Allow non-constant calls to constexpr functions. (#7067)

These turn up frequently in real-world code, for example when converting
a mutable global `Cpp.std.string_view` to a `Cpp.std.string`. Only
reject a non-constant call if the callee is `consteval`, not if it's
`constexpr`.

toolchain/check/cpp/constant.cpp

@@ -267,9 +267,12 @@ auto EvalCppCall(Context& context, SemIR::LocId loc_id,
   clang::Expr::EvalResult eval_result;
   eval_result.Diag = &notes;
   if (!call_expr->EvaluateAsConstantExpr(eval_result, context.ast_context())) {
+    if (!function_decl->isConsteval()) {
+      return SemIR::ConstantId::NotConstant;
+    }
     context.clang_sema().Diag(call_expr->getBeginLoc(),
                               clang::diag::err_invalid_consteval_call)
-        << function_decl << function_decl->isConsteval();
+        << function_decl << /*is consteval*/ true;
     for (const auto& note : notes) {
       context.clang_sema().Diag(note.first, note.second);
     }

toolchain/check/testdata/interop/cpp/constexpr.carbon

@@ -3,6 +3,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // INCLUDE-FILE: toolchain/testing/testdata/min_prelude/full.carbon
+// EXTRA-ARGS: --clang-arg=--std=c++20
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
@@ -120,14 +121,58 @@ constexpr int f(int a) {
 }
 ''';
 
-// CHECK:STDERR: fail_invalid_constant_eval.carbon:[[@LINE+10]]:26: error: call to immediate function 'f' is not a constant expression [CppInteropParseError]
-// CHECK:STDERR:    21 | let a: array(i32, Cpp.f(5)) = (1, 2, 3);
-// CHECK:STDERR:       |                          ^
-// CHECK:STDERR: fail_invalid_constant_eval.carbon:[[@LINE-7]]:10: note: cannot refer to element 5 of array of 1 element in a constant expression [CppInteropParseNote]
-// CHECK:STDERR:     7 |   return arr[a];
-// CHECK:STDERR:       |          ^
-// CHECK:STDERR: fail_invalid_constant_eval.carbon:[[@LINE+4]]:26: note: in call to 'f(5)' [CppInteropParseNote]
-// CHECK:STDERR:    21 | let a: array(i32, Cpp.f(5)) = (1, 2, 3);
-// CHECK:STDERR:       |                          ^
+// CHECK:STDERR: fail_invalid_constant_eval.carbon:[[@LINE+4]]:19: error: array bound is not a constant [InvalidArrayExpr]
+// CHECK:STDERR: let a: array(i32, Cpp.f(5)) = (1, 2, 3);
+// CHECK:STDERR:                   ^~~~~~~~
 // CHECK:STDERR:
 let a: array(i32, Cpp.f(5)) = (1, 2, 3);
+
+// --- nonconstant_constexpr_call.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+var a: i32 = 0;
+
+inline Cpp '''
+constexpr int f(const int &r) { return r; }
+''';
+
+// OK, runtime call.
+var b: i32 = Cpp.f(a);
+
+// --- fail_nonconstant_consteval_call.carbon
+
+library "[[@TEST_NAME]]";
+
+import Cpp;
+
+var a: i32 = 0;
+
+// TODO: This is failing when building the thunk, not when calling it.
+inline Cpp '''
+// CHECK:STDERR: fail_nonconstant_consteval_call.carbon:[[@LINE+9]]:15: error: call to consteval function 'f' is not a constant expression [CppInteropParseError]
+// CHECK:STDERR:    19 | consteval int f(const int &r) { return r; }
+// CHECK:STDERR:       |               ^
+// CHECK:STDERR: fail_nonconstant_consteval_call.carbon:[[@LINE+6]]:15: note: function parameter 'r' with unknown value cannot be used in a constant expression [CppInteropParseNote]
+// CHECK:STDERR:    19 | consteval int f(const int &r) { return r; }
+// CHECK:STDERR:       |               ^
+// CHECK:STDERR: fail_nonconstant_consteval_call.carbon:[[@LINE+3]]:15: note: declared here [CppInteropParseNote]
+// CHECK:STDERR:    19 | consteval int f(const int &r) { return r; }
+// CHECK:STDERR:       |               ^
+consteval int f(const int &r) { return r; }
+''';
+
+// CHECK:STDERR: fail_nonconstant_consteval_call.carbon:[[@LINE+11]]:14: note: in thunk for C++ function used here [InCppThunk]
+// CHECK:STDERR: var b: i32 = Cpp.f(a);
+// CHECK:STDERR:              ^~~~~~~~
+// CHECK:STDERR:
+// CHECK:STDERR: fail_nonconstant_consteval_call.carbon:[[@LINE+7]]:14: error: non-constant call to compile-time-only function [NonConstantCallToCompTimeOnlyFunction]
+// CHECK:STDERR: var b: i32 = Cpp.f(a);
+// CHECK:STDERR:              ^~~~~~~~
+// CHECK:STDERR: fail_nonconstant_consteval_call.carbon:[[@LINE-10]]:15: note: compile-time-only function declared here [CompTimeOnlyFunctionHere]
+// CHECK:STDERR: consteval int f(const int &r) { return r; }
+// CHECK:STDERR:               ^
+// CHECK:STDERR:
+var b: i32 = Cpp.f(a);