <typename G> inline static auto var = 5;
+};
+
+template <typename> struct PartialInlineAuto {
+  template <typename, typename> inline static auto var = 6;
+  template <typename T> inline static auto var<int, T> = 7;
+};
+
+int inline_auto = InlineAuto<int>::var<int>;
+int partial_inline_auto = PartialInlineAuto<int>::var<int, int>;
+
+}
+
+namespace GH140773 {
+template <class T> class ConstString { // #ConstString
+  ConstString(typename T::type) {} // #ConstString-Ctor
+};
+
+template <class = int>
+struct Foo {
+  template <char>
+  static constexpr ConstString kFilename{[] { // #kFileName
+    return 42;
+  }};
+};
+
+// We don't want to instantiate the member template until it's used!
+Foo<> foo;
+
+auto X = Foo<>::kFilename<'a'>;
+// expected-error@#kFileName {{no viable constructor}}
+// expected-note@-2 {{in instantiation of static data member}}
+// expected-note@#ConstString-Ctor {{candidate template ignored}}
+// expected-note@#ConstString-Ctor {{implicit deduction guide}}
+// expected-note@#ConstString {{candidate template ignored}}
+// expected-note@#ConstString {{implicit deduction guide}}
+
+}