[Clang] Only check exprs that might be immediate escalating in evaluated contexts (#93187)

As per https://eel.is/c++draft/expr.const#17

Fixes #91308

---------

Co-authored-by: Mariya Podchishchaeva <[email protected]>

Author: cor3ntin

clang/docs/ReleaseNotes.rst

@@ -769,6 +769,8 @@ Bug Fixes to C++ Support
 - Fixed a crash when trying to emit captures in a lambda call operator with an explicit object
   parameter that is called on a derived type of the lambda.
   Fixes (#GH87210), (GH89541).
+- Clang no longer tries to check if an expression is immediate-escalating in an unevaluated context.
+  Fixes (#GH91308).
 
 Bug Fixes to AST Handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^

clang/include/clang/Sema/Sema.h

@@ -5112,6 +5112,13 @@ class Sema final : public SemaBase {
              Context == ExpressionEvaluationContext::UnevaluatedList;
     }
 
+    bool isPotentiallyEvaluated() const {
+      return Context == ExpressionEvaluationContext::PotentiallyEvaluated ||
+             Context ==
+                 ExpressionEvaluationContext::PotentiallyEvaluatedIfUsed ||
+             Context == ExpressionEvaluationContext::ConstantEvaluated;
+    }
+
     bool isConstantEvaluated() const {
       return Context == ExpressionEvaluationContext::ConstantEvaluated ||
              Context == ExpressionEvaluationContext::ImmediateFunctionContext;
@@ -5146,6 +5153,12 @@ class Sema final : public SemaBase {
     return ExprEvalContexts.back();
   };
 
+  const ExpressionEvaluationContextRecord &parentEvaluationContext() const {
+    assert(ExprEvalContexts.size() >= 2 &&
+           "Must be in an expression evaluation context");
+    return ExprEvalContexts[ExprEvalContexts.size() - 2];
+  };
+
   bool isBoundsAttrContext() const {
     return ExprEvalContexts.back().ExprContext ==
            ExpressionEvaluationContextRecord::ExpressionKind::

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -4788,8 +4788,13 @@ TemplateDeductionResult Sema::DeduceTemplateArguments(
       DeduceReturnType(Specialization, Info.getLocation(), false))
     return TemplateDeductionResult::MiscellaneousDeductionFailure;
 
+  // [C++26][expr.const]/p17
+  // An expression or conversion is immediate-escalating if it is not initially
+  // in an immediate function context and it is [...]
+  // a potentially-evaluated id-expression that denotes an immediate function.
   if (IsAddressOfFunction && getLangOpts().CPlusPlus20 &&
       Specialization->isImmediateEscalating() &&
+      parentEvaluationContext().isPotentiallyEvaluated() &&
       CheckIfFunctionSpecializationIsImmediate(Specialization,
                                                Info.getLocation()))
     return TemplateDeductionResult::MiscellaneousDeductionFailure;

clang/test/SemaCXX/cxx2b-consteval-propagate.cpp

@@ -446,3 +446,11 @@ int h(int x) {
 }
 
 #endif
+
+
+namespace GH91308 {
+    constexpr void f(auto) {
+        static_assert(false);
+    }
+    using R1 = decltype(&f<int>);
+}