[Clang] Ignore assumptions with side effects at compile time (#85534)

Fixes #85519.

Author: Sirraide

clang/lib/AST/ExprConstant.cpp

@@ -5594,6 +5594,9 @@ static EvalStmtResult EvaluateStmt(StmtResult &Result, EvalInfo &Info,
         if (Assumption->isValueDependent())
           return ESR_Failed;
 
+        if (Assumption->HasSideEffects(Info.getCtx()))
+          continue;
+
         bool Value;
         if (!EvaluateAsBooleanCondition(Assumption, Value, Info))
           return ESR_Failed;

clang/test/SemaCXX/cxx23-assume.cpp

@@ -126,3 +126,13 @@ static_assert(f5<D>() == 1); // expected-note 3 {{while checking constraint sati
 static_assert(f5<double>() == 2);
 static_assert(f5<E>() == 1); // expected-note {{while checking constraint satisfaction}} expected-note {{in instantiation of}}
 static_assert(f5<F>() == 2); // expected-note {{while checking constraint satisfaction}} expected-note {{in instantiation of}}
+
+// Do not validate assumptions whose evaluation would have side-effects.
+constexpr int foo() {
+  int a = 0;
+  [[assume(a++)]] [[assume(++a)]]; // expected-warning 2 {{has side effects that will be discarded}} ext-warning 2 {{C++23 extension}}
+  [[assume((a+=1))]]; // expected-warning {{has side effects that will be discarded}} ext-warning {{C++23 extension}}
+  return a;
+}
+
+static_assert(foo() == 0);