fix Issue 23622 - ImportC #defines conflict with declarations

compiler/src/dmd/cparse.d

@@ -5306,6 +5306,8 @@ final class CParser(AST) : Parser!AST
 
         void addVar(AST.VarDeclaration v)
         {
+            //printf("addVar() %s\n", v.toChars());
+            v.isCmacro(true);           // mark it as coming from a C #define
             /* If it's already defined, replace the earlier
              * definition
              */

compiler/src/dmd/declaration.d

@@ -1148,6 +1148,7 @@ extern (C++) class VarDeclaration : Declaration
         bool doNotInferReturn;  /// do not infer 'return' for this variable
 
         bool isArgDtorVar;      /// temporary created to handle scope destruction of a function argument
+        bool isCmacro;          /// it is a C macro turned into a C declaration
     }
 
     import dmd.common.bitfields : generateBitFields;

compiler/src/dmd/declaration.h

@@ -270,6 +270,8 @@ public:
     bool doNotInferReturn(bool v);
     bool isArgDtorVar() const; // temporary created to handle scope destruction of a function argument
     bool isArgDtorVar(bool v);
+    bool isCmacro() const; // if a C macro turned into a C variable
+    bool isCmacro(bool v);
     static VarDeclaration *create(const Loc &loc, Type *t, Identifier *id, Initializer *init, StorageClass storage_class = STCundefined);
     VarDeclaration *syntaxCopy(Dsymbol *) override;
     void setFieldOffset(AggregateDeclaration *ad, FieldState& fieldState, bool isunion) override final;

compiler/src/dmd/dsymbol.d

@@ -2620,6 +2620,12 @@ Dsymbol handleSymbolRedeclarations(ref Scope sc, Dsymbol s, Dsymbol s2, ScopeDsy
 
     auto vd = s.isVarDeclaration(); // new declaration
     auto vd2 = s2.isVarDeclaration(); // existing declaration
+
+    if (vd && vd.isCmacro())
+        return vd2;
+
+    assert(!(vd2 && vd2.isCmacro()));
+
     if (vd && vd2)
     {
         /* if one is `static` and the other isn't, the result is undefined

compiler/src/dmd/frontend.h

@@ -5944,6 +5944,8 @@ public:
     bool doNotInferReturn(bool v);
     bool isArgDtorVar() const;
     bool isArgDtorVar(bool v);
+    bool isCmacro() const;
+    bool isCmacro(bool v);
 private:
     uint16_t bitFields;
 public:

compiler/test/compilable/test23622.c

@@ -0,0 +1,7 @@
+// https://issues.dlang.org/show_bug.cgi?id=23622
+
+int FP_NAN = 0;
+#define FP_NAN 0
+
+enum E { ENUM = 0 };
+#define ENUM 0